Work on error messages

Signed-off-by: Marcel Müller <neikos@neikos.email>

Cargo.lock

@@ -240,6 +240,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "document-features"
+version = "0.2.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d4b8a88685455ed29a21542a33abd9cb6510b6b129abadabdcef0f4c55bc8f61"
+dependencies = [
+ "litrs",
+]
+
 [[package]]
 name = "either"
 version = "1.15.0"
@@ -420,6 +429,12 @@ version = "0.12.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "32a66949e030da00e8c7d4434b251670a91556f4144941d37452769c25d58a53"
 
+[[package]]
+name = "litrs"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "11d3d7f243d5c5a8b9bb5d6dd2b1602c0cb0b9db1621bafc7ed66e35ff9fe092"
+
 [[package]]
 name = "log"
 version = "0.4.29"
@@ -439,7 +454,9 @@ dependencies = [
  "annotate-snippets",
  "criterion",
  "displaydoc",
+ "document-features",
  "insta",
+ "nomo",
  "serde",
  "serde_json",
  "test_each_file",

Cargo.toml

@@ -16,9 +16,14 @@ harness = false
 [profile.bench]
 debug = true
 
+[lints.rust]
+unsafe_code = "forbid"
+missing_docs = "warn"
+
 [dependencies]
 annotate-snippets = "0.12.13"
 displaydoc = "0.2.5"
+document-features = { version = "0.2.12", optional = true }
 serde_json = { version = "1.0.149", optional = true }
 thiserror = "2.0.18"
 winnow = { version = "0.7.14", features = ["unstable-recover"] }
@@ -30,11 +35,19 @@ insta = { version = "1.46.3", features = ["glob", "serde"] }
 serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.149"
 test_each_file = "0.3.7"
+nomo = { path = ".", features = ["unstable-pub"] }
 
 [profile.dev.package]
 insta.opt-level = 3
 similar.opt-level = 3
 
+[package.metadata.docs.rs]
+features = ["document-features"]
+
 [features]
 default = ["serde_json"]
+## Add support for inserting [`serde_json::Value`]s into [`Context`] objects
 serde_json = ["dep:serde_json"]
+## Get access to the internals of the crate ⚠️ This is a perma-unstable feature ⚠️
+unstable-pub = []
+document-features = ["dep:document-features"]

benches/asting.rs

@@ -1,3 +1,5 @@
+#![allow(missing_docs)]
+
 use criterion::BenchmarkId;
 use criterion::Criterion;
 use criterion::criterion_group;
@@ -20,7 +22,7 @@ fn asting_benchmark(c: &mut Criterion) {
         parsing.bench_with_input(BenchmarkId::from_parameter(size), &input, |b, input| {
             b.iter(|| {
                 let tokens = nomo::lexer::parse(input.clone()).unwrap();
-                let _ast = nomo::parser::parse(tokens.tokens()).unwrap();
+                let _ast = nomo::parser::parse(input.clone(), tokens.tokens()).unwrap();
             });
         });
     }
@@ -44,7 +46,7 @@ fn asting_nested(c: &mut Criterion) {
         parsing.bench_with_input(BenchmarkId::from_parameter(size), &input, |b, input| {
             b.iter(|| {
                 let tokens = nomo::lexer::parse(input.clone()).unwrap();
-                let _ast = nomo::parser::parse(tokens.tokens()).unwrap();
+                let _ast = nomo::parser::parse(input.clone(), tokens.tokens()).unwrap();
             });
         });
     }

benches/parsing.rs

@@ -1,3 +1,5 @@
+#![allow(missing_docs)]
+
 use criterion::BenchmarkId;
 use criterion::Criterion;
 use criterion::criterion_group;

fuzz/Cargo.lock

@@ -112,7 +112,7 @@ checksum = "f8ca58f447f06ed17d5fc4043ce1b10dd205e060fb3ce5b979b8ed8e59ff3f79"
 
 [[package]]
 name = "nomo"
-version = "0.1.0"
+version = "0.0.1"
 dependencies = [
  "annotate-snippets",
  "displaydoc",

fuzz/Cargo.toml

@@ -12,6 +12,7 @@ libfuzzer-sys = "0.4"
 
 [dependencies.nomo]
 path = ".."
+features = ["unstable-pub"]
 
 [[bin]]
 name = "fuzz_target_1"

src/compiler/mod.rs

@@ -63,6 +63,7 @@ pub enum Instruction {
         slot: VariableSlot,
     },
     PushScope {
+        #[allow(unused)]
         inherit_parent: bool,
     },
     Abort,
@@ -93,6 +94,7 @@ pub enum Instruction {
         value_slot: VariableSlot,
     },
     LoadLiteralToSlot {
+        #[allow(unused)]
         source: TemplateToken,
         value: NomoValue,
         slot: VariableSlot,
@@ -577,14 +579,15 @@ fn emit_expr_load(
 #[cfg(test)]
 mod tests {
     use crate::compiler::emit_machine;
+    use crate::input::NomoInput;
 
     #[test]
     fn check_simple_variable_interpolation() {
-        let input = "Hello {{= world }}";
+        let input = NomoInput::from("Hello {{= world }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = crate::parser::parse(parsed.tokens()).unwrap();
+        let ast = crate::parser::parse(input, parsed.tokens()).unwrap();
 
         let emit = emit_machine(ast);
 
@@ -617,11 +620,12 @@ mod tests {
 
     #[test]
     fn check_if_else_if() {
-        let input = "{{ if foo }} foo {{ else if bar }} bar {{ else }} foobar {{ end }}";
+        let input =
+            NomoInput::from("{{ if foo }} foo {{ else if bar }} bar {{ else }} foobar {{ end }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = crate::parser::parse(parsed.tokens()).unwrap();
+        let ast = crate::parser::parse(input, parsed.tokens()).unwrap();
 
         let emit = emit_machine(ast);
 
@@ -630,11 +634,11 @@ mod tests {
 
     #[test]
     fn check_function_call() {
-        let input = "{{ if foo(23) }} bar {{ else }} foobar {{ end }}";
+        let input = NomoInput::from("{{ if foo(23) }} bar {{ else }} foobar {{ end }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = crate::parser::parse(parsed.tokens()).unwrap();
+        let ast = crate::parser::parse(input, parsed.tokens()).unwrap();
 
         let emit = emit_machine(ast);
 

src/errors.rs

@@ -0,0 +1,153 @@
+use std::sync::Arc;
+
+use annotate_snippets::AnnotationKind;
+use annotate_snippets::Level;
+use annotate_snippets::Patch;
+use annotate_snippets::Renderer;
+use annotate_snippets::Snippet;
+use thiserror::Error;
+use winnow::stream::Offset;
+
+use crate::input::NomoInput;
+use crate::lexer::ParseError;
+use crate::parser::AstError;
+
+/// An error occurred while producing an Ast
+#[derive(Debug, Error)]
+pub struct AstFailure {
+    errors: Vec<AstError>,
+    input: NomoInput,
+}
+
+impl std::fmt::Display for AstFailure {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.render(f)
+    }
+}
+
+impl AstFailure {
+    pub(crate) fn from_errors(errors: Vec<AstError>, source: NomoInput) -> AstFailure {
+        AstFailure {
+            errors,
+            input: source,
+        }
+    }
+
+    /// Create a CLI printable report
+    pub fn to_report(&self) -> String {
+        self.to_string()
+    }
+
+    /// Render this failure to the given formatter
+    ///
+    /// Note, you can also use [`to_string`](ToString::to_string), as this type also implements
+    /// [`Display`](std::fmt::Display).
+    pub fn render(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        let renderer = annotate_snippets::Renderer::styled()
+            .decor_style(annotate_snippets::renderer::DecorStyle::Unicode);
+
+        for error in &self.errors {
+            let mut snippets = vec![
+                annotate_snippets::Level::ERROR
+                    .primary_title(
+                        error
+                            .message
+                            .as_deref()
+                            .unwrap_or("An error occurred while producing an Ast"),
+                    )
+                    .element(
+                        annotate_snippets::Snippet::source(self.input.as_str()).annotation(
+                            annotate_snippets::AnnotationKind::Primary.span(
+                                constrain_without_whitespace(
+                                    self.input.as_ref(),
+                                    error.span.clone().map(|s| s.range).unwrap_or_else(|| 0..0),
+                                ),
+                            ),
+                        ),
+                    )
+                    .elements(
+                        error
+                            .help
+                            .as_ref()
+                            .map(|help| annotate_snippets::Level::HELP.message(help)),
+                    ),
+            ];
+            if let Some((range, help)) = &error.replacement {
+                snippets.push(
+                    annotate_snippets::Level::NOTE
+                        .secondary_title("Try adding it")
+                        .element(
+                            Snippet::source(self.input.as_str())
+                                .patch(Patch::new(range.range.clone(), help.clone())),
+                        ),
+                );
+            }
+
+            writeln!(f, "{}", renderer.render(&snippets))?;
+            writeln!(f)?;
+        }
+
+        Ok(())
+    }
+}
+
+fn constrain_without_whitespace(
+    input: &str,
+    range: std::ops::Range<usize>,
+) -> std::ops::Range<usize> {
+    let trimmed = input[range].trim();
+    let start = trimmed.offset_from(&input);
+    let end = start + trimmed.len();
+
+    start..end
+}
+
+/// An error occurred during lexing
+#[derive(Debug, Error)]
+pub struct ParseFailure {
+    input: Arc<str>,
+    errors: Vec<ParseError>,
+}
+
+impl std::fmt::Display for ParseFailure {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(&self.to_report())
+    }
+}
+
+impl ParseFailure {
+    pub(crate) fn from_errors(errors: Vec<ParseError>, input: NomoInput) -> ParseFailure {
+        ParseFailure {
+            input: Arc::from(input.to_string()),
+            errors,
+        }
+    }
+
+    /// Produce a CLi printable report
+    pub fn to_report(&self) -> String {
+        let reports = self
+            .errors
+            .iter()
+            .map(|error| {
+                Level::ERROR
+                    .primary_title(
+                        error
+                            .message
+                            .as_deref()
+                            .unwrap_or("An error occurred while parsing"),
+                    )
+                    .element(
+                        Snippet::source(self.input.as_ref()).annotation(
+                            AnnotationKind::Primary
+                                .span(error.span.clone().map(|s| s.range).unwrap_or_else(|| 0..0)),
+                        ),
+                    )
+                    .elements(error.help.as_ref().map(|help| Level::HELP.message(help)))
+            })
+            .collect::<Vec<_>>();
+
+        let renderer =
+            Renderer::styled().decor_style(annotate_snippets::renderer::DecorStyle::Unicode);
+        renderer.render(&reports)
+    }
+}

src/eval/mod.rs

@@ -297,14 +297,15 @@ mod tests {
     use crate::eval::execute;
     use crate::functions::FunctionMap;
     use crate::functions::NomoFunctionError;
+    use crate::input::NomoInput;
 
     #[test]
     fn check_simple_variable_interpolation() {
-        let input = "Hello {{= world }}";
+        let input = NomoInput::from("Hello {{= world }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = crate::parser::parse(parsed.tokens()).unwrap();
+        let ast = crate::parser::parse(input, parsed.tokens()).unwrap();
 
         let emit = crate::compiler::emit_machine(ast);
 
@@ -321,11 +322,11 @@ mod tests {
 
     #[test]
     fn check_method_call() {
-        let input = "Hello {{= foo(world) }}";
+        let input = NomoInput::from("Hello {{= foo(world) }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = crate::parser::parse(parsed.tokens()).unwrap();
+        let ast = crate::parser::parse(input, parsed.tokens()).unwrap();
 
         let emit = crate::compiler::emit_machine(ast);
 
@@ -348,11 +349,11 @@ mod tests {
 
     #[test]
     fn check_conditional_access() {
-        let input = "Hello {{= unknown? }}";
+        let input = NomoInput::from("Hello {{= unknown? }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = crate::parser::parse(parsed.tokens()).unwrap();
+        let ast = crate::parser::parse(input, parsed.tokens()).unwrap();
 
         let emit = crate::compiler::emit_machine(ast);
 

src/functions.rs

@@ -7,42 +7,57 @@ use thiserror::Error;
 use crate::NomoValueError;
 use crate::value::NomoValue;
 
+/// Possible errors while executing a function
 #[derive(Debug, Error, Display)]
 pub enum NomoFunctionError {
     /// Received {received} arguments, but this function only takes {expected}
+    #[expect(missing_docs)]
     WrongArgumentCount { received: usize, expected: usize },
 
     /// The argument at this position is of the wrong type
+    #[expect(missing_docs)]
     InvalidArgumentType { index: usize },
 
     /// A user-provided error
     #[error(transparent)]
     CustomError {
+        #[expect(missing_docs)]
         custom: Box<dyn std::error::Error + Send + Sync>,
     },
 }
 
+/// A function that can be used inside a template
 pub trait NomoFunction<T>: 'static + Send + Sync {
+    /// Call the function with the given arguments
     fn call(&self, args: Vec<NomoValue>) -> Result<NomoValue, NomoFunctionError>;
 }
 
+#[cfg(feature = "unstable-pub")]
 #[derive(Default)]
+#[expect(missing_docs)]
 pub struct FunctionMap {
     funcs: HashMap<String, ErasedNomoFunction>,
 }
 
+#[cfg(not(feature = "unstable-pub"))]
+#[derive(Default)]
+pub(crate) struct FunctionMap {
+    funcs: HashMap<String, ErasedNomoFunction>,
+}
+
 impl FunctionMap {
+    #[expect(missing_docs)]
     pub fn register<NF: NomoFunction<T>, T>(&mut self, name: impl Into<String>, func: NF) {
         self.funcs
             .insert(name.into(), ErasedNomoFunction::erase(func));
     }
 
-    pub fn get(&self, name: impl AsRef<str>) -> Option<&ErasedNomoFunction> {
+    pub(crate) fn get(&self, name: impl AsRef<str>) -> Option<&ErasedNomoFunction> {
         self.funcs.get(name.as_ref())
     }
 }
 
-pub struct ErasedNomoFunction {
+pub(crate) struct ErasedNomoFunction {
     func: Box<dyn Any + Send + Sync>,
     call_fn: fn(&dyn Any, Vec<NomoValue>) -> Result<NomoValue, NomoFunctionError>,
 }

src/input.rs

@@ -8,6 +8,7 @@ use winnow::stream::Offset;
 use winnow::stream::Stream;
 use winnow::stream::StreamIsPartial;
 
+/// The input for templates in [nomo](crate)
 #[derive(Clone, PartialEq, Eq)]
 pub struct NomoInput {
     backing: Arc<str>,
@@ -15,19 +16,26 @@ pub struct NomoInput {
 }
 
 impl NomoInput {
+    /// Manually create an input
+    ///
+    /// While it is not unsafe to pass in mismatched informations, the outcome will most likely not
+    /// work and possibly even panic later
     pub fn from_parts(backing: Arc<str>, range: Range<usize>) -> NomoInput {
         NomoInput { backing, range }
     }
 
+    /// Turn the input into its parts
     pub fn into_parts(self) -> (Arc<str>, Range<usize>) {
         (self.backing, self.range)
     }
 
+    /// Get the range this input covers
     pub fn get_range(&self) -> Range<usize> {
         self.range.clone()
     }
 }
 
+#[doc(hidden)]
 #[derive(Debug, Clone)]
 pub struct NomoInputCheckpoint {
     range: Range<usize>,
@@ -78,6 +86,7 @@ impl Deref for NomoInput {
 }
 
 impl NomoInput {
+    /// Get the input as a [`str`]
     pub fn as_str(&self) -> &str {
         self.deref()
     }
@@ -101,6 +110,8 @@ impl Offset for NomoInput {
     }
 }
 
+
+#[doc(hidden)]
 pub struct NomoInputIter {
     idx: usize,
     input: NomoInput,

src/lexer/mod.rs

@@ -1,10 +1,3 @@
-use std::sync::Arc;
-
-use annotate_snippets::AnnotationKind;
-use annotate_snippets::Level;
-use annotate_snippets::Renderer;
-use annotate_snippets::Snippet;
-use thiserror::Error;
 use winnow::LocatingSlice;
 use winnow::Parser;
 use winnow::RecoverableParser;
@@ -39,60 +32,13 @@ use winnow::token::take_until;
 use winnow::token::take_while;
 
 use crate::SourceSpan;
+use crate::errors::ParseFailure;
 use crate::input::NomoInput;
 use crate::resume_after_cut;
 
 type Input<'input> = Recoverable<LocatingSlice<NomoInput>, ParseError>;
 type PResult<'input, T> = Result<T, ParseError>;
 
-#[derive(Debug, Error)]
-pub struct ParseFailure {
-    input: Arc<str>,
-    errors: Vec<ParseError>,
-}
-
-impl std::fmt::Display for ParseFailure {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.write_str(&self.to_report())
-    }
-}
-
-impl ParseFailure {
-    fn from_errors(errors: Vec<ParseError>, input: NomoInput) -> ParseFailure {
-        ParseFailure {
-            input: Arc::from(input.to_string()),
-            errors,
-        }
-    }
-
-    pub fn to_report(&self) -> String {
-        let reports = self
-            .errors
-            .iter()
-            .map(|error| {
-                Level::ERROR
-                    .primary_title(
-                        error
-                            .message
-                            .as_deref()
-                            .unwrap_or("An error occurred while parsing"),
-                    )
-                    .element(
-                        Snippet::source(self.input.as_ref()).annotation(
-                            AnnotationKind::Primary
-                                .span(error.span.clone().map(|s| s.range).unwrap_or_else(|| 0..0)),
-                        ),
-                    )
-                    .elements(error.help.as_ref().map(|help| Level::HELP.message(help)))
-            })
-            .collect::<Vec<_>>();
-
-        let renderer =
-            Renderer::styled().decor_style(annotate_snippets::renderer::DecorStyle::Unicode);
-        renderer.render(&reports)
-    }
-}
-
 #[derive(Debug, Clone)]
 pub struct ParseError {
     pub(crate) message: Option<String>,

src/lib.rs

@@ -1,3 +1,102 @@
+//! # Nomo, the templating library
+//!
+//! To get started, add the crate to your project:
+//!
+//! ```bash
+//! cargo add nomo
+//! ```
+//!
+//! Then, load some templates:
+//!
+//! ```rust
+//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
+//! let mut templates = nomo::Nomo::new();
+//! templates.add_template("index.html", "<!DOCTYPE html><h1>Hello {{= name }}!</h1>");
+//!
+//! let mut context = nomo::Context::new();
+//! context.insert("name", "World");
+//! let result = templates.render("index.html", &context)?;
+//!
+//! assert_eq!(result, "<!DOCTYPE html><h1>Hello World!</h1>");
+//! # Ok(()) }
+//! ```
+//!
+//! The crate has the following feature flags:
+#![cfg_attr(
+    feature = "document-features",
+    cfg_attr(doc, doc = ::document_features::document_features!())
+)]
+//!
+//! ## Syntax
+//!
+//! Nomo tries to keep a consistent syntax across its features.
+//! The main concepts are:
+//!
+//! [`nomo`](crate) uses `{{ }}` as its identifiers. Each of them forms a 'block'.
+//!
+//! There are two kinds of blocks:
+//!
+//! - `{{= <expr> }}` Interpolations
+//! - `{{ <control> }}` everything else
+//!
+//! ### Expressions
+//!
+//! An expression in [`nomo`](crate) is anything that produces a value. Notably, control structures
+//! _do not_ create values. This is different to rust.
+//!
+//! So for example this does not work:
+//!
+//! ```nomo
+//! {{= if is_active "Active" else "Inactive" end }}
+//! ```
+//!
+//! If you wish to conditionally output something, you would write:
+//!
+//! ```nomo
+//! {{ if is_active }}
+//!     Active
+//! {{ else }}
+//!     Inactive
+//! {{ end }}
+//! ```
+//!
+//! In expressions you can write:
+//! - Mathematical expressions (`+-*/`)
+//! - Logical/Binary expressions (`&&`, `||`, `>`, ...)
+//! - Literals (`12`, `292.21`, `"Hello"`)
+//!
+//! ### Interpolations
+//!
+//! Interpolations is how one prints out data. A [`NomoValue`] can be printed if it is a:
+//!
+//! - String
+//! - Integer/SignedInteger
+//! - Bool
+//! - Float
+//!
+//! All other values will result in an error.
+//!
+//! ### Control Structures
+//!
+//! [`Nomo`](crate) supports several control structures:
+//!
+//! **Conditions `if/else`**:
+//!
+//! ```nomo
+//! {{ if <expr> }}
+//! {{ else if <expr> }}
+//! {{ else }}
+//! {{ end }}
+//! ```
+//!
+//! **Loops `for .. in`**
+//!
+//! ```nomo
+//! {{ for <identifier> in <expr> }}
+//! {{ else }}
+//! {{ end }}
+//! ```
+
 use std::collections::HashMap;
 
 use displaydoc::Display;
@@ -5,34 +104,71 @@ use thiserror::Error;
 
 use crate::compiler::VMInstructions;
 use crate::functions::FunctionMap;
+use crate::functions::NomoFunction;
 use crate::input::NomoInput;
 use crate::value::NomoValue;
 use crate::value::NomoValueError;
 
-pub mod compiler;
+macro_rules! unstable_pub {
-pub mod eval;
+    ($(#[$m:meta])* mod $name:ident) => {
+        $(#[$m])*
+        #[cfg(feature = "unstable-pub")]
+        #[allow(missing_docs)]
+        pub mod $name;
+        #[cfg(not(feature = "unstable-pub"))]
+        mod $name;
+    };
+}
+
+unstable_pub!(
+    /// The compiler internals
+    mod compiler
+);
+unstable_pub!(
+    /// Evaluation internals
+    mod eval
+);
+unstable_pub!(
+    /// Lexer internals
+    mod lexer
+);
+unstable_pub!(
+    /// Parser internals
+    mod parser
+);
+
+mod winnow_ext;
+
+/// Errors in this library
+pub mod errors;
+
+/// Nomo Functions
 pub mod functions;
+/// Input for nomo
 pub mod input;
-pub mod lexer;
+/// Values used in Nomo
-pub mod parser;
 pub mod value;
 
+/// Errors related to parsing and evaluating templates
 #[derive(Debug, Error, Display)]
 pub enum NomoError {
     /// Could not parse the given template
     ParseError {
         #[from]
-        source: lexer::ParseFailure,
+        #[expect(missing_docs)]
+        source: errors::ParseFailure,
     },
     /// Invalid Template
     AstError {
         #[from]
-        source: parser::AstFailure,
+        #[expect(missing_docs)]
+        source: errors::AstFailure,
     },
 
     /// An error occurred while evaluating
     EvaluationError {
         #[from]
+        #[expect(missing_docs)]
         source: eval::EvaluationError,
     },
 
@@ -40,6 +176,7 @@ pub enum NomoError {
     UnknownTemplate(String),
 }
 
+/// The main struct and entry point for the [`nomo`](crate)
 pub struct Nomo {
     templates: HashMap<String, Template>,
     function_map: FunctionMap,
@@ -52,6 +189,7 @@ impl Default for Nomo {
 }
 
 impl Nomo {
+    /// Create a new Nomo Instance
     pub fn new() -> Nomo {
         Nomo {
             templates: HashMap::new(),
@@ -59,6 +197,7 @@ impl Nomo {
         }
     }
 
+    /// Add a new template
     pub fn add_template(
         &mut self,
         name: impl Into<String>,
@@ -66,7 +205,7 @@ impl Nomo {
     ) -> Result<(), NomoError> {
         let source = value.into();
         let parse = lexer::parse(source.clone())?;
-        let ast = parser::parse(parse.tokens())?;
+        let ast = parser::parse(source.clone(), parse.tokens())?;
 
         let instructions = compiler::emit_machine(ast);
 
@@ -76,6 +215,17 @@ impl Nomo {
         Ok(())
     }
 
+    /// Register a function to make it available when rendering
+    pub fn register_function<A>(&mut self, name: impl Into<String>, f: impl NomoFunction<A>) {
+        self.function_map.register(name, f);
+    }
+
+    /// List of currently available templates
+    pub fn templates(&self) -> Vec<String> {
+        self.templates.keys().cloned().collect()
+    }
+
+    /// Render a specific template
     pub fn render(&self, name: &str, ctx: &Context) -> Result<String, NomoError> {
         let template = self
             .templates
@@ -92,6 +242,7 @@ struct Template {
     instructions: VMInstructions,
 }
 
+/// The context for a given render call
 pub struct Context {
     values: HashMap<String, NomoValue>,
 }
@@ -103,12 +254,16 @@ impl Default for Context {
 }
 
 impl Context {
+    /// Create new Context
     pub fn new() -> Context {
         Context {
             values: HashMap::new(),
         }
     }
 
+    /// Add a value into the map
+    ///
+    /// If you enable the `serde_json` feature, you can insert [`serde_json::Value`]s
     pub fn try_insert(
         &mut self,
         key: impl Into<String>,
@@ -119,17 +274,19 @@ impl Context {
         Ok(())
     }
 
+    /// Add a value into the map, panic if you can't
     pub fn insert(&mut self, key: impl Into<String>, value: impl Into<NomoValue>) {
         self.values.insert(key.into(), value.into());
     }
 
+    /// Access the values inside this context
     pub fn values(&self) -> &HashMap<String, NomoValue> {
         &self.values
     }
 }
 
 #[derive(Debug, Clone)]
-pub struct SourceSpan {
+struct SourceSpan {
     pub range: std::ops::Range<usize>,
 }
 

src/parser/mod.rs

@@ -1,4 +1,3 @@
-use thiserror::Error;
 use winnow::Parser;
 use winnow::RecoverableParser;
 use winnow::combinator::Infix::Left;
@@ -21,6 +20,7 @@ use winnow::error::AddContext;
 use winnow::error::FromRecoverableError;
 use winnow::error::ModalError;
 use winnow::error::ParserError;
+use winnow::stream::Location;
 use winnow::stream::Offset;
 use winnow::stream::Recoverable;
 use winnow::stream::Stream;
@@ -28,6 +28,8 @@ use winnow::stream::TokenSlice;
 use winnow::token::any;
 
 use crate::SourceSpan;
+use crate::errors::AstFailure;
+use crate::input::NomoInput;
 use crate::lexer::TemplateToken;
 use crate::lexer::TokenKind;
 use crate::lexer::TokenOperator;
@@ -50,6 +52,7 @@ pub struct AstError {
     pub(crate) message: Option<String>,
     pub(crate) help: Option<String>,
     pub(crate) span: Option<crate::SourceSpan>,
+    pub(crate) replacement: Option<(SourceSpan, String)>,
 
     is_fatal: bool,
 }
@@ -60,6 +63,7 @@ impl AstError {
             message: None,
             help: None,
             span: None,
+            replacement: None,
 
             is_fatal: false,
         }
@@ -74,6 +78,11 @@ impl AstError {
         self.help = Some(help.to_string());
         self
     }
+
+    fn replacement(mut self, span: SourceSpan, replacement: &str) -> Self {
+        self.replacement = Some((span, replacement.to_string()));
+        self
+    }
 }
 
 impl ModalError for AstError {
@@ -104,13 +113,14 @@ impl FromRecoverableError<Input<'_>, AstError> for AstError {
                 .filter(|t| t.kind() != TokenKind::Whitespace);
             let last = tokens.next();
             let first = tokens.last();
+
             match (last, first) {
                 (None, None) => None,
                 (None, Some(single)) | (Some(single), None) => Some(SourceSpan {
                     range: single.source().get_range(),
                 }),
                 (Some(last), Some(first)) => {
-                    let start = first.source().get_range().start;
+                    let start = first.source().get_range().end;
                     let end = last.source().get_range().end;
 
                     Some(SourceSpan { range: start..end })
@@ -131,6 +141,7 @@ impl AddContext<Input<'_>, AstError> for AstError {
     ) -> Self {
         self.message = context.message.or(self.message);
         self.help = context.help.or(self.help);
+        self.replacement = context.replacement.or(self.replacement);
         self
     }
 }
@@ -151,55 +162,6 @@ impl ParserError<Input<'_>> for AstError {
     }
 }
 
-#[derive(Debug, Error)]
-pub struct AstFailure {
-    errors: Vec<AstError>,
-}
-
-impl std::fmt::Display for AstFailure {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.write_str("TODO")
-    }
-}
-
-impl AstFailure {
-    fn from_errors(errors: Vec<AstError>) -> AstFailure {
-        AstFailure { errors }
-    }
-
-    pub fn to_report(&self, source: &str) -> String {
-        let reports = self
-            .errors
-            .iter()
-            .map(|error| {
-                annotate_snippets::Level::ERROR
-                    .primary_title(
-                        error
-                            .message
-                            .as_deref()
-                            .unwrap_or("An error occurred while producing an Ast"),
-                    )
-                    .element(
-                        annotate_snippets::Snippet::source(source).annotation(
-                            annotate_snippets::AnnotationKind::Primary
-                                .span(error.span.clone().map(|s| s.range).unwrap_or_else(|| 0..0)),
-                        ),
-                    )
-                    .elements(
-                        error
-                            .help
-                            .as_ref()
-                            .map(|help| annotate_snippets::Level::HELP.message(help)),
-                    )
-            })
-            .collect::<Vec<_>>();
-
-        let renderer = annotate_snippets::Renderer::styled()
-            .decor_style(annotate_snippets::renderer::DecorStyle::Unicode);
-        renderer.render(&reports)
-    }
-}
-
 type Input<'input> = Recoverable<TokenSlice<'input, TemplateToken>, AstError>;
 
 impl<'input> Parser<Input<'input>, TemplateToken, AstError> for TokenKind {
@@ -210,7 +172,7 @@ impl<'input> Parser<Input<'input>, TemplateToken, AstError> for TokenKind {
     }
 }
 
-pub fn parse(input: &[TemplateToken]) -> Result<TemplateAst<'_>, AstFailure> {
+pub fn parse(source: NomoInput, input: &[TemplateToken]) -> Result<TemplateAst<'_>, AstFailure> {
     let (_remaining, val, errors) = parse_asts.recoverable_parse(TokenSlice::new(input));
 
     if errors.is_empty()
@@ -218,7 +180,7 @@ pub fn parse(input: &[TemplateToken]) -> Result<TemplateAst<'_>, AstFailure> {
     {
         Ok(TemplateAst { root: val })
     } else {
-        Err(AstFailure::from_errors(errors))
+        Err(AstFailure::from_errors(errors, source))
     }
 }
 
@@ -261,6 +223,7 @@ pub enum TemplateAstExpr<'input> {
     ElseConditional {
         expression: Option<Box<TemplateAstExpr<'input>>>,
     },
+    #[allow(unused)]
     Invalid(&'input [TemplateToken]),
     MathOperation {
         op: TokenOperator,
@@ -356,26 +319,42 @@ fn parse_action<'input>(input: &mut Input<'input>) -> Result<TemplateAstExpr<'in
         alt((
             parse_conditional_chain,
             parse_for_chain,
-            (parse_block(
+            parse_block(parse_unknown_action)
-                cut_err(not(repeat_till(
-                    0..,
-                    any,
-                    peek((ws, TokenKind::RightDelim)),
-                )
-                .map(|((), _)| ())))
-                .context(
-                    AstError::ctx()
-                        .msg("Standlone action block")
-                        .help("If you want to output this expression, add a '=' to the block"),
-                )
                 .take()
                 .map(TemplateAstExpr::Invalid),
         )),
-        )),
     )
     .parse_next(input)
 }
 
+fn parse_unknown_action<'input>(
+    input: &mut Input<'input>,
+) -> Result<TemplateAstExpr<'input>, AstError> {
+    let expression = peek(parse_expression).parse_next(input);
+
+    let is_expr = match expression {
+        Ok(_) => true,
+        Err(err) if err.is_backtrack() => true,
+        _ => false,
+    };
+
+    if is_expr {
+        return Err(AstError::ctx()
+            .msg("Standlone action block")
+            .help("If you want to output this expression, add a '=' to the block")
+            .cut());
+    }
+
+    let keywords = peek(parse_keyword).parse_next(input);
+
+    let is_keyword = keywords.is_ok();
+
+    if is_keyword {
+        return Err(AstError::ctx().msg("Unexpected keyword").cut());
+    }
+
+    Err(AstError::ctx().cut())
+}
 fn parse_for_chain<'input>(input: &mut Input<'input>) -> Result<TemplateAstExpr<'input>, AstError> {
     trace("for_loop", |input: &mut Input<'input>| {
         let for_block = parse_for_loop.map(Box::new).parse_next(input)?;
@@ -390,7 +369,7 @@ fn parse_for_chain<'input>(input: &mut Input<'input>) -> Result<TemplateAstExpr<
 
         let (content, taken) = resume_after_cut(
             repeat_till(0.., parse_ast, loop_end),
-            repeat_till(0.., any, parse_end).map(|((), _)| ()),
+            repeat_till(0.., parse_ast, parse_end).map(|((), _)| ()),
         )
         .with_taken()
         .parse_next(input)?;
@@ -415,24 +394,34 @@ fn parse_for_chain<'input>(input: &mut Input<'input>) -> Result<TemplateAstExpr<
 fn parse_for_loop<'input>(input: &mut Input<'input>) -> Result<TemplateAstExpr<'input>, AstError> {
     trace(
         "for_loop_inner",
-        parse_block(
+        parse_block(|input: &mut Input<'input>| {
-            (
+            let _ignored = surrounded(ws, TokenKind::For).parse_next(input)?;
-                ws,
+
-                TokenKind::For,
+            let value_ident =
-                ws,
+                cut_err(TokenKind::Ident.context(AstError::ctx().msg("Missing ident here")))
-                TokenKind::Ident,
+                    .parse_next(input)?;
-                ws,
+
-                TokenKind::In,
+            let _ignored = cut_err(
-                ws,
+                preceded(ws, TokenKind::In).context(
-                parse_expression.map(Box::new),
+                    AstError::ctx()
+                        .msg("Missing `in` in `for .. in ` loop")
+                        .replacement(
+                            SourceSpan {
+                                range: input.current_token_start()..(input.current_token_start()),
+                            },
+                            " in",
+                        ),
+                ),
             )
-                .map(|(_, _for, _, value_ident, _, _in, _, value_expression)| {
+            .parse_next(input)?;
-                    TemplateAstExpr::For {
+
+            let value_expression = parse_expression.map(Box::new).parse_next(input)?;
+
+            Ok(TemplateAstExpr::For {
                 value_ident,
                 value_expression,
-                    }
+            })
         }),
-        ),
     )
     .parse_next(input)
 }
@@ -649,11 +638,37 @@ where
 fn parse_operand<'input>(input: &mut Input<'input>) -> Result<TemplateAstExpr<'input>, AstError> {
     trace(
         "operand",
-        alt((parse_function, parse_variable_access, parse_literal)),
+        alt((
+            parse_keywords_fail,
+            parse_function,
+            parse_variable_access,
+            parse_literal,
+        )),
     )
     .parse_next(input)
 }
 
+fn parse_keyword<'input>(input: &mut Input<'input>) -> Result<TemplateToken, AstError> {
+    alt((
+        TokenKind::ConditionalIf,
+        TokenKind::ConditionalElse,
+        TokenKind::For,
+        TokenKind::End,
+        TokenKind::In,
+    ))
+    .parse_next(input)
+}
+
+fn parse_keywords_fail<'input>(
+    input: &mut Input<'input>,
+) -> Result<TemplateAstExpr<'input>, AstError> {
+    let _value = parse_keyword.parse_next(input)?;
+
+    Err(AstError::ctx()
+        .msg("Found literal, expected expression")
+        .cut())
+}
+
 fn parse_literal<'input>(input: &mut Input<'input>) -> Result<TemplateAstExpr<'input>, AstError> {
     trace(
         "literal",
@@ -757,6 +772,7 @@ mod tests {
     use winnow::combinator::fail;
     use winnow::stream::TokenSlice;
 
+    use crate::input::NomoInput;
     use crate::lexer::TokenKind;
     use crate::parser::AstError;
     use crate::parser::AstFailure;
@@ -766,25 +782,22 @@ mod tests {
     use crate::parser::parse_block;
     use crate::parser::parse_end;
 
-    fn panic_pretty<'a>(
+    fn panic_pretty<'a>(tokens: Result<TemplateAst<'a>, AstFailure>) -> TemplateAst<'a> {
-        input: &'_ str,
-        tokens: Result<TemplateAst<'a>, AstFailure>,
-    ) -> TemplateAst<'a> {
         match tokens {
             Ok(ast) => ast,
             Err(failure) => {
-                panic!("{}", failure.to_report(input));
+                panic!("{}", failure);
             }
         }
     }
 
     #[test]
     fn check_only_content() {
-        let input = "Hello World";
+        let input = NomoInput::from("Hello World");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = parse(parsed.tokens()).unwrap();
+        let ast = parse(input, parsed.tokens()).unwrap();
 
         insta::assert_debug_snapshot!(ast, @r#"
         TemplateAst {
@@ -799,11 +812,11 @@ mod tests {
 
     #[test]
     fn check_simple_variable_interpolation() {
-        let input = "Hello {{= world }}";
+        let input = NomoInput::from("Hello {{= world }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = parse(parsed.tokens()).unwrap();
+        let ast = parse(input, parsed.tokens()).unwrap();
 
         insta::assert_debug_snapshot!(ast, @r#"
         TemplateAst {
@@ -827,11 +840,11 @@ mod tests {
 
     #[test]
     fn check_simple_if() {
-        let input = "{{ if foo }} Hiii {{ end }}";
+        let input = NomoInput::from("{{ if foo }} Hiii {{ end }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast, @r#"
         TemplateAst {
@@ -872,35 +885,39 @@ mod tests {
 
     #[test]
     fn check_invalid_action() {
-        let input = r#"{{ value }}
+        let input = NomoInput::from(
+            r#"{{ value }}
         {{ value }}
         {{ value }}
         {{ value }}
-        {{ value }}"#;
+        {{ value }}"#,
+        );
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = parse(parsed.tokens()).unwrap_err();
+        let ast = parse(input, parsed.tokens()).unwrap_err();
 
-        insta::assert_snapshot!(ast.to_report(input));
+        insta::assert_snapshot!(ast);
     }
 
     #[test]
     fn check_nested_simple_if() {
-        let input = r#"{{ if foo }}
+        let input = NomoInput::from(
+            r#"{{ if foo }}
             {{ if bar }}
                 Hiii
             {{ end }}
         {{ end }}
 
         {{= value }}
-        "#;
+        "#,
+        );
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
         insta::assert_debug_snapshot!("simple_if_tokens", parsed);
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!("simple_if_ast", ast);
     }
@@ -909,9 +926,9 @@ mod tests {
     fn check_parsing_block() {
         use winnow::RecoverableParser;
 
-        let input = "{{ foo }}";
+        let input = NomoInput::from("{{ foo }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input).unwrap();
 
         let result = alt((
             parse_end,
@@ -943,9 +960,10 @@ mod tests {
                     help: None,
                     span: Some(
                         SourceSpan {
-                            range: 0..6,
+                            range: 2..6,
                         },
                     ),
+                    replacement: None,
                     is_fatal: false,
                 },
             ],
@@ -955,11 +973,11 @@ mod tests {
 
     #[test]
     fn check_empty_if_output() {
-        let input = "{{ if foo }}{{ end }}";
+        let input = NomoInput::from("{{ if foo }}{{ end }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast, @r#"
         TemplateAst {
@@ -992,99 +1010,101 @@ mod tests {
 
     #[test]
     fn check_if_else() {
-        let input = "{{ if foo }} foo {{ else }} bar {{ end }}";
+        let input = NomoInput::from("{{ if foo }} foo {{ else }} bar {{ end }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }
 
     #[test]
     fn check_if_else_if() {
-        let input = "{{ if foo }} foo {{ else if bar }} bar {{ end }}";
+        let input = NomoInput::from("{{ if foo }} foo {{ else if bar }} bar {{ end }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }
 
     #[test]
     fn check_trim_whitespace() {
-        let input = "{{ if foo -}} foo {{- else if bar -}} bar {{- end }}";
+        let input = NomoInput::from("{{ if foo -}} foo {{- else if bar -}} bar {{- end }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }
 
     #[test]
     fn check_for_loop() {
-        let input = "{{ for value in array }} Hi: {{= value }} {{ else }} No Content :C {{ end }}";
+        let input = NomoInput::from(
+            "{{ for value in array }} Hi: {{= value }} {{ else }} No Content :C {{ end }}",
+        );
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }
 
     #[test]
     fn check_math_expression() {
-        let input = "{{= 5 * 3 + 2 / 3 }}";
+        let input = NomoInput::from("{{= 5 * 3 + 2 / 3 }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }
 
     #[test]
     fn check_logical_expression() {
-        let input = "{{= true && false || 3 >= 2 && 5 == 2 }}";
+        let input = NomoInput::from("{{= true && false || 3 >= 2 && 5 == 2 }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }
 
     #[test]
     fn check_function_call() {
-        let input = "{{= foo(2 * 3, bar(2 + baz)) }}";
+        let input = NomoInput::from("{{= foo(2 * 3, bar(2 + baz)) }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }
 
     #[test]
     fn check_conditional_access() {
-        let input = "{{= foo? }}";
+        let input = NomoInput::from("{{= foo? }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }
 
     #[test]
     fn check_access_operator() {
-        let input = "{{= foo?.bar }}";
+        let input = NomoInput::from("{{= foo?.bar }}");
 
-        let parsed = crate::lexer::parse(input.into()).unwrap();
+        let parsed = crate::lexer::parse(input.clone()).unwrap();
 
-        let ast = panic_pretty(input, parse(parsed.tokens()));
+        let ast = panic_pretty(parse(input, parsed.tokens()));
 
         insta::assert_debug_snapshot!(ast);
     }

src/parser/snapshots/nomo__parser__tests__check_invalid_action.snap

@@ -1,6 +1,6 @@
 ---
 source: src/parser/mod.rs
-expression: ast.to_report(input)
+expression: ast
 ---
 error: Standlone action block
   ╭▸ 
@@ -8,23 +8,31 @@ expression: ast.to_report(input)
   │    ━━━━━
   │
   ╰ help: If you want to output this expression, add a '=' to the block
+
 error: Standlone action block
   ╭▸ 
 2 │         {{ value }}
   │            ━━━━━
+  │
   ╰ help: If you want to output this expression, add a '=' to the block
+
 error: Standlone action block
   ╭▸ 
 3 │         {{ value }}
   │            ━━━━━
+  │
   ╰ help: If you want to output this expression, add a '=' to the block
+
 error: Standlone action block
   ╭▸ 
 4 │         {{ value }}
   │            ━━━━━
+  │
   ╰ help: If you want to output this expression, add a '=' to the block
+
 error: Standlone action block
   ╭▸ 
 5 │         {{ value }}
   │            ━━━━━
+  │
   ╰ help: If you want to output this expression, add a '=' to the block

src/value.rs

@@ -1,11 +1,12 @@
 use std::borrow::Cow;
 use std::collections::BTreeMap;
 
-#[cfg(feature = "serde_json")]
 use displaydoc::Display;
 use thiserror::Error;
 
+/// Values to be used inside templates
 #[derive(Clone)]
+#[expect(missing_docs)]
 pub enum NomoValue {
     String {
         value: Cow<'static, str>,
@@ -35,6 +36,7 @@ pub enum NomoValue {
 }
 
 impl NomoValue {
+    /// Return a str if there is one inside
     pub fn as_str(&self) -> Option<&str> {
         if let Self::String { value } = self {
             Some(value)
@@ -43,6 +45,7 @@ impl NomoValue {
         }
     }
 
+    /// Return an arry if there is one inside
     pub fn as_array(&self) -> Option<&[NomoValue]> {
         if let Self::Array { value } = self {
             Some(value)
@@ -51,6 +54,7 @@ impl NomoValue {
         }
     }
 
+    /// Return a bool if there is one inside
     pub fn as_bool(&self) -> Option<bool> {
         if let Self::Bool { value } = self {
             Some(*value)
@@ -61,6 +65,7 @@ impl NomoValue {
         }
     }
 
+    /// Return an object if there is one inside
     pub fn as_object(&self) -> Option<&BTreeMap<String, NomoValue>> {
         if let Self::Object { value } = self {
             Some(value)
@@ -69,6 +74,7 @@ impl NomoValue {
         }
     }
 
+    /// Return an integer if there is one inside
     pub fn as_integer(&self) -> Option<u64> {
         if let Self::Integer { value } = self {
             Some(*value)
@@ -77,6 +83,7 @@ impl NomoValue {
         }
     }
 
+    /// Return a float if there is one inside
     pub fn as_float(&self) -> Option<f64> {
         if let Self::Float { value } = self {
             Some(*value)
@@ -85,6 +92,7 @@ impl NomoValue {
         }
     }
 
+    /// Return the iterator if there is one inside
     pub fn as_iterator(&self) -> Option<&dyn CloneIterator<Item = NomoValue>> {
         if let Self::Iterator { value } = self {
             Some(value)
@@ -93,6 +101,7 @@ impl NomoValue {
         }
     }
 
+    /// Return the iterator mutably if there is one inside
     pub fn as_iterator_mut(&mut self) -> Option<&mut dyn CloneIterator<Item = NomoValue>> {
         if let Self::Iterator { value } = self {
             Some(value)
@@ -237,19 +246,21 @@ impl NomoValue {
     pub(crate) fn try_to_string(&self) -> Option<String> {
         match self {
             NomoValue::String { value } => Some(value.to_string()),
-            NomoValue::Array { .. } => None,
             NomoValue::Bool { value } => Some(value.to_string()),
-            NomoValue::Object { .. } => None,
             NomoValue::Integer { value } => Some(value.to_string()),
             NomoValue::SignedInteger { value } => Some(value.to_string()),
             NomoValue::Float { value } => Some(value.to_string()),
+            NomoValue::Array { .. } => None,
+            NomoValue::Object { .. } => None,
             NomoValue::Iterator { .. } => None,
             NomoValue::Undefined => None,
         }
     }
 }
 
+/// Marker trait for iterators that can be cloned
 pub trait CloneIterator: Iterator<Item = NomoValue> {
+    /// Create a new instance of the iterator inside a [Box]
     fn clone_box(&self) -> Box<dyn CloneIterator<Item = NomoValue>>;
 }
 

src/winnow_ext.rs

@@ -0,0 +1,51 @@
+use std::marker::PhantomData;
+
+use winnow::Parser;
+use winnow::error::AddContext;
+use winnow::error::ParserError;
+use winnow::stream::Stream;
+
+pub trait ParserExt<I, O, E>: Parser<I, O, E> {
+    fn with_context<F, C>(self, context: F) -> WithContext<Self, I, O, E, F, C>
+    where
+        Self: Sized,
+        I: Stream,
+        E: AddContext<I, C> + ParserError<I>,
+        F: Fn(&<I as Stream>::Slice, &E) -> C,
+    {
+        WithContext {
+            parser: self,
+            context,
+            _pd: PhantomData,
+        }
+    }
+}
+
+pub struct WithContext<P, I, O, E, F, C> {
+    parser: P,
+    context: F,
+    _pd: PhantomData<(I, O, E, C)>,
+}
+
+impl<P, I, O, E, F, C> Parser<I, O, E> for WithContext<P, I, O, E, F, C>
+where
+    P: Parser<I, O, E>,
+    I: Stream,
+    E: AddContext<I, C> + ParserError<I>,
+    F: Fn(&<I as Stream>::Slice, &E) -> C,
+{
+    fn parse_next(&mut self, input: &mut I) -> winnow::Result<O, E> {
+        let start = input.checkpoint();
+
+        let res = self.parser.parse_next(input);
+
+        res.map_err(|err| {
+            let offset = input.offset_from(&start);
+            input.reset(&start);
+            let taken = input.next_slice(offset);
+
+            let context = (self.context)(&taken, &err);
+            err.add_context(input, &start, context)
+        })
+    }
+}

tests/checks.rs

@@ -1,3 +1,5 @@
+#![allow(missing_docs)]
+
 #[test]
 fn check_files() {
     let files = std::fs::read_dir("tests/checks/").unwrap();
@@ -5,11 +7,13 @@ fn check_files() {
     for file in files {
         let input = std::fs::read_to_string(file.unwrap().path()).unwrap();
 
-        let Ok(parsed) = nomo::lexer::parse(input.into()) else {
+        let input = nomo::input::NomoInput::from(input);
+
+        let Ok(parsed) = nomo::lexer::parse(input.clone()) else {
             continue;
         };
 
-        let Ok(ast) = nomo::parser::parse(parsed.tokens()) else {
+        let Ok(ast) = nomo::parser::parse(input, parsed.tokens()) else {
             continue;
         };
 

tests/errors/invalid_controls.1-parsed.snap

@@ -0,0 +1,119 @@
+---
+source: tests/file_tests.rs
+expression: parsed
+info:
+  input: "{{ if }} {{ end }}\n{{ if if }} {{ end }}\n{{ if if }} {{ for foo in bar }} {{ end }} {{ end }}\n{{ for in bar }} {{ end }}\n{{ for blah bar }} {{ end }}\n{{ for blah}} {{ end }}\n{{ else }}"
+  context: {}
+---
+ParsedTemplate {
+    tokens: [
+        [LeftDelim]"{{" (0..2),
+        [Whitespace]" " (2..3),
+        [ConditionalIf]"if" (3..5),
+        [Whitespace]" " (5..6),
+        [RightDelim]"}}" (6..8),
+        [Whitespace]" " (8..9),
+        [LeftDelim]"{{" (9..11),
+        [Whitespace]" " (11..12),
+        [End]"end" (12..15),
+        [Whitespace]" " (15..16),
+        [RightDelim]"}}" (16..18),
+        [Whitespace]"\n" (18..19),
+        [LeftDelim]"{{" (19..21),
+        [Whitespace]" " (21..22),
+        [ConditionalIf]"if" (22..24),
+        [Whitespace]" " (24..25),
+        [ConditionalIf]"if" (25..27),
+        [Whitespace]" " (27..28),
+        [RightDelim]"}}" (28..30),
+        [Whitespace]" " (30..31),
+        [LeftDelim]"{{" (31..33),
+        [Whitespace]" " (33..34),
+        [End]"end" (34..37),
+        [Whitespace]" " (37..38),
+        [RightDelim]"}}" (38..40),
+        [Whitespace]"\n" (40..41),
+        [LeftDelim]"{{" (41..43),
+        [Whitespace]" " (43..44),
+        [ConditionalIf]"if" (44..46),
+        [Whitespace]" " (46..47),
+        [ConditionalIf]"if" (47..49),
+        [Whitespace]" " (49..50),
+        [RightDelim]"}}" (50..52),
+        [Whitespace]" " (52..53),
+        [LeftDelim]"{{" (53..55),
+        [Whitespace]" " (55..56),
+        [For]"for" (56..59),
+        [Whitespace]" " (59..60),
+        [Ident]"foo" (60..63),
+        [Whitespace]" " (63..64),
+        [In]"in" (64..66),
+        [Whitespace]" " (66..67),
+        [Ident]"bar" (67..70),
+        [Whitespace]" " (70..71),
+        [RightDelim]"}}" (71..73),
+        [Whitespace]" " (73..74),
+        [LeftDelim]"{{" (74..76),
+        [Whitespace]" " (76..77),
+        [End]"end" (77..80),
+        [Whitespace]" " (80..81),
+        [RightDelim]"}}" (81..83),
+        [Whitespace]" " (83..84),
+        [LeftDelim]"{{" (84..86),
+        [Whitespace]" " (86..87),
+        [End]"end" (87..90),
+        [Whitespace]" " (90..91),
+        [RightDelim]"}}" (91..93),
+        [Whitespace]"\n" (93..94),
+        [LeftDelim]"{{" (94..96),
+        [Whitespace]" " (96..97),
+        [For]"for" (97..100),
+        [Whitespace]" " (100..101),
+        [In]"in" (101..103),
+        [Whitespace]" " (103..104),
+        [Ident]"bar" (104..107),
+        [Whitespace]" " (107..108),
+        [RightDelim]"}}" (108..110),
+        [Whitespace]" " (110..111),
+        [LeftDelim]"{{" (111..113),
+        [Whitespace]" " (113..114),
+        [End]"end" (114..117),
+        [Whitespace]" " (117..118),
+        [RightDelim]"}}" (118..120),
+        [Whitespace]"\n" (120..121),
+        [LeftDelim]"{{" (121..123),
+        [Whitespace]" " (123..124),
+        [For]"for" (124..127),
+        [Whitespace]" " (127..128),
+        [Ident]"blah" (128..132),
+        [Whitespace]" " (132..133),
+        [Ident]"bar" (133..136),
+        [Whitespace]" " (136..137),
+        [RightDelim]"}}" (137..139),
+        [Whitespace]" " (139..140),
+        [LeftDelim]"{{" (140..142),
+        [Whitespace]" " (142..143),
+        [End]"end" (143..146),
+        [Whitespace]" " (146..147),
+        [RightDelim]"}}" (147..149),
+        [Whitespace]"\n" (149..150),
+        [LeftDelim]"{{" (150..152),
+        [Whitespace]" " (152..153),
+        [For]"for" (153..156),
+        [Whitespace]" " (156..157),
+        [Ident]"blah" (157..161),
+        [RightDelim]"}}" (161..163),
+        [Whitespace]" " (163..164),
+        [LeftDelim]"{{" (164..166),
+        [Whitespace]" " (166..167),
+        [End]"end" (167..170),
+        [Whitespace]" " (170..171),
+        [RightDelim]"}}" (171..173),
+        [Whitespace]"\n" (173..174),
+        [LeftDelim]"{{" (174..176),
+        [Whitespace]" " (176..177),
+        [ConditionalElse]"else" (177..181),
+        [Whitespace]" " (181..182),
+        [RightDelim]"}}" (182..184),
+    ],
+}

tests/errors/invalid_controls.2-ast.snap

@@ -0,0 +1,51 @@
+---
+source: tests/file_tests.rs
+expression: ast
+info:
+  input: "{{ if }} {{ end }}\n{{ if if }} {{ end }}\n{{ if if }} {{ for foo in bar }} {{ end }} {{ end }}\n{{ for in bar }} {{ end }}\n{{ for blah bar }} {{ end }}\n{{ for blah}} {{ end }}\n{{ else }}"
+  context: {}
+---
+error: Expected an expression after 'if'
+  ╭▸ 
+1 │ {{ if }} {{ end }}
+  ╰╴   ━━
+
+error: Expected an expression after 'if'
+  ╭▸ 
+2 │ {{ if if }} {{ end }}
+  ╰╴      ━━
+
+error: Expected an expression after 'if'
+  ╭▸ 
+3 │ {{ if if }} {{ for foo in bar }} {{ end }} {{ end }}
+  ╰╴      ━━
+
+error: Missing ident here
+  ╭▸ 
+4 │ {{ for in bar }} {{ end }}
+  ╰╴       ━━━━━━
+
+error: Missing `in` in `for .. in ` loop
+  ╭▸ 
+5 │ {{ for blah bar }} {{ end }}
+  │        ━━━━━━━━
+  ╰╴
+note: Try adding it
+  ╭╴
+5 │ {{ for blah in bar }} {{ end }}
+  ╰╴            ++
+
+error: Missing `in` in `for .. in ` loop
+  ╭▸ 
+6 │ {{ for blah}} {{ end }}
+  │        ━━━━
+  ╰╴
+note: Try adding it
+  ╭╴
+6 │ {{ for blah in}} {{ end }}
+  ╰╴            ++
+
+error: Unexpected keyword
+  ╭▸ 
+7 │ {{ else }}
+  ╰╴   ━━━━

tests/errors/invalid_controls.nomo

@@ -0,0 +1,9 @@
+{}
+---
+{{ if }} {{ end }}
+{{ if if }} {{ end }}
+{{ if if }} {{ for foo in bar }} {{ end }} {{ end }}
+{{ for in bar }} {{ end }}
+{{ for blah bar }} {{ end }}
+{{ for blah}} {{ end }}
+{{ else }}

tests/errors/invalid_if.1-parsed.snap

@@ -0,0 +1,24 @@
+---
+source: tests/file_tests.rs
+expression: parsed
+info:
+  input: "{{ if if }} {{ end }}"
+  context: {}
+---
+ParsedTemplate {
+    tokens: [
+        [LeftDelim]"{{" (0..2),
+        [Whitespace]" " (2..3),
+        [ConditionalIf]"if" (3..5),
+        [Whitespace]" " (5..6),
+        [ConditionalIf]"if" (6..8),
+        [Whitespace]" " (8..9),
+        [RightDelim]"}}" (9..11),
+        [Whitespace]" " (11..12),
+        [LeftDelim]"{{" (12..14),
+        [Whitespace]" " (14..15),
+        [End]"end" (15..18),
+        [Whitespace]" " (18..19),
+        [RightDelim]"}}" (19..21),
+    ],
+}

tests/errors/invalid_if.2-ast.snap

@@ -0,0 +1,11 @@
+---
+source: tests/file_tests.rs
+expression: ast
+info:
+  input: "{{ if if }} {{ end }}"
+  context: {}
+---
+error: Expected an expression after 'if'
+  ╭▸ 
+1 │ {{ if if }} {{ end }}
+  ╰╴      ━━

tests/errors/invalid_if.nomo

@@ -0,0 +1,3 @@
+{}
+---
+{{ if if }} {{ end }}

tests/file_tests.rs

@@ -1,11 +1,16 @@
+#![allow(missing_docs)]
+
 use std::collections::HashMap;
 use std::path::Path;
 
 use nomo::Context;
 use nomo::functions::FunctionMap;
+use nomo::input::NomoInput;
 
 test_each_file::test_each_path! { for ["nomo"] in "./tests/cases/" as cases => check_for_input }
 
+test_each_file::test_each_path! { for ["nomo"] in "./tests/errors/" as error_cases => check_errors }
+
 #[derive(serde::Serialize)]
 struct Info {
     input: String,
@@ -39,16 +44,18 @@ fn check_for_input([path]: [&Path; 1]) {
         context.try_insert(k, v).unwrap();
     }
 
-    let parsed = nomo::lexer::parse(input.into()).unwrap();
+    let input = NomoInput::from(input);
+
+    let parsed = nomo::lexer::parse(input.clone()).unwrap();
 
     let _guard = settings.bind_to_scope();
 
     insta::assert_debug_snapshot!(format!("{basename}.1-parsed"), parsed);
 
-    let ast = match nomo::parser::parse(parsed.tokens()) {
+    let ast = match nomo::parser::parse(input, parsed.tokens()) {
         Ok(ast) => ast,
         Err(err) => {
-            eprintln!("{}", err.to_report(input));
+            eprintln!("{}", err);
             panic!("Could not evaluate ast");
         }
     };
@@ -63,3 +70,69 @@ fn check_for_input([path]: [&Path; 1]) {
 
     insta::assert_debug_snapshot!(format!("{basename}.4-output"), output);
 }
+
+fn check_errors([path]: [&Path; 1]) {
+    let mut settings = insta::Settings::clone_current();
+    settings.set_snapshot_path("errors");
+    settings.set_prepend_module_to_snapshot(false);
+
+    let basename = path.file_stem().unwrap().to_string_lossy();
+    let input = std::fs::read_to_string(path).unwrap();
+
+    let (context, input) = input.split_once("\n---\n").unwrap_or_else(|| ("", &input));
+
+    let map = if !context.is_empty() {
+        serde_json::from_str::<HashMap<String, serde_json::Value>>(context).unwrap()
+    } else {
+        HashMap::new()
+    };
+
+    settings.set_info(&Info {
+        input: input.to_string(),
+        context: map.clone(),
+    });
+
+    let mut context = Context::new();
+
+    for (k, v) in map {
+        context.try_insert(k, v).unwrap();
+    }
+
+    let _guard = settings.bind_to_scope();
+
+    let input = NomoInput::from(input);
+
+    let parsed = nomo::lexer::parse(input.clone()).map_err(|err| err.to_report());
+
+    match &parsed {
+        Ok(parsed) => insta::assert_debug_snapshot!(format!("{basename}.1-parsed"), parsed),
+        Err(parsed) => insta::assert_snapshot!(format!("{basename}.1-parsed"), parsed),
+    }
+
+    let Ok(parsed) = parsed else {
+        return;
+    };
+
+    let ast = nomo::parser::parse(input, parsed.tokens()).map_err(|err| err.to_string());
+
+    match &ast {
+        Ok(ast) => insta::assert_debug_snapshot!(format!("{basename}.2-ast"), ast),
+        Err(ast) => insta::assert_snapshot!(format!("{basename}.2-ast"), ast),
+    }
+
+    let Ok(ast) = ast else {
+        return;
+    };
+
+    let emit = nomo::compiler::emit_machine(ast);
+
+    insta::assert_debug_snapshot!(format!("{basename}.3-instructions"), emit);
+
+    let output = nomo::eval::execute(&FunctionMap::default(), &emit, &context)
+        .map_err(|err| err.to_string());
+
+    match &output {
+        Ok(output) => insta::assert_debug_snapshot!(format!("{basename}.4-output"), output),
+        Err(output) => insta::assert_snapshot!(format!("{basename}.4-output"), output),
+    }
+}