Add first working pipeline of parse -> ast -> instr -> render

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

src/eval/mod.rs

@@ -1,7 +1,70 @@
-pub struct EvalStack {
-    stack: Vec<Evaluation>,
+use std::collections::HashMap;
+
+use displaydoc::Display;
+use thiserror::Error;
+
+use crate::Context;
+use crate::emit::Instruction;
+
+#[derive(Debug, Error, Display)]
+enum EvalError {
+    /// An unknown variable was encountered: .0
+    UnknownVariable(String),
+    /// An explicit abort was requested
+    ExplicitAbort,
 }
 
-pub enum Evaluation {
-    AppendContent { content: String },
+fn execute(instructions: &[Instruction], global_context: &Context) -> Result<String, EvalError> {
+    let mut output = String::new();
+
+    let mut scopes: HashMap<crate::emit::VariableSlot, serde_json::Value> = HashMap::new();
+
+    for instr in instructions {
+        match instr {
+            Instruction::AppendContent { content } => output.push_str(content),
+            Instruction::LoadFromContextToSlot { name, slot } => {
+                let value = global_context
+                    .values
+                    .get(name)
+                    .ok_or(EvalError::UnknownVariable(name.clone()))?;
+
+                scopes.insert(*slot, value.clone());
+            }
+            Instruction::EmitFromSlot { slot } => {
+                let value = scopes.get(slot).unwrap().as_str().unwrap();
+                output.push_str(value);
+            }
+            Instruction::PushScope { inherit_parent: _ } => todo!(),
+            Instruction::Abort => return Err(EvalError::ExplicitAbort),
+        }
+    }
+
+    Ok(output)
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::Context;
+    use crate::eval::execute;
+
+    #[test]
+    fn check_simple_variable_interpolation() {
+        let input = "Hello {{= world }}";
+
+        let parsed = crate::parser::parse(input).unwrap();
+
+        let ast = crate::ast::parse(parsed.tokens()).unwrap();
+
+        let emit = crate::emit::emit_machine(ast);
+
+        let mut context = Context::new();
+        context.insert("world", "World");
+        let output = execute(&emit, &context);
+
+        insta::assert_debug_snapshot!(output, @r#"
+        Ok(
+            "Hello World",
+        )
+        "#);
+    }
 }