Add native AXIOM runtime slice

Author: axiom-systemstech

native/README.md

@@ -6,6 +6,12 @@ from the Python bootstrap so both implementations can be compared before
 migration. The current native pipeline covers lexing, parsing, and the first
 semantic checks.
 
+The native runtime can execute the initial literal-printing slice:
+
+```rust
+axiom_native::runtime::run("fn main() { print(\"Hello AXIOM\") }")?;
+```
+
 ```bash
 cargo test --manifest-path native/Cargo.toml
 ```

native/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod parser;
+pub mod runtime;
 pub mod semantic;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]

native/src/runtime.rs

@@ -0,0 +1,62 @@
+use crate::parser::{Expression, Program, Statement};
+use crate::{parser, semantic};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RuntimeError {
+    pub message: String,
+}
+
+pub fn run(source: &str) -> Result<String, RuntimeError> {
+    let program = parser::parse(source).map_err(|error| RuntimeError {
+        message: format!("{} at {}:{}", error.message, error.line, error.column),
+    })?;
+    semantic::analyze(&program).map_err(|error| RuntimeError {
+        message: error.message,
+    })?;
+    execute(&program)
+}
+
+pub fn execute(program: &Program) -> Result<String, RuntimeError> {
+    let main = program
+        .functions
+        .iter()
+        .find(|function| function.name == "main")
+        .ok_or_else(|| RuntimeError {
+            message: "program must define 'main'".into(),
+        })?;
+    let mut output = String::new();
+    for statement in &main.body {
+        let Statement::Call(call) = statement;
+        if call.name == "print" {
+            let value = match &call.arguments[0] {
+                Expression::String(value) => value.clone(),
+                Expression::Integer(value) => value.to_string(),
+                Expression::Boolean(value) => value.to_string(),
+            };
+            output.push_str(&value);
+            output.push('\n');
+        }
+    }
+    Ok(output)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn runs_hello_program() {
+        assert_eq!(
+            run("fn main() { print(\"Hello AXIOM\") }").unwrap(),
+            "Hello AXIOM\n"
+        );
+    }
+
+    #[test]
+    fn runs_typed_literals() {
+        assert_eq!(
+            run("fn main() { print(42); print(true) }").unwrap(),
+            "42\ntrue\n"
+        );
+    }
+}