feat(tvix/eval): introduce mechanism for defining builtins

Adds a new builtins module in which builtins can be constructed. The
functions in this module should return a correctly structured value to
be passed to the compiler's `globals`.

This is wired up all the way to the compiler with an example
`toString` builtin, available as a global. Note that this does not yet
actually behave like the real toString, which has some differences
from `Display`.

Change-Id: Ibb5f6fbe6207782fdf2434435567fc1bd80039a5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6254
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -0,0 +1,26 @@
+//! This module implements the builtins exposed in the Nix language.
+//!
+//! See //tvix/eval/docs/builtins.md for a some context on the
+//! available builtins in Nix.
+
+use std::collections::HashMap;
+
+use crate::value::{Builtin, Value};
+
+macro_rules! builtin {
+    ( $map:ident, $name:literal, $arity:literal, $body:expr ) => {
+        $map.insert($name, Value::Builtin(Builtin::new($name, $arity, $body)));
+    };
+}
+
+/// Set of Nix builtins that are globally available.
+pub fn global_builtins() -> HashMap<&'static str, Value> {
+    let mut globals = HashMap::new();
+
+    builtin!(globals, "toString", 1, |args| {
+        // TODO: toString is actually not the same as Display
+        Ok(Value::String(format!("{}", args[0]).into()))
+    });
+
+    globals
+}

tvix/eval/src/compiler.rs

@@ -1029,7 +1029,11 @@ fn prepare_globals(additional: HashMap<&'static str, Value>) -> GlobalsMap {
     globals
 }
 
-pub fn compile(expr: ast::Expr, location: Option<PathBuf>) -> EvalResult<CompilationResult> {
+pub fn compile(
+    expr: ast::Expr,
+    location: Option<PathBuf>,
+    globals: HashMap<&'static str, Value>,
+) -> EvalResult<CompilationResult> {
     let mut root_dir = match location {
         Some(dir) => Ok(dir),
         None => std::env::current_dir().map_err(|e| {
@@ -1044,12 +1048,9 @@ pub fn compile(expr: ast::Expr, location: Option<PathBuf>) -> EvalResult<Compila
         root_dir.pop();
     }
 
-    // TODO: accept globals as an external parameter
-    let globals = prepare_globals(HashMap::new());
-
     let mut c = Compiler {
         root_dir,
-        globals,
+        globals: prepare_globals(globals),
         contexts: vec![LambdaCtx::new()],
         warnings: vec![],
         errors: vec![],

tvix/eval/src/eval.rs

@@ -3,6 +3,7 @@ use std::path::PathBuf;
 use rnix;
 
 use crate::{
+    builtins::global_builtins,
     errors::{ErrorKind, EvalResult},
     value::Value,
 };
@@ -28,7 +29,7 @@ pub fn interpret(code: &str, location: Option<PathBuf>) -> EvalResult<Value> {
         println!("{:?}", root_expr);
     }
 
-    let result = crate::compiler::compile(root_expr, location)?;
+    let result = crate::compiler::compile(root_expr, location, global_builtins())?;
 
     #[cfg(feature = "disassembler")]
     crate::disassembler::disassemble_chunk(&result.lambda.chunk);

tvix/eval/src/lib.rs

@@ -1,3 +1,4 @@
+mod builtins;
 mod chunk;
 mod compiler;
 mod errors;

tvix/eval/src/value/builtin.rs

@@ -34,6 +34,19 @@ pub struct Builtin {
 }
 
 impl Builtin {
+    pub fn new(name: &'static str, arity: usize, func: BuiltinFn) -> Self {
+        Builtin {
+            name,
+            arity,
+            func,
+            partials: vec![],
+        }
+    }
+
+    pub fn name(&self) -> &'static str {
+        self.name
+    }
+
     /// Apply an additional argument to the builtin, which will either
     /// lead to execution of the function or to returning a partial
     /// builtin.