refactor(tvix/eval): handle forcing in Builtin::apply r/4918

Instead of arity, we pass a array reference to Builtin::new that
describes how many arguments there are and which of them need to be
forced, eliminating the need to force manually.

Note that this change doesn't fix some of the instances where the the
Builtin doesn't consider that the value could be a Thunk.

Change-Id: Iadb58bb79886c30dc6b09dcf0ffad8abf28036a1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6662
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

2 files changed, 58 insertions, 62 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 527a8f8530c4..10ec6c89f946 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -50,19 +50,18 @@ pub fn coerce_value_to_path(v: &Value, vm: &mut VM) -> Result<PathBuf, ErrorKind
 /// WASM).
 fn pure_builtins() -> Vec<Builtin> {
     vec![
-        Builtin::new("add", 2, |mut args, _| {
+        Builtin::new("add", &[true, true], |mut args, _| {
             let b = args.pop().unwrap();
             let a = args.pop().unwrap();
             arithmetic_op!(a, b, +)
         }),
-        Builtin::new("abort", 1, |mut args, _| {
+        Builtin::new("abort", &[true], |mut args, _| {
             return Err(ErrorKind::Abort(
                 args.pop().unwrap().to_str()?.as_str().to_owned(),
             ));
         }),
-        Builtin::new("attrNames", 1, |args, vm| {
-            let value = args[0].force(vm)?;
-            let xs = value.to_attrs()?;
+        Builtin::new("attrNames", &[true], |args, _| {
+            let xs = args[0].to_attrs()?;
             let mut output = Vec::with_capacity(xs.len());
 
             for (key, _val) in xs.iter() {
@@ -71,9 +70,8 @@ fn pure_builtins() -> Vec<Builtin> {
 
             Ok(Value::List(NixList::construct(output.len(), output)))
         }),
-        Builtin::new("attrValues", 1, |args, vm| {
-            let value = args[0].force(vm)?;
-            let xs = value.to_attrs()?;
+        Builtin::new("attrValues", &[true], |args, _| {
+            let xs = args[0].to_attrs()?;
             let mut output = Vec::with_capacity(xs.len());
 
             for (_key, val) in xs.iter() {
@@ -82,22 +80,16 @@ fn pure_builtins() -> Vec<Builtin> {
 
             Ok(Value::List(NixList::construct(output.len(), output)))
         }),
-        Builtin::new("bitAnd", 2, |args, vm| {
-            let x = args[0].force(vm)?;
-            let y = args[1].force(vm)?;
-            Ok(Value::Integer(x.as_int()? & y.as_int()?))
+        Builtin::new("bitAnd", &[true, true], |args, _| {
+            Ok(Value::Integer(args[0].as_int()? & args[1].as_int()?))
         }),
-        Builtin::new("bitOr", 2, |args, vm| {
-            let x = args[0].force(vm)?;
-            let y = args[1].force(vm)?;
-            Ok(Value::Integer(x.as_int()? | y.as_int()?))
+        Builtin::new("bitOr", &[true, true], |args, _| {
+            Ok(Value::Integer(args[0].as_int()? | args[1].as_int()?))
         }),
-        Builtin::new("bitXor", 2, |args, vm| {
-            let x = args[0].force(vm)?;
-            let y = args[1].force(vm)?;
-            Ok(Value::Integer(x.as_int()? ^ y.as_int()?))
+        Builtin::new("bitXor", &[true, true], |args, _| {
+            Ok(Value::Integer(args[0].as_int()? ^ args[1].as_int()?))
         }),
-        Builtin::new("catAttrs", 2, |mut args, _| {
+        Builtin::new("catAttrs", &[true, true], |mut args, _| {
             let list = args.pop().unwrap().to_list()?;
             let key = args.pop().unwrap().to_str()?;
             let mut output = vec![];
@@ -110,10 +102,10 @@ fn pure_builtins() -> Vec<Builtin> {
 
             Ok(Value::List(NixList::construct(output.len(), output)))
         }),
-        Builtin::new("compareVersions", 2, |mut args, vm| {
-            let s1 = args[0].force(vm)?.to_str()?;
+        Builtin::new("compareVersions", &[true, true], |args, _| {
+            let s1 = args[0].to_str()?;
             let s1 = VersionPartsIter::new(s1.as_str());
-            let s2 = args[1].force(vm)?.to_str()?;
+            let s2 = args[1].to_str()?;
             let s2 = VersionPartsIter::new(s2.as_str());
 
             match s1.cmp(s2) {
@@ -122,14 +114,13 @@ fn pure_builtins() -> Vec<Builtin> {
                 std::cmp::Ordering::Greater => Ok(Value::Integer(-1)),
             }
         }),
-        Builtin::new("div", 2, |mut args, _| {
+        Builtin::new("div", &[true, true], |mut args, _| {
             let b = args.pop().unwrap();
             let a = args.pop().unwrap();
             arithmetic_op!(a, b, /)
         }),
-        Builtin::new("elemAt", 2, |args, vm| {
-            let value = args[0].force(vm)?;
-            let xs = value.to_list()?;
+        Builtin::new("elemAt", &[true, true], |args, _| {
+            let xs = args[0].to_list()?;
             let i = args[1].as_int()?;
             if i < 0 {
                 Err(ErrorKind::IndexOutOfBounds { index: i })
@@ -140,63 +131,57 @@ fn pure_builtins() -> Vec<Builtin> {
                 }
             }
         }),
-        Builtin::new("length", 1, |args, vm| {
-            if let Value::Thunk(t) = &args[0] {
-                t.force(vm)?;
-            }
+        Builtin::new("length", &[true], |args, _| {
             Ok(Value::Integer(args[0].to_list()?.len() as i64))
         }),
-        Builtin::new("head", 1, |args, vm| {
-            let xs = args[0].force(vm)?;
-            match xs.to_list()?.get(0) {
-                Some(x) => Ok(x.clone()),
-                None => Err(ErrorKind::IndexOutOfBounds { index: 0 }),
-            }
+        Builtin::new("head", &[true], |args, _| match args[0].to_list()?.get(0) {
+            Some(x) => Ok(x.clone()),
+            None => Err(ErrorKind::IndexOutOfBounds { index: 0 }),
         }),
-        Builtin::new("isAttrs", 1, |args, _| {
+        Builtin::new("isAttrs", &[true], |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::Attrs(_))))
         }),
-        Builtin::new("isBool", 1, |args, _| {
+        Builtin::new("isBool", &[true], |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::Bool(_))))
         }),
-        Builtin::new("isFloat", 1, |args, _| {
+        Builtin::new("isFloat", &[true], |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::Float(_))))
         }),
-        Builtin::new("isFunction", 1, |args, _| {
+        Builtin::new("isFunction", &[true], |args, _| {
             Ok(Value::Bool(matches!(
                 args[0],
                 Value::Closure(_) | Value::Builtin(_)
             )))
         }),
-        Builtin::new("isInt", 1, |args, _| {
+        Builtin::new("isInt", &[true], |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::Integer(_))))
         }),
-        Builtin::new("isList", 1, |args, _| {
+        Builtin::new("isList", &[true], |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::List(_))))
         }),
-        Builtin::new("isNull", 1, |args, _| {
+        Builtin::new("isNull", &[true], |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::Null)))
         }),
-        Builtin::new("isPath", 1, |args, _| {
+        Builtin::new("isPath", &[true], |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::Path(_))))
         }),
-        Builtin::new("isString", 1, |args, _| {
+        Builtin::new("isString", &[true], |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::String(_))))
         }),
-        Builtin::new("mul", 2, |mut args, _| {
+        Builtin::new("mul", &[true, true], |mut args, _| {
             let b = args.pop().unwrap();
             let a = args.pop().unwrap();
             arithmetic_op!(a, b, *)
         }),
-        Builtin::new("sub", 2, |mut args, _| {
+        Builtin::new("sub", &[true, true], |mut args, _| {
             let b = args.pop().unwrap();
             let a = args.pop().unwrap();
             arithmetic_op!(a, b, -)
         }),
-        Builtin::new("substring", 3, |args, vm| {
-            let beg = args[0].force(vm)?.as_int()?;
-            let len = args[1].force(vm)?.as_int()?;
-            let x = args[2].force(vm)?.to_str()?;
+        Builtin::new("substring", &[true, true, true], |args, _| {
+            let beg = args[0].as_int()?;
+            let len = args[1].as_int()?;
+            let x = args[2].to_str()?;
 
             if beg < 0 {
                 return Err(ErrorKind::IndexOutOfBounds { index: beg });
@@ -221,8 +206,8 @@ fn pure_builtins() -> Vec<Builtin> {
                 x.as_str()[(beg as usize)..(end as usize)].into(),
             ))
         }),
-        Builtin::new("tail", 1, |args, vm| {
-            let xs = args[0].force(vm)?.to_list()?;
+        Builtin::new("tail", &[true], |args, _| {
+            let xs = args[0].to_list()?;
 
             if xs.len() == 0 {
                 Err(ErrorKind::TailEmptyList)
@@ -231,17 +216,20 @@ fn pure_builtins() -> Vec<Builtin> {
                 Ok(Value::List(NixList::construct(output.len(), output)))
             }
         }),
-        Builtin::new("throw", 1, |mut args, _| {
+        Builtin::new("throw", &[true], |mut args, _| {
             return Err(ErrorKind::Throw(
                 args.pop().unwrap().to_str()?.as_str().to_owned(),
             ));
         }),
-        Builtin::new("toString", 1, |args, vm| {
+        Builtin::new("toString", &[true], |args, vm| {
             args[0]
                 .coerce_to_string(CoercionKind::Strong, vm)
                 .map(Value::String)
         }),
-        Builtin::new("typeOf", 1, |args, vm| {
+        Builtin::new("typeOf", &[false], |args, vm| {
+            // We force manually here because it also unwraps the Thunk
+            // representation, if any.
+            // TODO(sterni): it'd be nice if we didn't have to worry about this
             let value = args[0].force(vm)?;
             Ok(Value::String(value.type_of().into()))
         }),
diff --git a/tvix/eval/src/value/builtin.rs b/tvix/eval/src/value/builtin.rs
index e79451392c2b..ca9924c5d5aa 100644
--- a/tvix/eval/src/value/builtin.rs
+++ b/tvix/eval/src/value/builtin.rs
@@ -36,7 +36,10 @@ pub type BuiltinFn = fn(arg: Vec<Value>, vm: &mut VM) -> Result<Value, ErrorKind
 #[derive(Clone)]
 pub struct Builtin {
     name: &'static str,
-    arity: usize,
+    /// Array reference that describes how many arguments there are (usually 1
+    /// or 2) and whether they need to be forced. `true` causes the
+    /// corresponding argument to be forced before `func` is called.
+    strict_args: &'static [bool],
     func: BuiltinFn,
 
     /// Partially applied function arguments.
@@ -44,10 +47,10 @@ pub struct Builtin {
 }
 
 impl Builtin {
-    pub fn new(name: &'static str, arity: usize, func: BuiltinFn) -> Self {
+    pub fn new(name: &'static str, strict_args: &'static [bool], func: BuiltinFn) -> Self {
         Builtin {
             name,
-            arity,
+            strict_args,
             func,
             partials: vec![],
         }
@@ -63,7 +66,12 @@ impl Builtin {
     pub fn apply(mut self, vm: &mut VM, arg: Value) -> Result<Value, ErrorKind> {
         self.partials.push(arg);
 
-        if self.partials.len() == self.arity {
+        if self.partials.len() == self.strict_args.len() {
+            for (idx, force) in self.strict_args.iter().enumerate() {
+                if *force {
+                    self.partials[idx].force(vm)?;
+                }
+            }
             return (self.func)(self.partials, vm);
         }