feat(tvix/eval): implement builtins.map

As we already have a VM passed to the builtins, we can simply execute
the provided closure/lambda in it for each value.

The primary annoyance with this is that we have to clone the upvalues
for each element, but we can try making this cheaper in the
future (it's also a general problem in the VM itself).

Change-Id: I5bcf56d58c509c0eb081e7cf52f6093216451ce4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6714
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -12,7 +12,8 @@ use std::{
 
 use crate::{
     errors::ErrorKind,
-    value::{Builtin, CoercionKind, NixAttrs, NixList, NixString, Value},
+    upvalues::UpvalueCarrier,
+    value::{Builtin, Closure, CoercionKind, NixAttrs, NixList, NixString, Value},
     vm::VM,
 };
 
@@ -144,6 +145,21 @@ fn pure_builtins() -> Vec<Builtin> {
         Builtin::new("length", &[true], |args, _| {
             Ok(Value::Integer(args[0].to_list()?.len() as i64))
         }),
+        Builtin::new("map", &[true, true], |args, vm| {
+            let list: NixList = args[1].to_list()?;
+            let func: Closure = args[0].to_closure()?;
+
+            list.into_iter()
+                .map(|val| {
+                    // Leave the argument on the stack before calling the
+                    // function.
+                    vm.push(val);
+                    vm.call(func.lambda(), func.upvalues().clone(), 1)
+                })
+                .collect::<Result<Vec<Value>, _>>()
+                .map(|list| Value::List(NixList::from(list)))
+                .map_err(Into::into)
+        }),
         Builtin::new("hasAttr", &[true, true], |args, _| {
             let k = args[0].to_str()?;
             let xs = args[1].to_attrs()?;