internal/rosa/azalea: store the zero value for nil

This is not final, but the behaviour is acceptable for now.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/azalea/evaluate.go

@@ -12,7 +12,7 @@ import (
 
 // Value are types supported by the language.
 type Value interface {
-	bool | int64 | string | []string | []int64 | [][2]string
+	bool | int64 | string | []string | []int64 | [][2]string | Nil
 }
 
 type (
@@ -50,6 +50,11 @@ func (args FArgs) Apply(v map[unique.Handle[Ident]]any) error {
 			}
 			return UndefinedError(arg.K.Value())
 		}
+
+		if arg.V == (Nil{}) {
+			reflect.ValueOf(r).Elem().SetZero()
+			continue
+		}
 		err := storeE(r, arg.V)
 		if err != nil {
 			return err
@@ -115,7 +120,7 @@ func (e TypeError) Is(err error) bool {
 }
 
 // storeE is a convenience function to set the value of a result pointer.
-func storeE(rp any, r any) error {
+func storeE(rp, r any) error {
 	pv := reflect.ValueOf(rp).Elem()
 	v := reflect.ValueOf(r)
 	pt, vt := pv.Type(), v.Type()
@@ -166,6 +171,9 @@ var (
 	ErrInvalidSpecial = errors.New("special must not be common or bound to scope")
 )
 
+// Nil represents an untyped nil.
+type Nil struct{}
+
 // evaluateAny implements [Evaluate].
 func evaluateAny(d PF, s []Frame, expr, rp any) bool {
 	defer func() {
@@ -226,6 +234,9 @@ func evaluateAny(d PF, s []Frame, expr, rp any) bool {
 				case "false":
 					store(rp, false)
 					return true
+				case "nil":
+					store(rp, Nil{})
+					return true
 				default:
 					return evaluateAny(d, s, v, rp)
 				}

internal/rosa/azalea/evaluate_test.go

@@ -100,15 +100,19 @@ func TestEvaluate(t *testing.T) {
 			Err:  UndefinedError("f"),
 		}},
 
-		{"error wrap deep", `f { v = nil; }`, makeStackCheck(func(
+		{"error wrap deep", `f { v = nonexistent; }`, makeStackCheck(func(
 			FArgs,
 		) (any, error) {
 			panic("unreachable")
 		}), "", EvaluationError{
-			Expr: Ident("nil"),
+			Expr: Ident("nonexistent"),
-			Err:  UndefinedError("nil"),
+			Err:  UndefinedError("nonexistent"),
 		}},
 
+		{"nil", `f { v = nil; }`, checkArgs(FArgs{
+			{K: unique.Make(Ident("v")), V: Nil{}},
+		}), "\xfd", nil},
+
 		{"common inputs", `package name { inputs, v = []; }`, nil, "", EvaluationError{
 			Expr: Func{
 				Ident:   Ident("name"),