internal/rosa/azalea: implement nil identifier

This also implements a nil-capable string type.

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 (
@@ -33,6 +33,29 @@ type (
 		F func(args FArgs) (v any, set bool, err error)
 		V map[unique.Handle[Ident]]any
 	}
+
+	// Nil represents an untyped nil.
+	Nil struct{}
+
+	// AString represents an argument string that may be nil.
+	AString struct {
+		V   string
+		Nil bool
+	}
+)
+
+// NilError is returned for an invalid nil assignment.
+type NilError struct{ reflect.Type }
+
+func (e NilError) Error() string {
+	return fmt.Sprintf("attempting to assign nil to %s", e.Type)
+}
+
+var (
+	// nilStringV is the [reflect.Value] of an Azalea nil assigned to an [AString].
+	nilStringV = reflect.ValueOf(AString{Nil: true})
+	// nilStringT is the [reflect.Type] of nilStringV.
+	nilStringT = nilStringV.Type()
 )
 
 // Apply applies named arguments and rejects unused arguments.
@@ -50,8 +73,8 @@ func (args FArgs) Apply(v map[unique.Handle[Ident]]any) error {
 			}
 			return UndefinedError(arg.K.Value())
 		}
-		err := storeE(r, arg.V)
-		if err != nil {
+
+		if err := storeE(r, arg.V); err != nil {
 			return err
 		}
 	}
@@ -115,11 +138,38 @@ 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()
+	pt := pv.Type()
+	as := nilStringT.AssignableTo(pt)
+
+	if r == (Nil{}) {
+		switch kind := pt.Kind(); kind {
+		case reflect.Bool, reflect.String:
+			return NilError{pt}
+		default:
+			if rv, ok := rp.(*any); ok && !reflect.ValueOf(*rv).IsValid() {
+				*rv = Nil{}
+			} else if as {
+				pv.Set(nilStringV)
+			} else {
+				pv.SetZero()
+			}
+			return nil
+		}
+	}
+
 	v := reflect.ValueOf(r)
-	pt, vt := pv.Type(), v.Type()
-	if !vt.AssignableTo(pt) {
+	if vt := v.Type(); !vt.AssignableTo(pt) {
+		if as {
+			var asv AString
+			if err := storeE(&asv.V, r); err != nil {
+				return err
+			}
+			pv.Set(reflect.ValueOf(asv))
+			return nil
+		}
+
 		return TypeError{vt, pt}
 	}
 	pv.Set(v)
@@ -226,6 +276,9 @@ 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 +100,79 @@ 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"),
-			Err:  UndefinedError("nil"),
+			Expr: Ident("nonexistent"),
+			Err:  UndefinedError("nonexistent"),
 		}},
 
+		{"nil", `f { v = nil; }`, checkArgs(FArgs{
+			{K: unique.Make(Ident("v")), V: Nil{}},
+		}), "\xfd", nil},
+
+		{"apply nil string", `f { v = nil; }`, makeStackCheck(func(
+			args FArgs,
+		) (v any, err error) {
+			var s string
+			err = args.Apply(map[unique.Handle[Ident]]any{
+				unique.Make(Ident("v")): &s,
+			})
+			v = s
+			return
+		}), "", EvaluationError{
+			Expr: Func{
+				Ident: Ident("f"),
+				Args: []Arg{
+					{K: []Ident{"v"}, V: Val{Ident("nil")}},
+				},
+			},
+			Err: NilError{Type: reflect.TypeFor[string]()},
+		}},
+
+		{"apply nil AString", `f { v = nil; }`, makeStackCheck(func(
+			args FArgs,
+		) (v any, err error) {
+			s := AString{}
+			err = args.Apply(map[unique.Handle[Ident]]any{
+				unique.Make(Ident("v")): &s,
+			})
+			v = s.V
+			if !s.Nil {
+				err = fmt.Errorf("got non-nil string %q", s.V)
+			}
+			return
+		}), "", nil},
+
+		{"apply AString", `f { v = "\xfd"; }`, makeStackCheck(func(
+			args FArgs,
+		) (v any, err error) {
+			s := AString{}
+			err = args.Apply(map[unique.Handle[Ident]]any{
+				unique.Make(Ident("v")): &s,
+			})
+			v = s.V
+			if s.Nil {
+				err = fmt.Errorf("unexpected nil %q", s.V)
+			}
+			return
+		}), "\xfd", nil},
+
+		{"apply nil", `f { v = nil; }`, makeStackCheck(func(
+			args FArgs,
+		) (v any, err error) {
+			s := make([]struct{}, 9)
+			err = args.Apply(map[unique.Handle[Ident]]any{
+				unique.Make(Ident("v")): &s,
+			})
+			if s != nil {
+				err = fmt.Errorf("got non-nil value %#v", s)
+			}
+			return
+		}), "", nil},
+
 		{"common inputs", `package name { inputs, v = []; }`, nil, "", EvaluationError{
 			Expr: Func{
 				Ident:   Ident("name"),
@@ -214,8 +278,8 @@ func TestEvaluate(t *testing.T) {
 			}
 
 			var errEquals bool
-			if _, ok := errors.AsType[TypeError](err); ok {
-				errEquals = errors.Is(err, tc.err)
+			if e, ok := errors.AsType[TypeError](err); ok {
+				errEquals = e.Is(tc.err)
 			} else {
 				errEquals = reflect.DeepEqual(err, tc.err)
 			}