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)
 				}