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

@@ -115,7 +115,7 @@ 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()
@@ -226,6 +226,9 @@ func evaluateAny(d PF, s []Frame, expr, rp any) bool {
 				case "false":
 					store(rp, false)
 					return true
+				case "nil":
+					reflect.ValueOf(rp).Elem().SetZero()
+					return true
 				default:
 					return evaluateAny(d, s, v, rp)
 				}

internal/rosa/azalea/evaluate_test.go

@@ -100,13 +100,13 @@ 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"),
 		}},
 
 		{"common inputs", `package name { inputs, v = []; }`, nil, "", EvaluationError{