Add ++i and --i support

[wrbl606]

No description provided.

[wrbl606]

Resolves #4

[ethanblake4]

Looks pretty good! Two comments; also, your tests should probably test the return values of the postfix/prefix expressions since that's the main difference between them.

[ethanblake4]

What would happen if L.name was null? like if you did:

final list = [1]; 
final result = ++list[0];
print(result);

[ethanblake4]

I'm realizing that you just took that from the postfix.dart code so this one's my fault actually, sorry! Still, if you wouldn't mind that check should be removed from both prefix and postfix, it's incorrect

[wrbl606]

Yeah, for ++list[0], this happens: See below

dart_eval runtime exception: type 'int' is not a subtype of type '$Value'
#0      BoxList.run (package:dart_eval/src/eval/runtime/ops/primitives.dart:255:69)
#1      Runtime.execute (package:dart_eval/src/eval/runtime/runtime.dart:787:12)
#2      Runtime.executeLib (package:dart_eval/src/eval/runtime/runtime.dart:775:12)

[wrbl606]

After removal of if (L.name != null)... plus_plus and minus_minus works for both prefix and postfix, but there's an issue with boxing I think?

As you can see in the failed test, doing

List<int> list = [0];
print(list[0]++);
print(list[0]);

causes

dart_eval runtime exception: type 'int' is not a subtype of type '$Value?' in type cast
#0      InvokeExternal.run (package:dart_eval/src/eval/runtime/ops/bridge.dart:101:34)
#1      Runtime.execute (package:dart_eval/src/eval/runtime/runtime.dart:787:12)
#2      Runtime.executeLib (package:dart_eval/src/eval/runtime/runtime.dart:775:12)

where

List<int> list = [0];
print(list[0]);
print(list[0]++);

works fine.

Not sure how to fix that, also doing any other list reading with eg. print(list)/print(list.first) after plus_plus/minus_minus prefix/postfix also causes similar issues:

print(list):

dart_eval runtime exception: type 'int' is not a subtype of type '$Value'
#0      BoxList.run (package:dart_eval/src/eval/runtime/ops/primitives.dart:255:69)
#1      Runtime.execute (package:dart_eval/src/eval/runtime/runtime.dart:787:12)
#2      Runtime.executeLib (package:dart_eval/src/eval/runtime/runtime.dart:775:12)

print(list.first):

dart_eval runtime exception: type 'int' is not a subtype of type '$Value'
#0      BoxList.run (package:dart_eval/src/eval/runtime/ops/primitives.dart:255:69)
#1      Runtime.execute (package:dart_eval/src/eval/runtime/runtime.dart:787:12)
#2      Runtime.executeLib (package:dart_eval/src/eval/runtime/runtime.dart:775:12)

[wrbl606]

Above also happens for current master so we can rule out the possibility of introducing this issue with changes in this PR

[ethanblake4]

Ok that turned out to be a bit of a rabbit hole that I wouldn't expect you to dive into at this point, so I went ahead and pushed a fix to reference.dart in the v0.7.0 branch. If you pull it in, you should be able to do return V.setValue(ctx, result); on line 82 of prefix.dart and everything will work correctly.

TLDR: lists in dart_eval are expected to contain only boxed types like $int, but we were passing an unboxed int to setValue and it was just setting that directly into the list. You should be able to pass anything to setValue and let it figure it out, so I made setValue box/unbox the argument you pass it automatically. The caller of setValue needs to be informed about this change though otherwise it will think the variable is still in its previous boxing state, so I also made it return the argument.

[wrbl606]

With your changes, list access is now working correctly, thanks!