tighten `MapBuilder` factory parameter

[ahmednfwela]

MapBuilder currently has the factory: factory MapBuilder([map = const {}])
I think it should be

factory MapBuilder([map = const <K, V>{}])

there are also other instances where dynamic is used instead of strict types, is that intentional? e.g. #241

[davidmorgan]

Thanks Ahmed!

These APIs were designed way back in the Dart 1 days when type annotations didn't matter so much and there was no type inference.

I have a plan for a pile of changes like the one you suggest at some point, to modernize the APIs; but they are breaking changes so they will take some work to land. I'm not sure when that will happen.

Actually now that Dart 3 has launched the right time might be now, I will give it some thought.

Thanks!

[ahmednfwela]

that's really great! I will be happy to help contribute with some PRs once you share your plan 🙂

[mes71]

it can't be like

factory MapBuilder([map = const <K, V>{}])

1-Generics (K, V) are not inferred in a const expression
The const keyword requires compile-time constants, but since K and V are generic types, Dart doesn't allow them to be used in a const map without being explicitly specified.

2-Positional parameters can't have type-dependent default values
In Dart, default values for optional positional parameters must be compile-time constants, but <K, V>{} depends on generic types that are only resolved at runtime.