YamlMap does not sport any type parameters

[srawlins]

This makes it hard to call forEach, or other methods that take a callback, without using dynamic as bottom (dart-lang/sdk#29630):

YamlMap a = ...
a.forEach((String k, YamlMap v) { ... }); // uses dynamic as bottom.

[srawlins]

Same with YamlList.

[nex3]

I don't know if there's much we can do about this... there are no type guarantees about what these values contain. YAML even allows non-string keys. Ultimately, you have to make an assertion at some point that the values are the types you expect, and Dart as a language strongly encourages you to do that explicitly with as or Map.cast().

[srawlins]

Can we not just do:

class YamlMap<K, V> extends YamlNode with collection.MapMixin<K, V>, UnmodifiableMapMixin

?

[nex3]

Where would those type parameters ever be instantiated? YamlMap is only ever constructed by the YAML parser, which can make no guarantees about the heterogeneity of the objects the mapping contains.

[srawlins]

I'm just looking for an implicit cast (another Dart language feature):

YamlMap<String, YamlMap> a = ...
a.forEach((String k, YamlMap v) { ... });

How would you suggest using Map.cast?

[nex3]

Implicit/as casts on generic types like that are not runtime strong-mode safe unless the type is actually constructed with those generic types. It works in Dart 1 semantics, which had to be very lax around reified generics because the type system made it nearly impossible to guarantee that anything stayed reified, but in Dart 2 it will fail. That's why the cast() methods are being added: they change the reified type arguments that type-check values as they're accessed.

You should write something like:

var map = /* ... */.cast<String, YamlMap>();
map.forEach((key, value) { ... });