[Open Discussion] Cast Specification

[ncannasse]

ATM in Haxe 4 we have different ways to cast a type to another:

• the "safe cast", which is cast(expr,T)
  • at compilation : will ensure that the type of expr is a subtype of T so the cast is valid
  • at runtime : will perform the typecheck and throw if not satisified
• the "safe downcast", which is Std.downcast(expr,T)
  • very similar to the safe cast, but returns null instead of throwing an error if the cast is not satisfied.
  • can be used instead of a expr is T combined with a cast(expr,T)
• the "unsafe cast", which is var v : T = cast expr
  • no compile time check is performed
  • runtime check is only performed on static targets, which will throw an exception on these targets only. On JS for example no error will occur
  • sadly that's at the same time the most unsafe AND the shortest to write.
• additionally we have expr is T and Std.isOfType(v,T) to check if an expression is of a given type at runtime, the Std.isOfType allowing to specify the type as a variable.

I know it's a bit late but I think we should change this for Haxe 5, as this is the kind of big change we except and allow in a major release. My proposal would be to:

• make the current unsafe cast syntax do the same thing as the safe cast, so var v : T = cast expr would be the same as var v = cast(expr,T).
• introduce a Std.unsafeCast(expr) that would replace the unsafe cast, but then be much more explicit.
• promote downcast to a keyword and allow both var v : T = downcast expr and var v = downcast(expr,T) the same as the safe cast but with a null result.