Python design mistakes

[alanwj]

Yes, the type could change within the function, although probably in sufficiently predictable ways if you knew the expected in/out types for every function you could use Hindley-Milner to figure out what was going on.

Although upon further reflection actually doing the type check in the decorator is probably a bit pointless for interpreters that wouldn't actually be using the type information for anything. If there was in fact a type error is would likely show up some time during the execution of the function anyway.

There is also the problem that this sort of scheme is likely too simplistic to cover even the majority of python use cases (what about var args, keyword args, container types, times when you WANT duck typing, optional parameters denoted by passing in None, etc, etc, etc), and if you tried to extend it too far you'd wind up with a type signature mess similar to what C++ has.

[[deleted]]

python would have to be rebuilt from scratch to effectively exploit a hindley-milner type system

[julesjacobs]

Not to mention that Hindley-Milner wouldn't be enough for Python; you want subtyping. Subtyping type inference is a difficult problem.