[Haskell-cafe] Re: Equirecursive types?

[lee marks]

John Hughes wrote:
> ...
> >
> Indeed, error messages in common cases would be worsened significantly, 
> because as Ross says, common errors (such as forgetting a parameter in a 
> recursive call) would lead to legal definitions that cause a type error 
> when applied. Partly for this reason, OCaml permits recursion only in 
> object types, if I remember rightly, where it's most useful. In other 
> cases the occurs check is still used.
> 

Couldn't this be mitigated by requiring an explicit type annotation on the function being called
recursively? In other words infinite types are never silently inferred, but can only coincide with
explicit signatures.

So this is legal:

  type Fix s a = s a (Fix s a)

  fold :: Functor (s a) => (s a b -> b) -> Fix s a -> b
  fold f = f . fmap (fold f)

but this is not:

  fold f = f . fmap (fold f)


I guess it just seems that banning infinite types altogether is throwing the baby with the bath.



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