[Haskell-cafe] Re: Type errors, would extensions help?

[Gleb Alexeyev]

Mauricio wrote:
> Hi,
> 
> I have this problem trying to define a function
> inside a do expression. I tried this small code
> to help me check. This works well:
> 
> ---
> import Data.Ratio ;
> main = do {
>   printNumber <- let {
>       print :: (Num n,Show n) => n -> IO () ;
>       print n = do { putStrLn $ show n}
>     } in return print ;
>   print (1%5) ;
>   print 5.0
> }
> ---

I guess you intended to call printNumber in the quoted snippet?
There's a way to use GHC's extensions to do what you want, let me 
illustrate with simpler example:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ImpredicativeTypes #-}

t1 () = do f <- (return id :: IO (forall a. a->a))
            return (f "foo", f True)

However, I would call this style unnatural and unnecessary. What's wrong 
with plain 'let' or 'where' that work without any extensions?

t2 () = do let f = id
            return (f "foo", f True)