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)
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