[Haskell-cafe] Re: a simple question about types

Daniel Fischer writes:

On Wednesday 17 November 2010 19:09:16, Jerzy M wrote:

...

Hallo, let me take this simple function: (2*). If I check its type

:t (2*)

I'll obtain (2*) :: (Num a) => a -> a

But now it suffices to write g = (2*) and check

:t g

to obtain g :: Integer -> Integer

One more combination, now I write h x = (2*) x and check once more

:t h

to get h :: (Num a) => a -> a

So my question is: why (in this second example) Integer is inferred? What makes a difference?

The monomorphism restriction.

And default. If you load this into ghci module Main where default (Int) g = (2*) main = putStrLn "foo" and type :t g you'll get Int -> Int -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk