Re: [Haskell-cafe] Partial Application

On Jan 18, 2007, at 16:15 , Philippe de Rochambeau wrote:

When I read this, I thought that you could partially apply "multiply" by typing "multiply 2" at the ghci prompt. However, this generated an error:

<interactive>:1:0: No instance for (Show (Int -> Int)) arising from use of `print' at <interactive>:1:0-9 Possible fix: add an instance declaration for (Show (Int -> Int)) In the expression: print it In a 'do' expression: print it

Well, yes and no. Partial application at the prompt works fine; displaying the value of a partially applied function doesn't work so well, because un-applied or partially applied functions don't have nice printable representations (the "No instance for (Show (Int -> Int))" error) by default. You can actually change that to some extent, by loading something like this into ghci:

import Data.Typeable

instance (Typeable a, Typeable b) => Show (a -> b) where show f = "<" ++ show (typeOf f) ++ ">"

Now your partially applied function should print as Integer> instead of producing an error. (And because Haskell functions are curried and functions are themselves Typeable, this automatically generalizes to functions taking more than one argument. (This isn't perfect, since polymorphic functions are not Typeable.) -- brandon s. allbery [linux,solaris,freebsd,perl] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH