Mark T.B. Carroll wrote:

I have a little programme that doesn't compile:

module Example where

class (Show c, Ord c, Bounded c) => MyClass c

showThings :: MyClass c => c -> (String, String)

showThings x = let foo = maxBound :: c in (show x, show foo)

If I change that second-to-last line to,

let foo = max x maxBound

then it compiles. However, it's clearly silly to use "max" just to make the type of the maxBound be the same type as the x. (I'm assuming that the Ord and the Bounded instances of the type are sensibly consistent.)

What should I be writing if I want foo to be the maxBound applied to the type that x is?

You could use asTypeOf from the Prelude: let foo = maxBound `asTypeOf` x -- asTypeOf is a type-restricted version of const. It is usually used -- as an infix operator, and its typing forces its first argument -- (which is usually overloaded) to have the same type as the second. asTypeOf :: a -> a -> a asTypeOf = const Also, Hugs and GHC both support an extension which lets you put type annotations in patterns: showThings (x::c) = let foo = maxBound :: c in (show x, show foo) /Bjorn