On Fri, Jan 27, 2006 at 07:04:43PM +0100, Benjamin Franksen wrote:
All normal bindings (i.e. using '=') should be as polymorphic and general as possible.
I agree with the position in the Ben's email. Also, especially since much of the discussion has considered the impact on beginners, I want to recall that the monomorphism restriction can cause highly confusing error messages. They do not necessarily mention "probably cause: monomorphism restriction", and one cannot always find the problem by looking at the error location and the definition. I have a feeling there should be a better example of this issue floating around, but not finding one I'll submit my own: bar :: IO Char bar = foo foo = return 'a' baz = "hello " ++ foo ghc 6.4.1 gives the error try.hs:2:6: Couldn't match `IO' against `[]' Expected type: IO Char Inferred type: [Char] In the definition of `bar': bar = foo If you imagine that baz is buried at the bottom of the module in some test code, you might see how this could be mystifying. After all, there's no list mentioned in foo or bar. (But perhaps the error message could be improved to mention the monomorphism restriction?) My feeling is that when a variable definition should be monomorphic (and thus shared), the programmer ought to give the monomorphic type explicitly, rather than count on some use of the variable (possibly far away in the code) to decide it implicitly. Andrew