On Thu, Jul 16, 2009 at 08:52:40PM +0100, Andrew Coppin wrote:
Ross Mellgren wrote:
It's not where -- let also works
Prelude> let { foo x = x } in (foo 1, foo True) (1,True)
Awesome. So by attempting to implement Haskell's type system, I have discovered that I actually don't understand Haskell's type system. Who'd have thought it?
Clearly I must go consult the Report and check precisely what the rules are...
actually, the rules are pretty straightforward. It doesn't matter where something is bound, just _how_ it is bound. Let-bound names (which includes 'where' and top-level definitions) can be polymorphic. lambda-bound or case-bound names (names bound as an argument to a function or that appear in a pattern) can only be monomorphic. And that's all there is to it. (the monomorphism restriction complicates it a little, but we don't need to worry about that for now) As an extension, ghc and jhc allow arguments and case bound variables to be polymorphic but only when explicitly declared so by a user supplied type annotation. (the exact rules for what 'explicitly declared' means can be a little complicated when formalized, but they match up enough with intuition that it isn't a problem in practice). They will never infer such a type on their own. John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/