On Wednesday 14 October 2009 6:15:11 am Roel van Dijk wrote:
If you declare a type for f5 then ghci must check if that type is correct, which triggers the explosion. If you don't declare a type then it won't infer the type until necessary. Basically, ghci is lazy
Well, that may be the explanation, but it's a bit more convoluted than I'd have initially thought. For instance, if I write: e = head () it fails immediately with a type error, so clearly it must be doing some level of checking immediately. However, it only needs to check 'head' and '()' to notice that they're incompatible, so I suppose it may not need to compute the type of e (were it well-typed). But then, adding: f6 = f5 . f5 produces no more delay than before. Nor does: g = snd . f5 So ghci is able to verify that f5 can be instantiated to the forms a -> b and b -> c in the first case, and that it is of the form a -> (b,c) in the second case, without blowing up. However, either of: e = f5 () e' () = f5 () does blow up, so it must be verifying more than the fact that f5 is of the form a -> b, where a can be instantiated to (), which is odd if it's smart enough to only compute the portion of the type necessary to verify that applications can be well typed. -- Dan