On Wed, May 27, 2009 at 9:59 PM, Bulat Ziganshin
Hello Simon,
Wednesday, May 27, 2009, 11:42:22 PM, you wrote:
while we are here - i always had problems understanding what is inferred and what is expected type. may be problem is just that i'm not native speaker
are other, especially beginners, had the same problem?
The inferred type of e is the type that the compiler thinks e has. The expected type is the type it *should* have, given its context. Consider: f :: Int -> Int then the expression (f True) has a type error: Couldn't match expected type `Int' against inferred type `Bool' In the first argument of `f', namely `True' GHC is saying the first argument of f *should* be an Int, but it seems to be a Bool. As to whether it's confusing, I sometimes have to read these messages a few times (sometimes it's unclear which expression is being referred to, or why GHC thinks that the expression has a certain type), but the words themselves are clear in their meaning to me. HTH, Max