Re: Re[2]: [Haskell-cafe] Strange type error with associated type synonyms
On Wed, May 27, 2009 at 9:59 PM, Bulat Ziganshin wrote: > 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Strange type error with associated type synonyms
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? > Claus made a suggestion about type error messages: > | Apart from bracketing fully applied type synonyms, the error message > | could be improved by providing the missing bit of information about > | 'Memo': > | > | D:\home\Haskell\tmp\desktop\types.hs:11:11: > | Couldn't match expected type `Memo d1' > |against inferred type `Memo d' > | (type Memo d :: * -> *) > | In the second argument of `(.)', namely `appl' > | In the expression: abst . appl > | In the definition of `f': f = abst . appl > I've implemented this idea; note the "NB" line below: > NoMatchErr.hs:20:11: > Couldn't match expected type `Memo d' >against inferred type `Memo d1' > NB: `Memo' is a (non-injective) type function > In the second argument of `(.)', namely `appl' > In the expression: abst . appl > In the definition of `f': f = abst . appl > (Rather than give its kind, I thought it was better to focus on the > reason for the mis-match, namely the non-injectivity.) > Simon > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
