Re: [Haskell-cafe] Code that doesn't compile - but should :)

16 Mar 2010


      The error message (obtained by loading the file with ghci) is:

GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             (
C:\Users\pulcy\Desktop\Papers\Monads\Objec
tiveMonad\HObject\Experiments\FunctorsProblems.hs, interpreted )

C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
Problems.hs:18:15:
    Could not deduce (Selector s (f a -> a))
      from the context (Selector s (a1 -> f1 a1), Converter a1 f1)
      arising from a use of `select'
                   at
C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObjec
t\Experiments\FunctorsProblems.hs:18:15-22
    Possible fix:
      add (Selector s (f a -> a)) to the context of
        the type signature for `f'
      or add an instance declaration for (Selector s (f a -> a))
    In the expression: select s
    In the definition of `method': method = select s
    In the expression:
        let method = select s in (\ x -> let res = ... in convert res)

C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
Problems.hs:21:11:
    Couldn't match expected type `a1' against inferred type `f a'
      `a1' is a rigid type variable bound by
           the type signature for `f'
             at
C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Expe
riments\FunctorsProblems.hs:16:18
    In the expression: convert res
    In the expression: let res = method x in convert res
    In the expression: (\ x -> let res = method x in convert res)
Failed, modules loaded: none.
Prelude>

On Tue, Mar 16, 2010 at 2:31 AM, Ivan Lazar Miljenovic <
ivan.miljenovic@gmail.com> wrote:
...
Giuseppe Maggiore  writes:
...
Hi! Can anyone tell me why this code does not work? I cannot seem to
figure why it is broken...
The error message (and how you got it) would help...
...
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances,
  UndecidableInstances, FlexibleContexts, EmptyDataDecls,
ScopedTypeVariables,
  TypeOperators, TypeSynonymInstances #-}
You sure you have enough language extensions there? ;-)
Barely :)
...
--
Ivan Lazar Miljenovic
Ivan.Miljenovic@gmail.com
IvanMiljenovic.wordpress.com http://ivanmiljenovic.wordpress.com/
-- 
Giuseppe Maggiore
Ph.D. Student (Languages and Games)
Microsoft Student Partner
Mobile: +393319040031
Office: +390412348444