On Sep 12, 2009, at 20:17 , Sean McLaughlin wrote:

However, if you replace the function f with

f :: (MonadState s m, Has Int s) => m Int
f = do x <- project
   y <- project
   return x

then it fails with the error

   Could not deduce (Has α s)
   from the context (MonadState s m, Has Int s)
   arising from a use of `project'
   at /Users/sean/uploads/Weird.hs:16:12-18

   Possible fix:
   add (Has α s) to the context of the type signature for `f'
   In a stmt of a 'do' expression: y <- project
   In the expression:
   do x <- project

   y <- project
   return x
   In the definition of `f':
   f = do x <- project
   y <- project
   return x

I think what this is really telling you is that there's no way for ghc to tell what type y has. x is fine because it's in the result (hence fixed by the type (m Int)), but y is just "there" and all ghc knows is that it has (Has α s) and (MonadState s m, Has Int s) but no way to get from one to the other (more specifically: it has no clue what α might be).

Note that project is explicitly polymorphic, so the fact that you used it to get an (Int) does not in any sense guarantee that the subsequent use *also* produces an Int. If you want this, you'll have to constrain the call somehow.

brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com

system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu

electrical and computer engineering, carnegie mellon university KF8NH