[GHC] #8119: better type inference for lambdas

8 Aug 2013

      #8119: better type inference for lambdas
-------------------------------------------+-------------------------------
       Reporter:  dmwit                    |             Owner:
           Type:  feature request          |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.6.3
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 With the following code in a file:

 {{{
 {-# LANGUAGE TypeFamilies #-}
 class Test a where test :: a
 instance (a ~ Int, b ~ Int) => Test (a -> b) where test = id
 }}}

 We can tell ghci that {{{test}}} is a function (and nothing more) in two
 different ways:

 {{{
 *Main> :t test `asTypeOf` (undefined :: a -> b)
 test `asTypeOf` (undefined :: a -> b) :: Int -> Int
 *Main> :t \x -> test x
 \x -> test x :: Test (t1 -> t) => t1 -> t
 }}}

 The type inferred for {{{\x -> test x}}} is very disappointing in
 comparison! A similar example can be cooked up with MPTCs and functional
 dependencies; see also http://lpaste.net/91534 for some (still quite
 abstract, of course) motivation.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8119>
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