[GHC] #10608: Compile error regression from GHC 7.10 to 7.11

6 Jul 2015

      #10608: Compile error regression from GHC 7.10 to 7.11
-------------------------------------+-------------------------------------
              Reporter:  hvr         |             Owner:  simonpj
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:  7.12.1
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Other
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Consider the following program,

 {{{#!hs
 {-# LANGUAGE OverloadedStrings #-}
 -- {-# LANGUAGE FlexibleContexts #-}

 chunksOf :: Int -> String -> [String]
 chunksOf n = go
   where
     -- go :: String -> [String]
     go "" = []
     go s@(_:_) = a : go b
       where
         (a,b) = splitAt n s
 }}}

 when compiled with GHC 7.8.4:

 {{{
 GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Main             ( chunksof.hs, interpreted )

 chunksof.hs:8:5: Warning:
     Pattern match(es) are overlapped
     In an equation for ‘go’: go s@(_ : _) = ...
 Ok, modules loaded: Main.
 λ:2>
 }}}

 when compiled with GHC 7.10:

 {{{
 GHCi, version 7.10.1.20150630: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( chunksof.hs, interpreted )

 chunksof.hs:8:5:
     Non type-variable argument in the constraint: Data.String.IsString [t]
     (Use FlexibleContexts to permit this)
     When checking that ‘go’ has the inferred type
       go :: forall t. (Eq t, Data.String.IsString [t]) => [t] -> [[t]]
     In an equation for ‘chunksOf’:
         chunksOf n
           = go
           where
               go "" = []
               go s@(_ : _)
                 = a : go b
                 where
                     (a, b) = splitAt n s
 Failed, modules loaded: none.
 }}}

 NB: `FlexibleContexts` is rightly suggested!

 However, when compiled with GHC HEAD:

 {{{
 GHCi, version 7.11.20150630: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( chunksof.hs, interpreted )

 chunksof.hs:8:8: error:
     Could not deduce (IsString [t]) arising from the literal ‘""’
     from the context: Eq t bound by the inferred type of go :: Eq t => [t]
 -> [[t]] at chunksof.hs:(8,5)-(11,27)
     In the pattern: ""
     In an equation for ‘go’: go "" = []
     In an equation for ‘chunksOf’:
         chunksOf n
           = go
           where
               go "" = []
               go s@(_ : _)
                 = a : go b
                 where
                     (a, b) = splitAt n s
 Failed, modules loaded: none.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10608>
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