GitLab

Consider the following program,

{-# 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.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture