New subject: [GHC] #9755: Unhelpful error message when -XScopedTypeVariables is omitted (was: Monomorphism related Ix/Vector error when code is loaded by GHCi/cabal repl)

3 Nov 2014

      #9755: Monomorphism related Ix/Vector error when code is loaded by GHCi/cabal repl
-------------------------------------+-------------------------------------
       Reporter:  bitemyapp          |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  GHCi               |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 {{{
 {-# LANGUAGE BangPatterns #-}

 import Data.Vector (Vector, (!))
 import qualified Data.Vector as Vec
 import Data.Ix (Ix)
 import qualified Data.Ix as Ix

 vecIndexIx  :: (Ix ix, Bounded ix) => Vector a -> ix -> a
 vecIndexIx vec ix = vec ! Ix.index (minBound :: ix, maxBound :: ix) ix

 vecCreateIx :: (Ix ix, Bounded ix) => (ix -> a) -> Vector a
 vecCreateIx f = Vec.fromListN (Ix.rangeSize bounds)
                   [ y | ix <- Ix.range bounds, let !y = f ix ]
   where
     bounds = (minBound, maxBound)
 }}}

 The following errors occur:

 {{{
 GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 Prelude> :l cabal
 cabal.sandbox.config  cabal_problem.hs
 Prelude> :l cabal_problem.hs
 [1 of 1] Compiling Main             ( cabal_problem.hs, interpreted )

 cabal_problem.hs:9:37:
     Could not deduce (Bounded ix1) arising from a use of ‘minBound’
     from the context (Ix ix, Bounded ix)
       bound by the type signature for
                  vecIndexIx :: (Ix ix, Bounded ix) => Vector a -> ix -> a
       at cabal_problem.hs:8:16-57
     Possible fix:
       add (Bounded ix1) to the context of
         an expression type signature: ix1
         or the type signature for
              vecIndexIx :: (Ix ix, Bounded ix) => Vector a -> ix -> a
     In the expression: minBound :: ix
     In the first argument of ‘Ix.index’, namely
       ‘(minBound :: ix, maxBound :: ix)’
     In the second argument of ‘(!)’, namely
       ‘Ix.index (minBound :: ix, maxBound :: ix) ix’

 cabal_problem.hs:9:53:
     Could not deduce (Bounded ix1) arising from a use of ‘maxBound’
     from the context (Ix ix, Bounded ix)
       bound by the type signature for
                  vecIndexIx :: (Ix ix, Bounded ix) => Vector a -> ix -> a
       at cabal_problem.hs:8:16-57
     Possible fix:
       add (Bounded ix1) to the context of
         an expression type signature: ix1
         or the type signature for
              vecIndexIx :: (Ix ix, Bounded ix) => Vector a -> ix -> a
     In the expression: maxBound :: ix
     In the first argument of ‘Ix.index’, namely
       ‘(minBound :: ix, maxBound :: ix)’
     In the second argument of ‘(!)’, namely
       ‘Ix.index (minBound :: ix, maxBound :: ix) ix’

 cabal_problem.hs:12:32:
     Could not deduce (Ix a0) arising from a use of ‘Ix.rangeSize’
     from the context (Ix ix, Bounded ix)
       bound by the type signature for
                  vecCreateIx :: (Ix ix, Bounded ix) => (ix -> a) -> Vector
 a
       at cabal_problem.hs:11:16-59
     The type variable ‘a0’ is ambiguous
     Note: there are several potential instances:
       instance Ix () -- Defined in ‘GHC.Arr’
       instance (Ix a, Ix b) => Ix (a, b) -- Defined in ‘GHC.Arr’
       instance (Ix a1, Ix a2, Ix a3) => Ix (a1, a2, a3)
         -- Defined in ‘GHC.Arr’
       ...plus 8 others
     In the first argument of ‘Vec.fromListN’, namely
       ‘(Ix.rangeSize bounds)’
     In the expression:
       Vec.fromListN
         (Ix.rangeSize bounds) [y | ix <- Ix.range bounds, let !y = f ix]
     In an equation for ‘vecCreateIx’:
         vecCreateIx f
           = Vec.fromListN
               (Ix.rangeSize bounds) [y | ix <- Ix.range bounds, let !y = f
 ix]
           where
               bounds = (minBound, maxBound)

 cabal_problem.hs:12:45:
     Could not deduce (Bounded a0) arising from a use of ‘bounds’
     from the context (Ix ix, Bounded ix)
       bound by the type signature for
                  vecCreateIx :: (Ix ix, Bounded ix) => (ix -> a) -> Vector
 a
       at cabal_problem.hs:11:16-59
     The type variable ‘a0’ is ambiguous
     Note: there are several potential instances:
       instance Bounded Data.Monoid.All -- Defined in ‘Data.Monoid’
       instance Bounded Data.Monoid.Any -- Defined in ‘Data.Monoid’
       instance Bounded a => Bounded (Data.Monoid.Dual a)
         -- Defined in ‘Data.Monoid’
       ...plus 23 others
     In the first argument of ‘Ix.rangeSize’, namely ‘bounds’
     In the first argument of ‘Vec.fromListN’, namely
       ‘(Ix.rangeSize bounds)’
     In the expression:
       Vec.fromListN
         (Ix.rangeSize bounds) [y | ix <- Ix.range bounds, let !y = f ix]
 Failed, modules loaded: none.
 }}}

 The remedy was in this pull request https://github.com/haskell/hackage-
 server/pull/273/files under the file
 "Distribution/Server/Features/Search/DocTermIds.hs"

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9755
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[GHC] #9755: Monomorphism related Ix/Vector error when code is loaded by GHCi/cabal repl

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