Unhelpful error message when -XScopedTypeVariables is omitted

{-# 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"

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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CC	hvr
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