[Haskell-cafe] GHC 7.4: Expected behavior or bug?

[Bas van Dijk]

On 27 December 2011 17:47, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
> I fixed a similar breakage in the hmatrix library:
>
> https://github.com/AlbertoRuiz/hmatrix/commit/a4f38eb196209436f72b938f6355f6e28474bef3

GHC-7.4.1-rc1 also reported another type error in code that was
accepted by GHC <= 7.2.2. These were the type errors I got:

[24 of 36] Compiling Numeric.LinearAlgebra.Algorithms (
lib/Numeric/LinearAlgebra/Algorithms.hs,
dist/build/Numeric/LinearAlgebra/Algorithms.o )

lib/Numeric/LinearAlgebra/Algorithms.hs:576:23:
    No instance for (RealFrac (RealOf t0))
      arising from a use of `floor'
    Possible fix:
      add an instance declaration for (RealFrac (RealOf t0))
    In the expression: floor
    In the second argument of `($)', namely
      `floor $ logBase 2 $ pnorm Infinity m'
    In the expression: max 0 $ floor $ logBase 2 $ pnorm Infinity m

lib/Numeric/LinearAlgebra/Algorithms.hs:576:31:
    No instance for (Floating (RealOf t0))
      arising from a use of `logBase'
    Possible fix:
      add an instance declaration for (Floating (RealOf t0))
    In the expression: logBase 2
    In the second argument of `($)', namely
      `logBase 2 $ pnorm Infinity m'
    In the second argument of `($)', namely
      `floor $ logBase 2 $ pnorm Infinity m'

lib/Numeric/LinearAlgebra/Algorithms.hs:576:39:
    No instance for (Num (RealOf t0))
      arising from the literal `2'
    Possible fix: add an instance declaration for (Num (RealOf t0))
    In the first argument of `logBase', namely `2'
    In the expression: logBase 2
    In the second argument of `($)', namely
      `logBase 2 $ pnorm Infinity m'

lib/Numeric/LinearAlgebra/Algorithms.hs:576:43:
    No instance for (Normed Matrix t0)
      arising from a use of `pnorm'
    Possible fix: add an instance declaration for (Normed Matrix t0)
    In the second argument of `($)', namely `pnorm Infinity m'
    In the second argument of `($)', namely
      `logBase 2 $ pnorm Infinity m'
    In the second argument of `($)', namely
      `floor $ logBase 2 $ pnorm Infinity m'

lib/Numeric/LinearAlgebra/Algorithms.hs:593:19:
    No instance for (Container Vector t0)
      arising from a use of `add'
    Possible fix: add an instance declaration for (Container Vector t0)
    In the expression: add
    In an equation for `|+|': |+| = add
    In an equation for `expGolub':
        expGolub m
          = iterate msq f !! j
          where
              j = max 0 $ floor $ logBase 2 $ pnorm Infinity m
              a = m */ fromIntegral ((2 :: Int) ^ j)
              q = geps eps
              eye = ident (rows m)
              ....

lib/Numeric/LinearAlgebra/Algorithms.hs:599:1:
    Couldn't match type `t0' with `t'
      because type variable `t' would escape its scope
    This (rigid, skolem) type variable is bound by
      the type signature for expm :: Field t => Matrix t -> Matrix t
    The following variables have types that mention t0
      expGolub :: Matrix t0 -> Matrix t0
        (bound at lib/Numeric/LinearAlgebra/Algorithms.hs:575:1)

Note that RealOf is a type family:

type family RealOf x

type instance RealOf Double = Double
type instance RealOf (Complex Double) = Double

type instance RealOf Float = Float
type instance RealOf (Complex Float) = Float

Adding the following explicit type signature fixed it:

expGolub :: ( Fractional t, Element t, Field t
            , Normed Matrix t
            , RealFrac (RealOf t)
            , Floating (RealOf t)
            ) => Matrix t -> Matrix t

I have no idea if this should be considered a bug.

Regards,

Bas