On 27 December 2011 17:47, Bas van Dijk wrote:

I fixed a similar breakage in the hmatrix library:

https://github.com/AlbertoRuiz/hmatrix/commit/a4f38eb196209436f72b938f6355f6...

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

Thanks for the explanation. 2011/12/27 José Pedro Magalhães :

Hi,

This is a change in behavior. Previously GHC was more liberal than Haskell 98 prescribed, and would not default the kind of otherwise unconstrained type variables to *. 7.4 does default to *, so you have to provide kind signatures when you want another kind (particularly in phantom type variables).

Cheers, Pedro

On Tue, Dec 27, 2011 at 16:47, Bas van Dijk wrote:

...

On 27 December 2011 17:38, Michael Snoyman wrote:

...

Thanks to Mark Wright for pointing this out[1].

We have the equivalent of the following code in persistent:

{-# LANGUAGE MultiParamTypeClasses #-} data Key backend entity = Key

class Monad (b m) => Foo b m where    func :: b m (Key b m)

This code works fine with GHC 7.0, but I get the following message from GHC 7.4:

   Expecting two more arguments to `b'    In the type `b m (Key b m)'    In the class declaration for `Foo'

Is this expected behavior, or a bug? If the former, what would be a possible workaround?

Thanks, Michael

[1] https://github.com/yesodweb/persistent/issues/31

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I fixed a similar breakage in the hmatrix library:

https://github.com/AlbertoRuiz/hmatrix/commit/a4f38eb196209436f72b938f6355f6...

I don't know if it's a bug in GHC, but the workaround is to add an explicit kind signature:

{-# LANGUAGE KindSignatures, MultiParamTypeClasses #-} data Key (backend :: * -> * -> *) entity = Key

class Monad (b m) => Foo b m where   func :: b m (Key b m)

Cheers,

Bas

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