On Fri, Dec 21, 2012 at 11:26:30AM +0000, Simon Peyton-Jones wrote:
> I think you need to remove the 'forall a' on the type signature for dP'.
> The 'a' you mean is the 'a' from the instance declaration, not a
> completely fresh 'a'.
This looks reasonable.
> Moreover I don't think you need the 'forall' on the 'instance' declaration.
> Just 'ScopedTypeVariables' should do it
All right, I try to follow both instructions:
-- docon.cabal -----------------------------------------------------
...
extensions:
TypeSynonymInstances UndecidableInstances FlexibleContexts
FlexibleInstances MultiParamTypeClasses OverlappingInstances
RecordWildCards NamedFieldPuns DoAndIfThenElse
.
ghc-options:
-fno-warn-overlapping-patterns -fwarn-unused-binds
-fwarn-unused-matches -fwarn-unused-imports
-XRankNTypes -- ** probably, it spoils nothing
-O
---------------------------------------------------------------
-- Pol3_.hs ----------------------------------------------------
...
{-# LANGUAGE ScopedTypeVariables #-} -- (1) **
instance (LinSolvRing (Pol a), CommutativeRing a) =>
LinSolvRing (UPol (Pol a))
where
gxBasis [] = ([], [])
gxBasis fs@(f:_) = (map back gs, mapmap back mt)
where
UPol _ p y dP = f
(o, n) = (pPPO p, genLength $ pVars p)
(toLex, fromLex) = (reordPol $ lexPPO n, reordPol o)
p' = (toLex p) `asTypeOf` p -- (2) **
dP' :: (LinSolvRing (Pol a), CommutativeRing a) => -- (3) **
Domains1 (Pol a)
dP' = upLinSolvRing p' Map.empty
s' = cToUPol y dP' p'
toOverP' = ct s' . map (\ (a, j) -> (toLex a, j)) . upolMons
fromOverP' = ct f . map (\ (a, j) -> (fromLex a, j)) . upolMons
back = fromOverP' . headVarPol dP
(gs, mt) = gxBasis $ map (fromHeadVarPol . toOverP') fs
-----------------------------------------------------------------
This does not help:
---------------------------------------------------------
Pol3_.hs:328:25:
Could not deduce (a ~ a1)
from the context (CommutativeRing (UPol (Pol a)),
MulMonoid (UPol (Pol a)),
LinSolvRing (Pol a),
CommutativeRing a)
bound by the instance declaration at Pol3_.hs:(313,10)-(314,72)
or from (LinSolvRing (Pol a1), CommutativeRing a1)
bound by the type signature for
dP' :: (LinSolvRing (Pol a1), CommutativeRing a1) =>
Domains1 (Pol a1)
at Pol3_.hs:327:12-71
`a' is a rigid type variable bound by
the instance declaration at Pol3_.hs:313:10
`a1' is a rigid type variable bound by
the type signature for
dP' :: (LinSolvRing (Pol a1), CommutativeRing a1) =>
Domains1 (Pol a1)
at Pol3_.hs:327:12
Expected type: Pol a1
Actual type: Pol a
In the first argument of `upLinSolvRing', namely p'
In the expression: upLinSolvRing p' Map.empty
In an equation for dP': dP' = upLinSolvRing p' Map.empty
Pol3_.hs:331:20:
Could not deduce (EuclideanRing a) arising from a use of dP'
from the context (CommutativeRing (UPol (Pol a)),
MulMonoid (UPol (Pol a)),
LinSolvRing (Pol a),
CommutativeRing a)
bound by the instance declaration at Pol3_.hs:(313,10)-(314,72)
Possible fix:
add (EuclideanRing a) to the context of the instance declaration
In the second argument of `cToUPol', namely dP'
In the expression: cToUPol y dP' p'
In an equation for s': s' = cToUPol y dP' p'
----------------------------------------------------------
Regards,
------
Sergei
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