[Haskell-cafe] Functional dependence nightmare
Edgar Gomes Araujo
talktoedgar at gmail.com
Sun Mar 27 01:59:28 CET 2011
Finally!!!! It's alive and working, huhhuu! After 3 long days...
As predictable, it was my fault. There was basically two big mistakes:
- splitAF::(SubUnit a b, Ord a)=>BoxPair -> ActiveSubUnit a -> StateMBC a
instead of "splitAF::(SubUnit a, Ord a)=> BoxPair -> a -> StateMBC a ()"
- type SetActiveSubUnits a = Set (ActiveSubUnit a)
instead of "type SetActiveSubUnits a = Set a"
After those corrections there was one error remaining:
Could not deduce (Unit a ~ Unit subUnit0)
from the context (SubUnit a, Ord a)
bound by the type signature for
mbc :: (SubUnit a, Ord a) => [Point] -> SetActiveSubUnits
a -> Box -> StateMBC a [Unit a]
NB: `Unit' is a type function, and may not be injective
Expected type: [Unit a]
Actual type: [Unit subUnit0]
Then I decided to go back to Fun Dep instead of GADT due the "injective"
class SubUnit subUnit unit | subUnit -> unit, unit -> subUnit where
class SubUnit subUnit where
type Unit subUnit :: *
And ... Bingo!
My last question, how can I make it work using GADT?
Thank so much for your kindly assistance.
On 26 March 2011 23:13, Stephen Tetley <stephen.tetley at gmail.com> wrote:
> Hi Edgar
> I think you have some errors of construction rather than just a
> problem with the type families / fun deps.
> Note, I've knocked off the class constraints on the data declarations
> on the copy I'm working with (e.g ActiveSubUnit) , this may or may not
> change things.
> This change to splitAF will type check, whether or not it does what you
> > splitAF::(SubUnit a, Ord a)=> BoxPair -> a -> StateMBC a ()
> > splitAF pairBox e = case subUnitPos pairBox e of
> Your code was:
> > splitAF::(SubUnit a)=>BoxPair -> a -> StateMBC a ()
> > splitAF pairBox e = case subUnitPos pairBox (activeUnit e) of
> getUnitsOnPlane will compile if you comment out the type sig, but GHC
> generates this type sig:
> getUnitsOnPlane ::
> (Unit su1 ~ Unit su,
> SubUnit (ActiveSubUnit su),
> SubUnit su1,
> SubUnit su,
> Ord su) =>
> -> BoxPair
> -> t
> -> StateT
> (SubUnitsSets (ActiveSubUnit su))
> [Unit su]
> -- Defined at DeUni.hs:324:1-15
> Note the class constraints are introducing a type variable *su1* that
> isn't mentioned in the function arguments, this strongly makes me
> think the function body isn't doing what it should. Also GHC infers a
> type var *t* rather than Plane for the second arg, again this makes me
> think the function body is not right.
> I'm a bit at a loss with the mbc function, if I comment out the
> function type sig and local type sigs GHC generates some errors
> mentioning Edge - as Edge is one of the instances of SubUnit, I think
> the function body is less polymorphic than you expect.
> Best wishes
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
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