[Haskell-cafe] Re: Functional Dependencies Help

[John Creighton]

The errors in the older code were due to not supplying enough input
arguments to all my class instance declarations. My final code works
and is pasted bellow:

{-# LANGUAGE EmptyDataDecls,
             MultiParamTypeClasses,
             ScopedTypeVariables,
             FunctionalDependencies,
             OverlappingInstances,
             FlexibleInstances,
             UndecidableInstances #-}

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}


data Noun = Noun deriving (Show) --10
data Verb = Verb deriving (Show) --
data Adjactive = Adjactive deriving (Show)

data Animal=Animal deriving (Show)
data Feline=Feline deriving (Show)
data Cat = Cat deriving (Show)

data Taby_Cat=Taby_Cat deriving (Show)
data T=T deriving (Show)
data F=F deriving (Show) --20


class Isa a b c | a b->c where isa::a->b->c

instance Isa Animal Noun T where isa a b = T --25

class IsSuperSet a b c | a b->c where -- General Definition
    isSuperSet :: a->b->c
class IsSuperSet' a b c | a b->c where -- Specific Cases
    isSuperSet :: a->b->c

instance IsSuperSet' Feline Cat T where --30
   isSuperSet a b=T
instance IsSuperSet' Animal Feline T where
   isSuperSet a b=T
instance IsSuperSet' a Animal F where
   isSuperSet a b=F --35


instance (IsSuperSet' d b c, --40
          IsSuperSet a d c,
         )=>
    IsSuperSet a b c where
      isSuperSet a b=undefined::c


class ToBool a where
   toBool :: a->Bool

instance ToBool T where
   toBool a = True

instance ToBool F where
   toBool a = False

myCat=Cat
bla=isSuperSet Animal Cat
bla2=isSuperSet Cat Animal

On May 1, 10:16 am, John Creighton <johns2... at gmail.com> wrote:
> On Apr 30, 6:18 pm, John Creighton <johns2... at gmail.com> wrote:
>
>
>
> > On Apr 29, 7:47 am, John Creighton <johns2... at gmail.com> wrote:
>
> > > I've been trying to apply some stuff I learned about functional
> > > dependencies, but I run into one of two problems. I either end up with
> > > inconsistent dependencies (OverlappingInstances  doesn't seem to
> > > apply) or I end up with infinite recursion. I want to be able to do
> > > simple things like if a is a subset of b and b is a subset of c then a
> > > is a subset of c. If a is a is a subset of b and b is a c then a is a
> > > c.
>
> > > Before I added the equality functions I had infinite recursion. Once I
> > > put them them in then I have trouble with overlapping instances.
>
> > I've been doing some reading and I think the following is an
> > improvement but I end up hanging the compiler so I can't tell what the
> > errors are. I'll see if their are any trace options that might be
> > helpfully for GHC.
>
> So bellow I'll post the latest version of my code but first the errors
> which seem very strange to me:
>
> --------------------------------------------------------------------
>
> could not deduce (IsSuperSet'
>                      isanimal iseq isanimal iseq1 (a -> b -> c3) )
>    from the context (IsSuperSet a b c2,
>                      Typeeq a b iseq1,
>                      TypeEq Animal b isaninmal,
>                      IsSuperSet' isanimal iseq1 a b c3)
>    arising from a use of 'isSuperSet'' at logicp2.hs:92:25-74
>    Possible fix:
>       add (IsSuperSet'
>               isanimal iseq isanimal iseq1 (a -> b -> c3)) to context
> of the declaration
>       or add an instance delaration for
>           (IsSuperSet' isanimal iseq isanimal iseq1 (a -> b -> c3))
>    In the expression:
>           (isSuperSet' (u :: isanimal) (u :: iseq) (a :: a)
> (b ::b)) :: c3
>    In the definition of 'isSuperset':
>        isSuperset a b
>                     = (isSuperSet' (u :: isanimal) (u :: iseq) (a ::
> a) (b :: b))
>    in the instance delaration for 'IsSuperSet a b c3'
>