[Haskell-cafe] Instances of constrained datatypes

[Simon Peyton-Jones]

I believe that you may find these papers relevant:

"Bulk types with class"
http://research.microsoft.com/%7Esimonpj/Papers/collections.ps.gz
"Restricted data types"
http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps

John's paper mentions that it would be great if the constraint
simplifier could generate recursive dictionaries --- and indeed GHC's
constraint simplifier now does exactly that.  So you can apply his
technique.

I'm not 100% certain this addresses your problem, but I think it does.

Simon

| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org
[mailto:haskell-cafe-bounces at haskell.org] On Behalf Of
| Keean Schupke
| Sent: 07 April 2005 09:30
| To: Keean Schupke
| Cc: haskell-cafe at haskell.org
| Subject: Re: [Haskell-cafe] Instances of constrained datatypes
| 
| One way to do roughly what you want is to pass the dictionary
yourself:
| 
|  >data EqDict a = EqDict {
|  >        leq :: a -> a -> Bool }
|  >
|  >data EqList a = EqList (EqDict a) [a]
|  >
|  >test :: EqList a -> EqList a -> Bool
|  >test (EqList dict (a0:as)) (EqList _ (b0:bs)) = (leq dict) a0 b0
| 
| In this way the definition of equality on elements of type 'a' is
passed
| with the list type, so it can be used wherever the list type is used,
| without requiring extra constraints.
| 
|    Keean.
| 
| Keean Schupke wrote:
| 
| > I think it is more a problem of imlpementation than one of what is
| > desirable. A Constrained data type:
| >
| > data (Eq v) => EqList v = EqList [v]
| >
| > The problem is how to get the dictionary for the class Eq to the
| > application site:
| >
| > f :: EqList v -> EqList v
| > f (EqList (u0:us)) (EqList (v0:vs)) | v0 == u0 = ...
| >
| > Which of course does not work... the constraint needs to be in the
| > function
| > type signature:
| >
| > f :: Eq v => EqList v -> EqList v
| >
| > Things are worse though, as even functions that use no methods of Eq
will
| > require the constraint.
| >
| > The constraint on the data type does not stop you construction
EqLists
| > from
| > non Eq members... of course this gets detected the moment you try
and
| > use it
| > in a constrained function.
| >
| >
| > In other words using the constraint in the data type does nothing...
| > you may as well just do:
| >
| > f :: Eq v => [v] -> [v]
| >
| >
| > Infact I believe it was decided to remove the feature from Haskell98
| > entirely, but there was apparently some use for the 'syntax'
although
| > with a different effect.
| >
| >    Keean.
| >
| > Cale Gibbard wrote:
| >
| >> I don't believe you can, but it would be nice. There are certain
| >> types, such as Set, where it's not really possible to just remove
the
| >> constraint from the data declaration, and yet it would be nice if
sets
| >> could be instances of Monad and Functor. Currently, to be an
instance
| >> of Functor or Monad, your type has to be a functor defined on the
| >> whole category of types.
| >>
| >> Could this issue be fixed somehow? Constrained instances would make
| >> various typeclass-based libraries more applicable. What would it
break
| >> to allow instances where the types of functions defined by the
| >> typeclass are further restricted? I suppose that checking that
types
| >> are correct becomes more difficult and non-local, because
functions
| >> which are defined using the typeclass won't already have that
| >> constraint for obvious reasons. Still, the constraint is in the
| >> instance, which must be around when the functions actually get
| >> applied. There are probably bad interactions with the module
system,
| >> but I'm not certain.
| >>
| >> People must have talked about this before... was a consensus
reached
| >> that I'm not aware of?
| >>
| >> - Cale
| >>
| >> On Apr 6, 2005 2:10 AM, Arjun Guha <guhaarju at grinnell.edu> wrote:
| >>
| >>
| >>> This is a contrived example, but contains the essence of what I'd
like
| >>> to do.  Suppose I have this datatype:
| >>>
| >>> > data (Eq v) => EqList v = EqList [v]
| >>>
| >>> I'd like to make it an instance of Functor.  However, fmap takes
an
| >>> arbitrary function  of type a -> b.  I need an Eq constraint on a
and
| >>> b.  Is there any way to do this without creating my own
`EqFunctor'
| >>> class with explicitly-kinded quantification:
| >>>
| >>> > class (Eq a) => EqFunctor (f :: * -> *) a where
| >>> >  eqfmap:: (Eq b) => (a -> b) -> f a -> f b
| >>>
| >>> Thanks.
| >>>
| >>> -Arjun
| >>>
| >>
| >
| > _______________________________________________
| > Haskell-Cafe mailing list
| > Haskell-Cafe at haskell.org
| > http://www.haskell.org/mailman/listinfo/haskell-cafe
| 
| 
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