[Haskell] Type problem
Emil Axelsson
emax at cs.chalmers.se
Tue Dec 13 04:20:46 EST 2005
I found an acceptable, but not too nice workaround:
* Add another class without methods:
class HasX a => HasX' a
* Make all types that may be unconstrained an instance of this class:
instance HasX' Number
* Make pairs an instance of HasX (this feels wrong):
instance HasX (a1,a2) where
xVal = undefined
* Add appropriate constraints to the GADT types (X has constraint HasX'):
data Val a where
P :: a -> Val a -- Primitive
T2 :: (HasX a1, HasX a2) => (Val a1, Val a2) -> Val (a1,a2)
X :: HasX' a => Val a -- Unconstrained
* Add (HasX a => ) to the value type.
At least this is safe. The undefined xVal will never be run.
I still wonder if the original idea couldn't work somehow...
Thank you,
/ Emil
Emil Axelsson skrev:
> Hello all,
>
> Could I please get some guidance with this?
>
> I'm working on implementing a simple relational language in Haskell.
> I'm trying to construct a data type that can represent values and
> patterns for a small set of supported types. See code below.
>
> HasX is a class of types that have an unconstrained value (xVal).
>
> Number is a typical member of that class.
>
> Val is my value/pattern data type.
> P represents a primitive value and T2 is used to make structure.
> X represents the unconstrained value or a wildcard pattern. It can only
> be used for types in HasX.
>
> The problem is the commented line in the value function. I want to use
> the xVal method to get the value for X. This is only allowed if I add
> the constraint (HasX a => ). But I don't want value to have that
> constraint, since then I cannot run it on pairs.
> Furthermore, it should be safe without the constraint. ex2 shows that we
> cannot use X to construct values that are not in HasX.
>
> Is this just a limitation of the current GATDs, or is it unreasonable of
> me to expect this to work?
>
> Is there any workaround, such as coercing the type of the value function?
>
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