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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