[Haskell-cafe] Expressing disjunctions of constraints
Olaf Klinke
olf at aatal-apotheke.de
Mon Jul 23 19:54:35 UTC 2018
Dear cafe,
today I wrote a type class with three parameters.
class Foo a b c | a b -> c where
foo :: a -> b -> c
instance Foo A R A where ...
instance Foo R A A where ...
In addition to the functional dependency I'd like to express that at least one of the parameters a and b is c, that is, Foo is the union of the two classes
class Foo1 a b where
foo1 :: a -> b -> a
class Foo2 a b where
foo2 :: a -> b -> b
and furthermore the choice between Foo1 and Foo2 determines one parameter to be a fixed type, say R. I understand that logical disjunction poses problems for instance resolution [1] because adding an instance may affect the instance dictionary lookup. This should not happen in my case because in the end this would be a one-parameter type class. The ultimate aim is to use the same symbol for both a function
foo :: R -> a -> a
and for flip foo. Of course writing flip foo is not that much of a nuisance, I just wonder whether Haskell is expressive enough to do this.
Olaf
[1] https://stackoverflow.com/questions/10255148/how-can-i-combine-two-type-constraints-with-a-logical-or-in-haskell
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