I'd like to express that at least one of the parameters a and b is c
Hello, Olaf! I never do it, but as idea: may be to use type-level lists, for example, and to add such condition in instance constraints? There is package type-lists with types "functions" like Find, etc (for member checking, appending, etc)... 23.07.2018 22:54, Olaf Klinke wrotes:
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-cons...
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