[Haskell-cafe] Expressing disjunctions of constraints

[Paul]

 > 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-constraints-with-a-logical-or-in-haskell
>
>
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