ZipList Bind/Monad

[joneshf]

If I understand the new programmable type errors, it seems like this instance can be added:

instance zipListIsNotBind
  :: Fail "ZipList is not Bind. Any implementation would break the associativity law"
  => Bind ZipList where
    bind = crashWith "bind: unreachable"

Also, does this stop the hierarchy? Or can someone take this Bind instance and use it to make a Monad ZipList and higher instances?

[paf31]

Yes, this seems right. Also for things like V from purescript-validation.

This should prevent anyone from implementing a (non-overlapping) Monad instance too.

[joneshf]

What about the two questions at the end?

[paf31]

I'm not sure exactly what you mean, but this instance should make it impossible to define a non-overlapping Monad instance, since any Monad instance will need a Bind instance too which forces the Fail constraint.

[joneshf]

Gotcha! Thanks, I'll submit a PR.