Overlapping instances

Help
#1 
class SomeClass (m :: Type -> Type)

instance someClassArray :: SomeClass Array

instance someClassNewtype :: (Newtype (t a) (m a), SomeClass m) => SomeClass t

This gives me the error

  Overlapping type class instances found for

    Control.MonadPlus.SomeClass t

  The following instances were found:

    Control.MonadPlus.someClassArray
    Control.MonadPlus.someClassNewtype


in type class instance

  Control.MonadPlus.SomeClass t
PureScript(OverlappingInstances)

I don’t understand this.

Newtype
Array

Array is not a Newtype. I don’t see how they’re overlapping. Does the compiler not see the (Newtype (t a) (m a), SomeClass m) dependency?

1 Like
#2

Correct, those constraints are checked after picking the instance based just on the part after the =>, so SomeClass t will overlap with everything.

There’s lots of discussion in this space at https://github.com/purescript/purescript/issues/3596

You can use instance chains to make the error go away, but it might not really be what you want

2 Likes
#3

Instance chains seems sufficient. Thank you, very cool.