Motivation of having Functional Dependencies

Question

What is the motivation of having functional dependencies in Haskell ?

One example of a functional dependency:

class (Monad m) => MonadSupply s m | m -> s where
  next :: m (Maybe s)

It is stated in the RWH book, that functional dependency helps the type checker. How does it actually help ?

Also, this piece of code actually compiles:

class (Monad m) => MonadSupply s m where
      next :: m (Maybe s)

But I guess, it will produce an runtime error.

Answer 1

It's perfectly fine to write code not using functional dependencies, it's just a pain to use since the inference sucks.

Basically without FDs, the function get :: MonadState m s => m s will have to figure out m and s independently. Usually m is quite easily inferred, but often s would require an explicit annotation.

Moreover, this is much more general than we need, so instead we can restrict our typechecker to say "For m, there is exactly 1 s", this way, once m is inferred, s is obvious to the type inference algorithm