[sparkie]

`String => Cat` and `String => Animal` are the types of the functions themselves - not the arg and return types.

If we consider inside the function:

    foo : arg:String => result:Animal
    foo = 
        ;; arg <: String
        ;; Animal <: result

However, outside of the function, when calling it, the opposite is true.

    let result = foo (arg)
        ;; String <: arg
        ;; result <: Animal

It's easy to get them mixed up when looking at it from the wrong perspective - but we should be looking at functions from the outside, not the inside - so yes, parameters should be contravariant and return types covariant.

[pansa2]

> parameters should be contravariant

Always? In that case, do you know why Dart has a `covariant` keyword [0], which makes function parameters covariant?

[0] https://dart.dev/language/type-system#covariant-keyword