Il 18 settembre 2021 alle 17:30 Galaxy Being ha scritto:
:t Just True Just True :: Maybe Bool :t Left True Left True :: Either Bool b :t Right False Right False :: Either a Bool
What am I being told here? It seems are both straightforward parameterized types, but Maybe doesn't give me a type parameter back, while Either does, and in different order, different names (a becomes b; b becomes a) depending on which variable I invoke. What deeper lore am I not seeing here?
When you ask the type of λ> :t Just True the interpreter *knows* that that `Maybe` is not just a `Maybe a` (so type constructor and its type parameter) but the /concrete/ type `Maybe Bool`. This would not be the case if we did λ> :t Nothing Nothing :: Maybe a Same story with `Either`. Each of the two data constructors (`Left` and `Right`) let the interpreter infer just *one* of the type parameters (the `a` and `b` in `Either a b`). Does this answer your question?