Either returns with its parameters, reversed, but Maybe did not. That's my
main question.
On Sat, Sep 18, 2021 at 5:43 PM Francesco Ariis
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? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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