To add, t is typically some kind of container although it doesn’t have to
be. For instance, “t a” could be a “list (of) cat(s)”, except it doesn’t
*have* to be a list (can be anything that’s an instance of Foldable), and
it doesn’t *have* to be cats (could be a giraffe or a piano or any other
type).
In “list (of) cat(s)”, list can be thought of as a type-level function that
takes a type (cat) to construct a concrete type (list cat), hence the
notation.
This shows you commonly defined instances of Foldable (notice that the very
first one is a list, denoted by [ ]).
https://hackage.haskell.org/package/Cabal-3.4.0.0/docs/Distribution-Compat-P...
—
RRI
On Tue, Aug 3, 2021 at 18:06 Francesco Ariis
Il 03 agosto 2021 alle 19:36 Galaxy Being ha scritto:
Thanks for the insights. So what then does the t a part mean? In the simpler version t a is [a]. Is t some container? It's not meant to be a function, is it?
`t` is any unary type constructor. Maybe, Tree, etc. all take one parameter (Maybe a, Tree a, etc.); in a similar fashion [] takes one parameter ([] a). [a] is just syntactic sugar
λ> [7] :: [Int] [7] λ> [7] :: [] Int [7]
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-- -- Ramnath R Iyer