Comfy Cats Effect room. Ask questions. Rant about config file formats. Be nice!
like, it should be capitalized
Real Stupid Question
I've been coming at cats piecemeal, and the one thing I never really see explained is F[_]
I believe that it is supposed to be some sort of typed function but am not really sure enough to stake my life one it
am I correct, and if not, where can I become less stupid about this in the future?
do you want to know what
F[_] means in terms of language feature
or the point of the pattern? (and what the pattern is, how it works)
have you ever heard the word
kind?
in the context of type systems, of course :smile:
and in short
F[_] is a higher kinded type parameter
but we can make it more precise if you want
whoops, already started :)
no need to backtrack now
you have types of kind
* (pronounced star, or type)
these are types that don't need any extra information
e.g.
Int String Option[Int], Option[List[Either[String, Int]]] etc
but some types do need extra information
in other words,
Option has kind * -> *, because it requires a proper type (say Int), and it gives you another proper type Option[Int]
Either is of kind * -> * -> * (they are curried, so you can think of it as having two arguments)
this language is quite rich, e.g. the kind of
Resource is (* -> *) -> * -> *
it takes a first argument of kind
* -> * and a second of kind * and it gives you something of kind *
e.g.
Resource[IO, Int]
a function on types, almost