On Sun, Nov 26, 2017 at 07:02:36PM +0100, mrx wrote:
What do you mean by parameter of Point a? Let's start with a type you probably know, Maybe:
data Maybe a = Just a | Nothing Sorry, i've not used Maybe yet. Chapter 3 that i'm trying to get through now mention a Maybe ever so briefly. I've heard of a Maybe monad, is
Den 2017-11-26 kl. 19:20, skrev Francesco Ariis: that it?
The `a` in `Maybe a` is a type parameter, as the whole thing can be a `Maybe Int`, `Maybe String`, etc.
Now let's check what `Point a` does
data Point a = Coordinate Double Double
Uhhh, suspicious, there is an `a` on the left side, but it's pretty useless, because there is no `a` on the right side. This is most likely not correct. Ah, i see. Thanks for the clarification. Better to write
Do you think it would be a mistake to simply skip writing the type declarations completely until I've reached type classes? As now you know how write signatures like `something :: Int -> [String]`, when you meet `Something a => etc.` tread with care until you reach
-- this, concrete data Point = Coordinate Double Double -- or parametric data Point a = Coordinate a a Does this mean that i can write `delta :: Point Double t -> Point Double t -> Direction d` as a type declaration. Then i would require `Coordinate Double Double` as in parameters. Correct? the chapter on typeclasses. When i write type declarations, then i should stick with the non-`(Foo f) =>` version until i've reached that chapter on type classes. It's stilla few chapters until i reach it, i'm on chapter 3 and type classes are chapter 6.
// Patrik