[Haskell-beginners] Type declarations

[Francesco Ariis]

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

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. Better to write

    -- this, concrete
    data Point = Coordinate Double Double
    -- or parametric
    data Point a = Coordinate a a

> 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
the chapter on typeclasses.