On Wed, Sep 23, 2015 at 2:51 PM, goforgit . <teztingit@gmail.com> wrote:

What about the following?

data List a = Empty | Add a (List a)

What does the a mean and why is it possible to put it there?

In addition to the good answers already given, it helps to think of it this way:

Here's a list of Bools:

data ListBool = EmptyListBool | AddListBool Bool ListBool

Here's a list of Chars:

data ListChar = EmptyListChar | AddListChar Char ListChar

Here's a list of Ints:

data ListInt = EmptyListInt | AddListInt Int ListInt

Well just look at all that repetition!

Surely there must be a way to keep DRY and abstract over all that?

Let's see: what's common to all of the above? What stays the same? What changes?

Here's something that tries to express and separate out what's "fixed" and what's "insert type here":

data ListX = Empty | Add X ListX

We're close.

That almost but doesn't quite work, because Haskell treats capital X as a concrete type, like Bool and Char and Int.

What we want is a type _variable_. And Haskell gives us that, if we use lower-case letters:

data List x = Empty | Add x (List x)

The parens is needed to distinguish against

data List x = Empty | Add x List x

which doesn't work for reasons you can probably guess.

Finally, it's convention to use type variables a b c and not x y z.

-- Kim-Ee

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