[Haskell-beginners] Numeric Integer vs Integer

[goforgit .]

Thank you guys, very good descriptions given on my question. Together with
your answers and the analogy of it being somewhat similar to something in
C++ made me understand how it works. Again many thanks!

On Wed, Sep 23, 2015 at 10:48 AM, Kim-Ee Yeoh <ky3 at atamo.com> wrote:

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