[Haskell-beginners] Numeric Integer vs Integer

[goforgit .]

I have a follow up question, I hope it's alright.

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?

On Tue, Sep 22, 2015 at 4:54 PM, goforgit . <teztingit at gmail.com> wrote:

> Thanks Brandon, that cleared things up!
>
> On Mon, Sep 21, 2015 at 8:39 PM, Brandon Allbery <allbery.b at gmail.com>
> wrote:
>
>>
>> On Mon, Sep 21, 2015 at 2:23 PM, goforgit . <teztingit at gmail.com> wrote:
>>
>>> Could someone explain to me the difference between the following?
>>>
>>> data Atype = Numeric Integer | A | B C
>>>
>>> and
>>>
>>> data Atype = Integer | A | B C
>>>
>>
>> The second one is an enumeration with three values: "Integer" (which is
>> *not* an integer, nor does it contain one), "A", and "B" which as written
>> there takes a value of some unspecified type C as a parameter.
>> The first one is a enumeration with three values: "Numeric" which takes
>> an Integer as a parameter, "A", and "B" which takes a value of some
>> unspecified type C as a parameter.
>>
>> Note that the "Integer" in the second one has *nothing whatsoever* to do
>> with the *type* Integer.
>>
>> Remember that you must always provide a data constructor with "data"; you
>> cannot simply say "data MyInt = Integer" to "wrap" an Integer, because you
>> have not said what to wrap it *in*. (You may have intended to create a type
>> alias, though; that would be "type", not "data".) A "data" always requires
>> a data constructor name, so the compiler can tell when you are talking
>> about a value of that type by looking for the constructor.
>>
>> --
>> brandon s allbery kf8nh                               sine nomine
>> associates
>> allbery.b at gmail.com
>> ballbery at sinenomine.net
>> unix, openafs, kerberos, infrastructure, xmonad
>> http://sinenomine.net
>>
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>
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