[Haskell-beginners] Numeric Integer vs Integer

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

What you wrote is equivalent to the following pseudo-haskell.

*List* takes a type and returns a new type, which represents a list of
elements of that type in this case. Thus *List* is a *type-constructor*.
It has two data-constructors, i.e. there are two ways to create elements of
the type *List a*, for any type a.

-- 'Empty' is a value of type 'List a', for all types 'a'. This is one
possible way to construct elements of type 'List a'.
Empty :: List a

-- 'Add' is a function that takes an element of type 'a', and a 'List a',
and constructs a 'List a' out of them.
Add :: a -> List a -> List a

Using the GHC extension, *GADTs*, it is possible to write code that
directly reflects this structure. Although that is considered an advanced
feature of the language.

{-# LANGUAGE GADTs #-}

data List a where
  Empty :: List a
  Add :: a -> List a -> List a

On 23 September 2015 at 13:21, goforgit . <teztingit at gmail.com> wrote:

> 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
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
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>>>
>>>
>>
>
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-- 
Regards

Sumit Sahrawat
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