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@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@gmail.com> wrote:
Thanks Brandon, that cleared things up!

On Mon, Sep 21, 2015 at 8:39 PM, Brandon Allbery <allbery.b@gmail.com> wrote:

On Mon, Sep 21, 2015 at 2:23 PM, goforgit . <teztingit@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@gmail.com                                  ballbery@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net

_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners




_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners




--
Regards

Sumit Sahrawat