[Haskell-beginners] Numeric Integer vs Integer

[Brandon Allbery]

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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