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

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