On Mon, Sep 21, 2015 at 2:23 PM, goforgit .
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