Question

I want to make a data type Var which represents a variable. A variable is a character that is a, b, c… z, lowercase or uppercase.

I can write this:

data Var = A | B | C -- continues to Z

but I can’t write this:

data Var = a | b | c -- continues to z

Presumably because they are lowercase. Is there an elegant way to make this data type?

Answer 1

One solution is to simply have

data Var = Var Char

While Var 'c' and Var 'D' will populate the type as desired, unfortunately Var '%' can populate it as well. There are two ways to fix this. One is to enumerate all constructors as you did:

data Var = A | B | C -- continues to Z
         | SmallA | SmallB | SmallC -- continues to z

The other way to only let correct values get into the type is to use smart constructors.