问题
For example:
fun example (a:'a list) : list = a
will have a signatures of:
'a list -> 'a list
What if I define it differently but with same content like
fun example (a : ''a list) : list = a
its signature will be:
''a list -> ''a list
What's the difference?
回答1:
A plain type variable like 'a can be substituted with an arbitrary type. The form ''a is a so-called equality type variable, which means that it can only be substituted by types that admit the use of the equality operator = (or <>) on their values.
For example, this function:
fun contains(x, []) = false
| contains(x, y::ys) = x = y orelse contains (x, ys)
cannot have type 'a * 'a list -> bool because it uses equality on x. It is given the more restrictive type ''a * ''a list -> bool.
Most types allow equality, but some don't, e.g., real, exn, and in particular, any function type t -> u. Composed types like records, tuples, or datatypes admit equality if all their components do.
Side remark: Haskell later generalised this concept to its notion of type classes, which allows arbitrary user-defined constraints of this sort on types. Equality type variables are replaced by the Eq type class.
来源：https://stackoverflow.com/questions/20437520/what-is-the-difference-between-a-and-a-in-sml