doc/guide/src/TypeConstructor.adoc

   1 TypeConstructor
   2 ===============
   3
   4 In <:StandardML:Standard ML>, a type constructor is a function from
   5 types to types.  Type constructors can be _nullary_, meaning that
   6 they take no arguments, as in `char`, `int`, and `real`.
   7 Type constructors can be _unary_, meaning that they take one
   8 argument, as in `array`, `list`, and `vector`.  A program
   9 can define a new type constructor in two ways: a `type` definition
  10 or a `datatype` declaration.  User-defined type constructors can
  11 can take any number of arguments.
  12
  13 [source,sml]
  14 ----
  15 datatype t = T of int * real            (* 0 arguments *)
  16 type 'a t = 'a * int                    (* 1 argument *)
  17 datatype ('a, 'b) t = A | B of 'a * 'b  (* 2 arguments *)
  18 type ('a, 'b, 'c) t = 'a * ('b  -> 'c)  (* 3 arguments *)
  19 ----
  20
  21 Here are the syntax rules for type constructor application.
  22
  23  * Type constructor application is written in postfix.  So, one writes
  24  `int list`, not `list int`.
  25
  26  * Unary type constructors drop the parens, so one writes
  27  `int list`, not `(int) list`.
  28
  29  * Nullary type constructors drop the argument entirely, so one writes
  30  `int`, not `() int`.
  31
  32  * N-ary type constructors use tuple notation; for example,
  33  `(int, real) t`.
  34
  35  * Type constructor application associates to the left.  So,
  36  `int ref list` is the same as `(int ref) list`.