type namedInt = (string * int);
val xBinding:namedInt = ("x", 15);
Type synonyms are simple shorthand for whatever's on the right hand side of the type synonym declaration. More interesting are user-defined data types:
(* Simple "enumeration" *)
datatype Colors = Red | Orange | Yellow | Green | Blue | Indigo | Violet;
(* A type with components *)
datatype MyNumber = MyInt of int | MyReal of real;
(* A recursive type *)
datatype StringTree =
Nothing
| StringTreeNode of string * StringTree * StringTree;
(* A recursive polymorphic type *)
datatype 'a Tree =
NullNode
| TreeNode of 'a * 'a Tree * 'a Tree;
(* Instantiating a tree node *)
val oneTree = TreeNode(4,
TreeNode(2, NullNode, NullNode),
TreeNode(7, NullNode, NullNode));
ML has a very powerful, flexible, and expressive module system. We won't have time to cover them fully this quarter, but here's the most basic use of modules:
structure MyStuff =
struct
datatype MyNum = MyInt of int | MyReal of real;
val myZero = MyInt(0);
fun myAdd(MyInt(x), MyInt(y)) = MyInt(x+y)
| myAdd(MyInt(x), MyReal(y)) = MyReal(real(x) + y)
| myAdd(MyReal(x), MyInt(y)) = MyReal(x + real(y))
| myAdd(MyReal(x), MyReal(y)) = MyReal(x + y);
end;
The above creates a struct (completely unlike a C struct in form and purpose!), and binds it to a structure name. Now, if any code outside this block wants to access any of the functions or types therein, that code must prefix the structure name:
val seal = MyStuff.MyReal(3.14159); val bear = MyStuff.MyInt(21); val theFreakishSealBear = MyStuff.myAdd(seal, bear);