Generalised Algebraic Datatypes

A Generalised Algebraic Datatype, or GADT, is an extension of the basic variant concept:

variant pair[T] =
| PUnit of unit => pair[unit]
| PInt[T] of int * pair[T] => pair[int * pair[T]]
| PFloat[T] of float * pair[T] => pair[float * pair[T]]
| PString[T] of string * pair[T] => pair[string * pair[T]]

This looks like an ordinary variant except there is an extra term on the RHS which is always the variant with some subscript.

With an ordinary variant of one type variable T the RHS constructor is always the variant type, in this case pair with its universal quantifiers, in this case type variable T.

With a GADT the subscript can be an arbitrary type function of the type variables instead of just T.

Given the above GADT here are some values:

var x1 : pair[unit] = #PUnit[unit];
var x2 : pair[int * pair[unit]] = PInt (1,x1);
var x3 = PFloat (42.33f, x2);

To use a GADT you need to write a generic function:

fun show [W:GENERIC] (x:W):string=
 match x with
 | PUnit => return "PUnit";
 | PInt (head, tail) => return "PInt(" + head.str+", " + ")";
 | PString (head, tail) => return "PString(" + head+", " + ")";
 | PFloat (head, tail) => return "PFloat(" + head.str+", " + ")";

println$ "x3=" +;

The reason for the generic function is that it provide static polymorphic recursion.

GADTs with existentials

A GAD constructor can introduce an extra type variable called an existential variable:

variant pair[T] =
| PUnit of unit => pair[unit]
| Pair[T,U] of U * pair[T] => pair[U * pair[T]]

var x1 = #PUnit[unit];
var x2 = Pair (22,x1);
var x3 = Pair (99.76,x2);

fun f[T:GENERIC] (x:T) = {
  match x with
  | Pair (a,b) => return a.str + ","+b.f;
  | PUnit => return "UNIT";

println$ f x3;

The advantage of this pair over the previous one is that it works for any type U, not just int, string or float. This GADT is actually defining a tuple recursively.

The function which analyses the GADT must be generic since the decoder requires polymorphic recursion. Note in particular the type of b on which f is called is not the same as x.

Another Example

This example is from Wikipedia:

variant Expr[T] =
  | EBool of bool => Expr[bool]
  | EInt of int => Expr[int]
  | EEqual of Expr[int] * Expr[int] => Expr[bool]

fun eval(e) => match e with
  | EBool a => a
  | EInt a => a
  | EEqual (a,b) => eval a == eval b

var expr1 = EEqual (EInt 2, EInt 3);
println$ eval expr1;

In this example we have boolean and integer values and an equality operator. The important thing is that equality only works on integers and returns a bool: without GADTs there is no type safe way to express this constraint.