Let us represent a rational number as a pair of integers. If we declare a boxed abstract type rat for values representing rational numbers, then each value of the type rat is stored in heap-allocated memory, which can only be reclaimed through garbage collection (GC). Instead, we follow an alternative approach by declaring rat as an unboxed abstract type. Therefore, a declaration like the following one is expected:

The problem with this declaration is that it is too abstract. As there is not information given about the size of the type rat, the ATS compiler does not even know how much memory is needed for storing a value of the type rat. However, the programmer should not assume that such a form of declaration is useless. There are realistic circumstances where a declaration of this form can be of great importance, and this is a topic I will cover elsewhere. For now, let us declare an unboxed abstract type as follows: This declaration simply informs the ATS compiler that the representation for values of the type rat is the same as the one for values of the type (int, int). However, this information is not made available to the typechecker of ATS. In particular, if a value of the type rat is treated as a pair of integers in a program, then a type-error will surely occur.The following code is contained in a file named
`ratmod.sats`, which is available
on-line.

exception Denominator exception DivisionByZero fun rat_make_int_int (p: int, q: int): rat fun ratneg: (rat) -> rat // negation fun ratadd: (rat, rat) -> rat // addition fun ratsub: (rat, rat) -> rat // subtraction fun ratmul: (rat, rat) -> rat // multiplication fun ratdiv: (rat, rat) -> rat // division

implement rat_make_int_int (p, q) = let fun make ( p: int, q: int ) : rat = let val r = gcd (p, q) in (p / r, q / r) end // end of [make] // val () = if q = 0 then $raise Denominator // in if q > 0 then make (p, q) else make (~p, ~q) end // end of [rat_make_int_int]

implement ratneg (x) = (~x.0, x.1) implement ratadd (x, y) = rat_make_int_int (x.0 * y.1 + x.1 * y.0, x.1 * y.1) // end of [ratadd] implement ratsub (x, y) = rat_make_int_int (x.0 * y.1 - x.1 * y.0, x.1 * y.1) // end of [ratsub] implement ratmul (x, y) = rat_make_int_int (x.0 * y.0, x.1 * y.1) implement ratdiv (x, y) = ( if y.0 > 0 then rat_make_int_int (x.0 * y.1, x.1 * y.0) else $raise DivisionByZero() // end of [if] ) (* end of [ratdiv] *)