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License: BSD 3-Clause "New" or "Revised" License
Implementation of type classes
License: BSD 3-Clause "New" or "Revised" License
an example in mlx1:
class<T> Num {
zero :: T,
sub :: T -> T -> T,
neg :: T -> T = sub zero
} in
impl Num for Int {
zero = 0,
sub x y = x - y
} in
neg 1
direct translation (mlx1-like syntax):
type Num = ΛT. constraint numD (T, T -> T -> T, T -> T) in
overload (∀'a. Num 'a) in
let zero = numD#0 in
let sub = numD#1 in
let neg = numD#2 in
instance (Num Int) = (0, sub_int, sub zero) in
neg 1
It is possible (by adding the instance itself to the context in translation of rhs of instance
) to type and translate this example to OCaml, however, the resulting translation cannot be compiled because a value recursion happens in the dictionary for Num Int
.
A possible translation is shown below. Here, numD_37
is occurred in the rhs of its definition.
let zero = (fun numD_22 -> fst3 (numD_22)) in
let sub = (fun numD_29 -> snd3 (numD_29)) in
let neg = (fun numD_36 -> thd3 (numD_36)) in
let numD_37 = (0, sub_int, sub (numD_37) (zero (numD_37))) in
neg (numD_37) (1)
Using lazy evaluation:
let zero = (fun numD_22 -> fst3 (numD_22)) in
let sub = (fun numD_29 -> snd3 (numD_29)) in
let neg = (fun numD_36 -> Lazy.force (thd3 (numD_36))) in
let rec numD_37 = (0, sub_int, lazy (sub (numD_37) (zero (numD_37)))) in
print_int (neg (numD_37) (1))
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