function_reference_dispatch.chr

:- module(function_reference_dispatch, [t/2, type(tags/0)]).
:- use_module(prelude).
:- chr_constraint t/2.
:- chr_type tags ---> unary ; binary ; identity ; via_var ; mixed.

:- function double/1, plus/2, identity/1, apply/2, apply2/3.

double(X)   -> X * 2.
plus(X, Y)  -> X + Y.
identity(X) -> X.

apply(F, X)     -> '$call'(F, X).
apply2(F, X, Y) -> '$call'(F, X, Y).

% Reference to unary function passed via fun name/1.
t(unary, R) <=>
    R is apply(fun double/1, 21).

% Reference to binary function via fun name/2.
t(binary, R) <=>
    R is apply2(fun plus/2, 3, 4).

% identity/1 — first-class function reference returns input.
t(identity, R) <=>
    R is apply(fun identity/1, quote(foo)).

% Reference threaded through a local variable.
t(via_var, R) <=>
    F is fun double/1,
    R is apply(F, 50).

% Reference vs lambda interchangeable: both work as first-class fns.
t(mixed, R) <=>
    R1 is apply(fun double/1, 5),
    R2 is apply(fun(X) -> X * 3 end, 5),
    R = pair(R1, R2).