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Prologue

What the hell is this? It's a tiny logic programming engine!

What is logic programming, you ask? Logic programming is a largely underrated paradigm that radically changes the way you write, think about, and run, programs.

Imagine your program as a bunch of relations. You can relate things to each other. Every time you would use an uni-directional assignment =, that assignment now becomes a bi-directional relation ==. It goes both ways. The program forms a chain of relations from one or more inputs to one or more outputs.

You can introduce logic variables that are unbound values (using fresh). These variables obey the constraints imposed on them through relations. This allows you to provide a fresh logic variable and see what it gets bound to. That's what you generally do to get an output from a logic program.

Through the use of conjunction ("and") and disjunction ("or"), you can form logical relations. This allows you to encode different possible flows of execution. The higher-level method for this is conde, which is a disjunction of conjunctions.

All of these logical relations form a tree. The execution of a program corresponds to a breadth-first search through the tree that unifies the provided arguments in accordance to the relations. This means that a program can discover more than one solution, or no solution at all.

This radical way of thinking about and writing programs allows for something very amazing: You can run your programs backwards.

What this means is that you can either give a program inputs and search for a corresponding output, but you can also provide outputs and ask for corresponding inputs.

Examples

eq

var_dump(run_star(function ($q) {
    return eq($q, 'corn');
}));

// => ['corn']

conde

var_dump(run_star(function ($q) {
    return conde([
        [eq($q, 'tea')],
        [eq($q, 'cup')],
    ]);
}));

// => ['tea', 'cup']

firsto

var_dump(run_star(function ($q) {
    return firsto(['a', 'c', 'o', 'r', 'n'], $q);
}));

// => ['a']

resto

var_dump(run_star(function ($q) {
    return resto(['a', 'c', 'o', 'r', 'n'], $q);
}));

// => [['c', 'o', 'r', 'n']]

all

var_dump(run_star(function ($q) {
    return all([
        firsto(['a', 'l'], $q),
        firsto(['a', 'x'], $q),
        firsto(['a', 'z'], $q),
    ]);
}));

// => ['a']

fresh

var_dump(run_star(function ($q) {
    return fresh(function ($x) use ($q) {
        return all([
            eq(['d', 'a', $x, 'c'], $q),
            conso($x, ['a', $x, 'c'], $q),
        ]);
    });
}));

// => ['d', 'a', 'd', 'c']

membero

function membero($x, $l) {
    return conde([
        [fresh(function ($d) use ($x, $l) {
            return conso($x, $d, $l);
         })],
        [fresh(function ($a, $d) use ($x, $l) {
            return all([
                conso($a, $d, $l),
                membero($x, $d),
            ]);
         })],
    ]);
}

var_dump(run_star(function ($q) {
    return all([
        membero($q, [1, 2, 3]),
        membero($q, [2, 3, 4]),
    ]);
}));

// => [2, 3]

run

var_dump(run(3, function ($q) {
    return membero('tofu', $q);
}));

// => [['tofu', '.', '_.0']
//     ['_.0', 'tofu', '.', '_.1']
//     ['_.0', '_.1', 'tofu', '.', '_.2']]

appendo

var_dump(run_star(function ($q) {
    return appendo([1, 2, 3], [4, 5, 6], $q);
}));

// => [[1, 2, 3, 4, 5, 6]]

neq (disequality)

var_dump(run_star(function ($q) {
    return all([
        membero($q, [1, 2, 3]),
        neq($q, 2),
    ]);
}));

// => [1, 3]

rembero

function rembero($x, $l, $out) {
    return conde([
        [eq([], $l), eq([], $out)],
        [fresh(function ($a, $d) use ($x, $l, $out) {
            return all([
                eq(pair($a, $d), $l),
                eq($a, $x),
                eq($d, $out),
            ]);
         })],
        [fresh(function ($a, $d, $res) use ($x, $l, $out) {
            return all([
                eq(pair($a, $d), $l),
                neq($a, $x),
                eq(pair($a, $res), $out),
                rembero($x, $d, $res),
            ]);
         })],
    ]);
}

var_dump(run_star(function ($q) {
    return rembero('b', ['a', 'b', 'c', 'b', 'd'], $q);
}));

// => [['a', 'c', 'b', 'd']]

igorw / reasoned

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