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aphyr/minikanren.pl

Created Oct 12, 2020
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Minikanren in Lisp in Prolog
:- use_module(library(pairs)).
:- use_module(library(reif)).
not_in_list(K, L) :-
if_((L = []),
true,
([X | More] = L,
dif(K, X),
not_in_list(K, More))).
not_in_env(K, Env) :-
/* \+ get_dict(K, Env, _) */
dict_pairs(Env, _, Pairs),
pairs_keys(Pairs, Keys),
not_in_list(K, Keys).
/* Binds a single binding form to a value. Symbols are assigned directly. */
bind(K, V, Env, Env2) :-
atom(K),
Env2 = Env.put(K, V).
/* Vectors are recursively destructured. &(V) denotes varargs. */
bind([], _, Env, Env).
bind([K|Ks], Vs, Env, Env3) :-
/* Varargs */
(K = &(K2),
bind(K2, Vs, Env, Env3), !) ;
/* Out of values */
([] = Vs,
bind(K, nil, Env, Env2),
bind(Ks, [], Env2, Env3));
/* Standard vector of bindings: bind first and recur */
([V1|V1s] = Vs,
bind(K, V1, Env, Env2),
bind(Ks, V1s, Env2, Env3)).
/* Self-evaluating literals */
eval_lit(Exp, _Env, Exp) :-
Exp = [] ;
Exp = nil ;
Exp = true ;
string(Exp) ;
number(Exp).
/* a bare variable x which is in scope */
eval_variable(Var, Env, R) :-
atom(Var),
get_dict(Var, Env, R).
/* Evaluates all elements of list */
eval_each([], _, []).
eval_each(Exp, Env, R) :-
[X | More] = Exp,
[XR | MoreR] = R,
eval_exp(X, Env, XR),
eval_each(More, Env, MoreR).
/* Binds vars to args in env, evaluates body, yielding R. */
call_closure(Vars, Args, Body, Env, R) :-
bind(Vars, Args, Env, Env2),
eval_exp(Body, Env2, R).
/* [f, a, b] */
eval_application([Fun | Args], Env, R) :-
dif(Fun, quote),
/* format("Trying to apply ~w to ~w~n", [Fun, Args]), */
/* Evaluate operator in env, yielding a closure */
eval_uexp(Fun, Env, λ(Type, Vars, Body, Env2)),
/* format("Evaluated fun.~n", []), */
/* Depending on the type of the closure... */
((Type = lambda,
/* Lambdas evaluate their arguments */
eval_each(Args, Env, ArgsR),
/* format("args evaluated~n", []), */
/* And invoke their body with those evaluated args. */
call_closure(Vars, ArgsR, Body, Env2, R), !) ;
(Type = macro,
/* Macros receive unevaluated arguments, and invoke their body to generate
an expression */
call_closure(Vars, Args, Body, Env2, Code),
/* Which is then evaluated */
eval_exp(Code, Env, R))).
/* [lambda [a b] body1 body2] or [macro ...]*/
eval_lambda([Type, Args | Body], Env, R) :-
/* Must be either macro or lambda, and not shadowed */
((Type = lambda, not_in_env(lambda, Env)) ;
(Type = macro, not_in_env(macro, Env))),
/* Construct a closure with the type, argument, body, and env. The
environment includes a `recur` target which is bound to the closure itself.
HOLY SHIT, you can just DO this and Prolog will LET you */
R = λ(Type, Args, [do | Body], Env.put(recur, R)).
eval_do([do], _Env, nil).
eval_do([do, X], Env, R) :-
eval_exp(X, Env, R).
eval_do([do, X1, X2 | More], Env, R) :-
eval_exp(X1, Env, _),
eval_do([do, X2 | More], Env, R).
/* [let [a val b val] body1 body2 ...] */
eval_let([let, [] | Body], Env, R) :-
eval_do([do | Body], Env, R).
eval_let([let, [K, V | More] | Body], Env, R) :-
eval_exp(V, Env, VR),
bind(K, VR, Env, Env2),
eval_let([let, More | Body], Env2, R).
/* [cond test branch test branch default] */
eval_cond([cond], _Env, nil).
eval_cond([cond, Default], Env, R) :-
eval_exp(Default, Env, R).
eval_cond([cond, Test, Branch | More], Env, R) :-
eval_uexp(Test, Env, TestR),
if_((nil = TestR),
eval_cond([cond | More], Env, R),
eval_exp(Branch, Env, R)).
/* [quote x] */
eval_quote([quote, R], Env, R) :-
/* Ensure quote isn't shadowed */
not_in_env(quote, Env).
/* M-M-M-METACIRCULARRRRR */
eval_eval([eval, X], Env, R) :-
eval_exp(X, Env, XR),
eval_exp(XR, Env, R).
/* Matches a typebox */
is_typed(τ(_,_)).
/* Every compound other than a lambda or typebox is a struct. */
is_struct(X) :-
compound(X),
compound_name_arity(X, T, _),
dif(T, λ),
dif(T, τ),
dif(T, '[|]').
/* Relational version */
is_typed_or_struct(X, R) :-
(is_typed(X) ; is_struct(X)), R = true , ! ;
R = false.
type(X, R) :-
(atomic(X),
([] = X , R = list) ;
(atom(X), R = atom) ;
(number(X), R = number) ;
(string(X), R = string)), ! ;
(compound(X),
((functor(X, '[|]', _), R = list), ! ;
(compound_name_arity(X, λ, _), R = function), ! ;
(compound_name_arguments(X, τ, [Type, _]), R = Type), ! ;
(compound_name_arity(X, Type, _), R = Type))).
/* True if X is of type Type; explodes with an error otherwise. */
typecheck(Type, X) :-
type(X, Type), ! ;
(format("Type error: expected ~p, received ~p~n",
[Type, X]),
false).
/* Unwraps type boxes */
untyped(X, R) :-
(X = τ(_, Value),
R = Value) ;
(dif(X, τ(_, _)),
R = X).
/* With one arg, type gets the type of an expression. */
eval_type([type, X], Env, R) :-
not_in_env(type, Env),
eval_exp(X, Env, XR),
type(XR, R).
/* This is the worst hack. (type cat x) serves both as a type-returning
* introspection, and also as a type assertion, both at runtime. If x is an
* untyped object, the type call succeeds and constructs a transparent type
* wrapper typed(name, X). This wrapper is transparent in that it is unwrapped
* within eval_exp--except, of course, for calls to eval_type!--most language
* features don't see the type-box's existence.
*
* When calling (type cat x) with a type-boxed x, this serves as a type
* *assertion*: x is returned unchanged iff its type is cat; otherwise, this
* explodes with a type error. */
eval_type([type, Name, X], Env, R) :-
not_in_env(type, Env),
eval_exp(X, Env, XR),
/* Type boxes and structs must match type Name; otherwise, wrap them in a
* type box. */
is_typed_or_struct(XR, Typed),
if_(Typed = true,
(typecheck(Name, XR),
R = XR),
/* Construct type box around untyped value. */
(R = τ(Name, XR))).
/* An explicit typecheck: like type, but does not promote untyped values to
* typed ones. Note that you can [check list []] transparently, but [type list []] introduces a type wrapper. */
eval_type([check, Name, X], Env, R) :-
not_in_env(check, Env),
eval_exp(X, Env, R),
typecheck(Name, R).
/* Used to unbox typed objects, allowing you to write map, filter, etc. lmao,
* not even 2 hours and already I need type polymorphism. */
eval_type([untype, X], Env, R) :-
not_in_env(untype, Env),
eval_exp(X, Env, XR),
untyped(XR, R).
/* [list x y] */
eval_list([list | Args], Env, R) :-
/* Ensure it's not shadowed */
not_in_env(list, Env),
/* And evaluate */
eval_each(Args, Env, R).
list_sense(X, R) :-
if_((nil = X), R = [], R = X).
eval_first([first, List], Env, R) :-
not_in_env(first, Env),
eval_uexp(List, Env, ListR),
if_(([] = ListR),
R = nil,
[R|_] = ListR).
eval_rest([rest, List], Env, R) :-
not_in_env(rest, Env),
eval_uexp(List, Env, ListR),
((ListR = [], R = nil) ;
(ListR = [_|R])).
eval_cons([cons, Head, Tail], Env, R) :-
not_in_env(cons, Env),
eval_exp(Head, Env, HeadR),
eval_exp(Tail, Env, TailR),
list_sense(TailR, TailL),
R = [HeadR | TailL].
eval_eq([eq, A, B], Env, R) :-
not_in_env(eq, Env),
eval_exp(A, Env, ARes),
eval_exp(B, Env, BRes),
if_((ARes = BRes), R = true, R = nil).
eval_plus([plus, A, B], Env, R) :-
eval_uexp(A, Env, AR),
eval_uexp(B, Env, BR),
R is AR + BR.
prn_helper([]) :-
format("~n").
prn_helper([X | Xs]) :-
format("~p ", [X]),
prn_helper(Xs).
eval_prn([prn | Args], Env, nil) :-
not_in_env(prn, Env),
eval_each(Args, Env, ArgsR),
prn_helper(ArgsR).
/* We use functors for structs */
eval_struct([struct, Type | Fields], Env, R) :-
not_in_env(struct, Env),
eval_each(Fields, Env, FieldsR),
compound_name_arguments(R, Type, FieldsR).
eval_struct([destruct, Type, Struct], Env, R) :-
not_in_env(destruct, Env),
eval_uexp(Struct, Env, StructR),
((atomic(StructR),
format("Can't destruct atom ~w~n", [StructR]),
false) ;
(compound(StructR),
compound_name_arguments(StructR, Type, R))).
eval_gensym([gensym, Prefix], _Env, R) :-
atom_concat(Prefix, '__auto__', Sym),
gensym(Sym, R).
/* Symbol concatenation */
eval_symcat([symcat, A, B], Env, R) :-
eval_exp(A, Env, AR),
eval_exp(B, Env, BR),
atom_concat(AR, BR, R).
eval_env(env, Env, Env) :-
not_in_env(env, Env).
eval_exp(Exp, Env, R) :-
/* format('eval ~w~n', [Exp]), */
eval_quote(Exp, Env, R), ! ;
eval_lambda(Exp, Env, R), ! ;
eval_do(Exp, Env, R), ! ;
eval_let(Exp, Env, R), ! ;
eval_cond(Exp, Env, R), ! ;
eval_eq(Exp, Env, R), ! ;
eval_list(Exp, Env, R), ! ;
eval_cons(Exp, Env, R), ! ;
eval_first(Exp, Env, R), ! ;
eval_rest(Exp, Env, R), ! ;
eval_plus(Exp, Env, R), ! ;
eval_struct(Exp, Env, R), ! ;
eval_prn(Exp, Env, R), ! ;
eval_env(Exp, Env, R), ! ;
eval_gensym(Exp, Env, R), ! ;
eval_symcat(Exp, Env, R), ! ;
eval_type(Exp, Env, R), ! ;
eval_eval(Exp, Env, R), ! ;
eval_lit(Exp, Env, R), ! ;
eval_variable(Exp, Env, R), ! ;
eval_application(Exp, Env, R), ! ;
(format('unable to eval ~p~n', [Exp]),
false).
/* Like eval_exp, but unwraps type boxes. This is where we ground out for
* prolog interop */
eval_uexp(Exp, Env, R) :-
eval_exp(Exp, Env, Typed),
untyped(Typed, R).
prologue([
/* Booleans ************************************************************/
and, [macro, [a, b],
[list, [quote, cond], a, b]],
or, [macro, [a, b],
[let, [a_, [gensym, a]],
[list, [quote, let],
[list, a_, a],
[cond, a_, a_, b]]]],
not, [lambda, [x], [cond, x, nil, true]],
/* Predicates ********************************************************/
is_null, [lambda, [x],
[let, [x, [untype, x]],
[or, [eq, x, []],
[eq, x, nil]]]],
is_empty, [lambda, [coll], [eq, [], [untype, coll]]],
is_list, [lambda, [x],
[eq, [type, [untype, x]], [quote, list]]],
is_pair, [lambda, [x],
[and, [not, [is_empty, x]],
[is_list, x]]],
is_fn, [lambda, [x],
[eq, [type, [untype, x]], [quote, function]]],
/* Basic sequence operations, polymorphic over all types. */
second, [lambda, [coll], [first, [rest, coll]]],
third, [lambda, [coll], [first, [rest, [rest, coll]]]],
count, [lambda, [coll],
[cond, [is_empty, coll],
0,
[plus, 1, [recur, [rest, coll]]]]],
map, [lambda, [f, coll],
[cond, [is_empty, coll],
[],
[cons, [f, [first, coll]],
[recur, f, [rest, coll]]]]],
filter, [lambda, [f, coll],
[cond, [is_empty, coll],
[],
[let, [x, [first, coll],
xs, [rest, coll]],
[cond, [f, x],
[cons, x, [recur, f, xs]],
[recur, f, xs]]]]],
fold, [lambda, [f, init, coll],
[cond, [is_empty, coll], init,
[recur, f, [f, init, [first, coll]], [rest, coll]]]],
rev, [lambda, [coll],
[fold, [lambda, [list, elem], [cons, elem, list]], [], coll]],
concat, [lambda, [as, bs],
[fold, [lambda, [res, a],
[cons, a, res]],
bs,
[rev, as]]],
/* Control flow *****************************************************/
/* Passes through nils, typechecks otherwise. */
check_option, [macro, [type, expr],
[let, [res_, [gensym, res]],
[list, [quote, let], [list, res_, expr],
[list, [quote, cond], [list, [quote, eq], nil, res_],
nil,
[list, [quote, check], type, res_]]]]],
assert, [macro, [expr, message],
[list, [quote, cond],
expr,
true,
[list, [quote, do],
[list, [quote, prn], "Assert failed:",
[list, [quote, quote], expr],
message],
[quote, throw]]]],
/* Apply a function to every node of a tree (list), preorder application,
* recursively */
treemap, [lambda, [f, x],
[let, [treemap, recur,
x2, [f, x]],
[cond, [is_pair, x2],
[cons, [f, [first, x2]], [recur, f, [rest, x2]]],
x2]]],
/* Utilities ***********************************************************/
/* Syntax-quote takes an expression and quotes it, except for [unquote, x]
* forms, which unquotes a single term. */
syntax_quote_fn, [lambda, [expr],
[let, [q, [quote, quote],
sq, recur],
[cond, [is_empty, expr], expr,
[is_list, expr],
[let, [f, [first, expr]],
[cond, /* Unquote means *don't* descend into this term. */
[eq, f, [quote, unquote]],
[second, expr],
/* Otherwise, recur into term and build up a list expr. */
[cons, [quote, list],
[map, sq, expr]]]],
/* Non-list terminals are all quoted. */
[list, q, expr]]]],
syntax_quote, [macro, [expr], [syntax_quote_fn, expr]],
syntax_quote_test, [macro, [x],
[syntax_quote, [prn, [list, 1], [unquote, x]]]],
/* Rewrites closures to be more readable. */
unfn, [lambda, [x],
[first,
[treemap, [lambda, [x],
/* Print functions without envs */
[cond, [is_fn, x],
[let, [[type, args, [do, &(body)]],
[destruct, λ, x]],
[struct, λ, args, body]],
x]],
[list, x]]]],
/* MICROKANREN ************************************************************/
/* Logic vars are their own struct type. We do this for safety--it's too easy
* to accidentally destructure/cons/etc lvars otherwise. */
lvar, [lambda, [num], [struct, lvar, num]],
is_lvar, [lambda, [x], [eq, [quote, lvar], [type, x]]],
lvar_num, [lambda, [x], [first, [destruct, foo, x]]],
/* We represent a binding as a struct as well. */
binding, [lambda, [var, val],
[struct, binding, var, val]],
is_binding, [lambda, [x], [eq, [type, x], [quote, binding]]],
bvar, [lambda, [b], [first, [destruct, binding, b]]],
bval, [lambda, [b], [second, [destruct, binding, b]]],
/* A substitution map is a list of bindings */
empty_subs, [type, subs, []],
/* Returns the binding of lvar num in substitution subs */
lvar_binding, [lambda, [num, subs],
[first, [filter, [lambda, [x],
[eq, num, [first, x]]],
subs]]],
/* A state is a substitution paired with a fresh variable counter. */
state, [lambda, [subs, counter],
[check, subs, subs],
[struct, state, subs, counter]],
is_state, [lambda, [x], [eq, [type, x], [quote, state]]],
empty_state, [state, empty_subs, 0],
st_subs, [lambda, [s], [check, subs, [first, [destruct, state, s]]]],
st_counter, [lambda, [s], [second, [destruct, state, s]]],
/* Search for the value of u in a given substitution. */
walk, [lambda, [u, subs],
[check, subs, subs],
[let, [pr, [and, [is_lvar, u],
[first, [filter, [lambda, [v],
/* u is equal to the binding's lvar */
[eq, u, [bvar, v]]],
subs]]],
res, [cond, pr,
[recur, [bval, pr], subs],
u]],
/* [prn, "walk", u, [quote, '->'], res, "in", subs], */
res]],
/* Adds a binding of x to v to the given substitution. */
ext_s, [lambda, [x, v, subs],
[check, subs, subs],
[type, subs, [cons, [binding, x, v], subs]]],
/* Unifies u with v in a substitution */
unify, [lambda, [u, v, subs],
[check, subs, subs],
/* Look up u and v in the substitution map */
[check_option, subs,
[let, [u, [walk, u, subs],
v, [walk, v, subs]],
/* If both are equal, return subs unchanged. This covers both normal
* values and lvars. */
[cond, [eq, u, v], subs,
/* If we have one lvar, and something else (either a value or a
* different lvar) extend the substitution with that
* association. */
[is_lvar, u], [ext_s, u, v, subs],
[is_lvar, v], [ext_s, v, u, subs],
/* If both are bindings, unify their vars and vals. The Scheme
* implementation gets around this by encoding bindings as
* pairs. TODO: when we're done being type-safe, go back to
* that. */
[and, [is_binding, u], [is_binding, v]],
[let, [subs, [recur, [bvar, u], [bvar, v], subs]],
[and, subs, [recur, [bval, u], [bval, v]], subs]],
/* We unify lists recursively. Note that this works for improper
* lists too! */
[and, [is_pair, u], [is_pair, v]],
[let, [subs, [recur, [first, u], [first, v], subs]],
[and, subs, [recur, [rest, u], [rest, v], subs]]]]]]],
/* M'zero, the empty stream */
mzero, [type, stream, []],
unit, [lambda, [state],
[assert, [is_state, state], "unit"],
[type, stream, [cons, state, mzero]]],
/* A goal constructor which asserts u and v are equal */
eql, [lambda, [u, v],
[type, goal,
[lambda, [st],
[assert, [is_state, st], "eql"],
[check, stream,
[let, [subs, [unify, u, v, [st_subs, st]]],
[cond, subs,
[unit, [state, subs, [st_counter, st]]],
mzero]]]]]],
≡, eql,
/* A goal which introduces a new logical variable. Takes a function [f, lvar]
which yields a goal, and constructs a goal using it. The constructed goal
takes a state sc, derives a goal with a fresh logic variable in that state,
and passes the (evolved) state to the goal that f yields. */
call_fresh, [lambda, [f],
[type, goal,
[lambda, [st],
[assert, [is_state, st], "call_fresh"],
/* Construct a new logic variable and call f with it, yielding a goal
* function. That goal function gets called with our state, evolved to
* account for the new variable. */
[check, stream,
[let, [c, [st_counter, st]],
[[check, goal, [f, [lvar, c]]],
[state, [st_subs, st], [plus, 1, c]]]]]]]],
/* Merges two alternative streams (i.e. representing the two forks of a
* disjunction) together. */
mplus, [lambda, [a, b],
[check, stream, a],
[check, stream, b],
[check, stream,
[let, [mplus, recur],
[cond, /* When one stream is exhausted, yield the other. */
[is_null, a], b,
/* A fn is a lazy stream; if given one, we yield our own lazy
* stream which, when evaluated, tries the opposite order: b,
* then the concrete stream [a]. Flipping a and b is what lets
* us trade off between streams, ensuring no single fork
* dominates the search. */
[is_fn, a], [type, stream, [lambda, [], [mplus, b, [a]]]],
/* a must be a concrete non-empty list. We yield its first
* state, followed by the disjunction of later states of a with
* b. */
[type, stream,
[cons, [first, a], [mplus, [rest, a], b]]]]]]],
/* Bind applies a goal (a function of states to streams) over a stream. */
bind, [lambda, [stream, goal],
[check, stream, stream],
[check, goal, goal],
[check, stream,
[let, [bind, recur],
[cond, /* If the stream is empty, we return the empty stream. */
[is_null, stream], mzero,
/* If the stream is lazy, we construct a lazy stream. */
[is_fn, stream], [type, stream,
[lambda, [], [bind, [stream], goal]]],
/* Otherwise, the stream is a concrete, non-empty stream. We
* have two choices now: either apply the goal to the first
* thing in the stream, or mapping the goal over the rest of the
* stream. We express these two options via mplus! */
[mplus, [goal, [first, stream]],
[bind, [type, stream, [rest, stream]], goal]]]]]],
/* Logical disjunction of two goals. */
disj, [lambda, [g1, g2],
[check, goal, g1],
[check, goal, g2],
[type, goal,
[lambda, [state],
[assert, [is_state, state], "disj"],
[check, stream,
[mplus, [g1, state], [g2, state]]]]]],
/* Logical conjunction of two goals */
conj, [lambda, [g1, g2],
[check, goal, g1],
[check, goal, g2],
[type, goal,
[lambda, [state],
[assert, [is_state, state], "conj"],
/* We apply the first goal to the state, yielding a stream. We then map
* the second goal over that stream, ensuring both goals hold. */
[check, stream,
[bind, [g1, state], g2]]]]],
/* Basic uKanren examples */
fives, [call_fresh, [lambda, [x], [≡, x, 5]]],
sixes, [call_fresh, [lambda, [x], [≡, x, 6]]],
either, [call_fresh, [lambda, [x], [disj, [≡, x, 3],
[≡, x, 4]]]],
alts, [conj, [call_fresh, [lambda, [x], [≡, x, 3]]],
/* This is a different logic variable! */
[call_fresh, [lambda, [x], [disj, [≡, x, 5], [≡, x, 6]]]]],
/* Evals until realized */
pull, [lambda, [stream],
[check, stream, stream],
[cond, [is_fn, stream],
[recur, [stream]],
stream]],
/* Limited take */
take, [lambda, [n, stream],
[cond, [eq, 0, n],
[],
[let, [stream, [pull, stream],
state, [first, stream]],
[cond, [is_fn, state],
[recur, n, state],
state,
[do, [assert, [is_state, state], "take"],
[cons, state, [recur, [plus, n, -1], [rest, stream]]]],
/* Out of elements */
[]]]]],
/* A basic function which takes n solutions from the goal on the empty state
* */
run_raw, [lambda, [n, goal],
[take, n, [goal, empty_state]]],
/* Reifier **************************************************************/
/* walkr reifies a term by replacing logic variables in it with their
* corresponding values in some substitution. */
walkr, [lambda, [v, subs],
/* Look up v in subs */
[let, [v, [walk, v, subs],
/* A little mutually-recursive helper to simplify traversal */
walkr, recur,
w, [lambda, [v2], [walkr, v2, subs]]],
[cond, /* If we have a logic variable, that's all we can do. */
[is_lvar, v], v,
/* If we have a binding, walk its var and val, and construct a new
* binding from that. TODO: if we use lists for bindings, this
* goes away too. */
[is_binding, v], [binding, [w, [bvar, v]], [w, [bval, v]]],
/* Walk lists recursively. */
[is_pair, v], [cons, [w, [first, v]],
[w, [rest, v]]],
/* A ground term */
v]]],
/* This is how we construct names for logic variables */
reify_name, [lambda, [n],
[symcat, [quote, '_'], n]],
/* Reifies a substitution map, by extending subs with extra entries for any
* unknowns in v. */
reify_s, [lambda, [v, subs],
/* Look up v in subs */
[let, [v, [walk, v, subs]],
[cond, /* If we have a logic variable, extend the substitution by binding
v to a new logic var named after the size of the substitution. */
[is_lvar, v],
[let, [n, [reify_name, [count, subs]]],
[ext_s, v, n, subs]],
/* If we have a binding, extend the substitution (recursively)
* with this variable name and value. */
[is_binding, v],
[recur, [bval, v], [recur, [bvar, v], subs]],
/* Recur through pairs */
[is_pair, v],
[recur, [rest, v], [recur, [first, v], subs]],
/* Ground out */
subs]]],
reify_state_first_var, [lambda, [state],
/* [prn],
[prn, "state", state], */
/* Look up the first logic variable in this state's substitution. */
[let, [v, [walkr, [lvar, 0], [st_subs, state]]],
/* Then reify the substitution for v, starting with an empty
* substitution, and walk v in *that*. Why? No clue. */
[prn, "v", v, "subs", [reify_s, v, empty_subs]],
[walkr, v, [reify_s, v, empty_subs]]]],
/* Reifies a state with respect to the first n vars */
reify_state_vars, [lambda, [num_vars, state],
/* [prn],
[prn, "state", state], */
[let, [subs, [st_subs, state]],
[[lambda, [n, r],
[cond, [eq, -1, n],
/* Done */
r,
/* Resolve the nth variable */
[let, [v, [walkr, [lvar, n], subs]],
/* And resolve that again... */
[recur, [plus, n, -1],
[cons, [walkr, v, [reify_s, v, empty_subs]],
r]]]]],
/* The first variable to resolve, and the list of our resolutions */
[plus, num_vars, -1],
[]]]],
/* Reifies states with respect to the first n vars. */
mk_reify, [lambda, [num_vars, states],
[prn],
[prn, "REIFY =============="],
[map, [lambda, [state],
[reify_state_vars, num_vars, state]],
states]],
/* Shortcut for running goals. Takes a number of solutions to find, a list of
* variable names which will be bound in the given goal, and reified when
* returned. */
run, [macro, [n, vars, &(goals)],
[syntax_quote,
[mk_reify, [unquote, [count, vars]],
[run_raw, [unquote, n],
[unquote,
[cons, [quote, fresh],
[cons, vars, goals]]]]]]],
/* Syntax! **************************************************************/
/* This defers evaluation of a goal via inverse-eta-delay */
zzz, [macro, [g],
[let, [state_, [gensym, state]],
[syntax_quote,
[type, goal,
[lambda, [[unquote, state_]],
[assert, [is_state, [unquote, state_]],
[list, "zzz", [unquote, state_]]],
[type, stream,
[lambda, [],
[check, stream,
[[unquote, g], [unquote, state_]]]]]]]]]],
/* conj or disj of multiple goals */
conjall, [macro, [&(gs)],
[let, [[g, &(gs)], [rev, gs],
code, [fold, [lambda, [form, goal],
[list, [quote, conj],
[list, [quote, zzz], goal],
form]],
/* Infinite loops? */
[list, [quote, zzz], g],
gs]],
code]],
disjall, [macro, [&(gs)],
[let, [[g, &(gs)], [rev, gs],
code, [fold, [lambda, [form, goal],
[list, [quote, disj],
[list, [quote, zzz], goal],
form]],
g,
gs]],
code]],
/* A disjunction of conjunctions */
conde, [macro, [&(clauses)],
[let, [code, [cons, [quote, disjall],
[map, [lambda, [goals],
[cons, [quote, conjall], goals]],
clauses]]],
code]],
fresh, [macro, [vars, &(goals)],
[let, [code, [fold, [lambda, [form, var],
/* Wrap in [call_fresh, [lambda, [x], ...]] */
[list, [quote, call_fresh],
[list, [quote, lambda],
[list, var],
[list, [quote, type],
[quote, goal],
form]]]],
/* Start with conjunction of all goals */
[cons, [quote, conjall], goals],
[rev, vars]]],
code]],
/* Sequence goals ****************************************************
*
* Adapted from https://github.com/mullr/micrologic/blob/master/src/micro_logic/sequence.clj */
/* Conso provides an abstraction for (cons head tail) = list. */
conso, [lambda, [hd, tl, lst],
[eql, [cons, hd, tl], lst]],
/* We can assert things about the first and rest of a list now */
firsto, [lambda, [head, list],
[fresh, [tail],
[conso, head, tail, list]]],
resto, [lambda, [tail, list],
[fresh, [head],
[conso, head, tail, list]]],
/* Here's sequence append */
/* From microkanren A Lucid Little Logic Language with a Simple Complete
* Search */
appendo, [lambda, [as, bs, combined],
[let, [appendo, recur],
[conde, [[eql, [], as],
[eql, bs, combined]],
[[fresh, [head, tail, rec],
/* Pull apart as */
[conso, head, tail, as],
/* Pull apart combined */
[conso, head, rec, combined],
/* And we can recur from there */
[appendo, tail, bs, rec]]]]]],
/* Peano numbers */
succ, [lambda, [n], [cons, [quote, 'S'], n]],
n0, [list, 0],
n1, [succ, n0],
n2, [succ, n1],
n3, [succ, n2],
/* Coercions to and from normal numbers */
peano, [lambda, [n],
[cond, [eq, 0, n],
n0,
[succ, [recur, [plus, -1, n]]]]],
unpeano1, [lambda, [n],
[cond, [eq, n, [list, 0]], 0,
[and, [is_pair, n], [eq, [quote, 'S'], [first, n]]],
[let, [m, [recur, [rest, n]]],
[cond, [eq, [type, m], [quote, number]],
[plus, 1, m],
[struct, 'S', m]]],
n]],
unpeano, [lambda, [x], [treemap, unpeano1, x]],
/* Successor and addition relations */
succo, [lambda, [n, next], [eql, [succ, n], next]],
pluso, [lambda, [a, b, sum],
[let, [pluso, recur],
[conde, [[eql, a, n0], [eql, b, sum]],
[[fresh, [a_, sum_],
[succo, a_, a],
[succo, sum_, sum],
[pluso, a_, b, sum_]]]]]],
lesso, [lambda, [lesser, greater],
[fresh, [n], [pluso, lesser, [succ, n], greater]]],
/* Maximum */
maxo, [lambda, [a, b, max],
[conde, [[eql, a, b], [eql, max, a]],
[[lesso, a, b], [eql, max, b]],
[[lesso, b, a], [eql, max, a]]]],
/* Ensures a and b are within 1 of each other. */
approxo, [lambda, [a, b],
[conde, [[eql, a, b]],
[[succo, a, b]],
[[succo, b, a]]]],
/* Construct a leaf node */
leaf, [lambda, [x],
[list, [quote, l], x]],
/* Construct a branch node with left and right parts */
branch, [lambda, [left, right],
[list, [quote, b], left, right]],
/* Converts a compact notation [l,r] to explicit leaf/branch */
tree, [lambda, [t],
[cond, [is_list, t],
[let, [[left, right], t],
[branch, [recur, left], [recur, right]]],
[leaf, t]]],
/* Maps explicit leaf/branch back to compact lists. */
untree, [lambda, [[type, a, b]],
[cond, [eq, type, [quote, b]], [list, [recur, a], [recur, b]],
[eq, type, [quote, l]], a]],
/* Rotates between two trees: [a, [b, c]] <-> [[a, b], c]*/
rot_lefto, [lambda, [t1, t2],
[fresh, [a, b, c],
[eql, t1, [branch, a, [branch, b, c]]],
[eql, t2, [branch, [branch, a, b], c]]]],
/* Left and right rotations */
rot_botho, [lambda, [t1, t2],
[conde, [[rot_lefto, t1, t2]],
[[rot_lefto, t2, t1]]]],
/* All possible rotations of a node at any depth between two trees. */
roto, [lambda, [t1, t2],
[let, [roto, recur],
/* Rotate this node */
[conde, [[rot_botho, t1, t2]],
/* Break apart this tree as a branch */
[[fresh, [t1l, t1r, t2l, t2r],
[eql, t1, [branch, t1l, t1r]],
[eql, t2, [branch, t2l, t2r]],
[conde,
/* And rotate left branch */
[[eql, t1r, t2r], [roto, t1l, t2l]],
/* Or right branch*/
[[eql, t1l, t2l], [roto, t1r, t2r]]]]]]]],
/* Any number of rotations at any level */
rotso, [lambda, [t1, t2],
[let, [rotso, recur],
[conde, [[eql, t1, t2]],
[[fresh, [t],
[roto, t1, t],
[rotso, t, t2]]]]]],
/* Height of a tree */
heighto, [lambda, [t, h],
[let, [heighto, recur],
[conde, /* Leaf */
[[fresh, [x],
[eql, t, [leaf, x]],
[eql, h, n0]]],
/* Branch */
[[fresh, [left, right, left_height, right_height, max_height],
[eql, t, [branch, left, right]],
[heighto, left, left_height],
[heighto, right, right_height],
[maxo, left_height, right_height, max_height],
[succo, max_height, h]]]]]],
/* Balanced tress have approximately equal heights at every level. Oh god,
* this is SO expensive. */
balancedo, [lambda, [t, h],
[let, [balancedo, recur],
[conde, /* Leaf nodes are trivially balanced */
[[fresh, [x], [eql, t, [leaf, x]]]],
/* Branch nodes are balanced if their left and right heights are
* approximately equal, and both left and right nodes are
* themselves balanced. */
[[fresh, [left, right, left_height, right_height],
[eql, t, [branch, left, right]],
[balancedo, left],
[balancedo, right],
[heighto, left, left_height],
[heighto, right, right_height],
[approxo, left_height, right_height]]]]]],
/* Takes any tree and finds an equivalent (via rotation) balanced tree. */
balanceo, [lambda, [tree, balanced],
[conde, [[rotso, tree, balanced],
[balancedo, balanced]]]]
/* Look up a variable in an association list */
lookupo, [lambda, [var, env, r],
[let, [lookupo, recur],
[fresh, [k, v, more],
[eql, env, [cons, [cons, k, v], more]],
/* Found it! */
[conde, [[eql, k, var],
[eql, v, r]],
/* Keep looking */
[[lookupo, var, more, r]]]]]],
/* How many symbols do you really need */
symbolo, [lambda, [var],
[conde, [[eql, var, [quote, f]]],
[[eql, var, [quote, x]]]]],
evalo, [lambda, [exp, env, r],
[let, [evalo, recur],
[conde,
/* Application */
[[fresh, [f, arg, var, body, env2, argr],
/* Break up [f arg] */
[eql, exp, [list, f, arg]],
/* Evaluate f to a closure */
[evalo, f, env, [list, [quote, λ], var, body, env2]],
/* And eval arg */
[evalo, arg, env, argr],
/* Then evaluate body with arg bound */
[evalo, body, [cons, [cons, var, argr], env2], r]]],
/* Lambda */
[[fresh, [var, body],
[eql, exp, [list, [quote, lambda], [list, var], body]],
[eql, r, [list, [quote, λ], var, body, env]]]],
/* A symbol */
[[symbolo, exp],
[lookupo, exp, env, r]]]]]
]).
demos(conde, [run, 2, [q,w,x,y],
[conde,
[[≡, [list, x, w, x], q],
[≡, y, w]],
[[≡, [list, w, x, w], q],
[≡, y, w]]]]).
demos(conde_anon, [run, 2, [q],
[fresh, [w, x, y],
[conde,
[[≡, [list, x, w, x], q],
[≡, y, w]],
[[≡, [list, w, x, w], q],
[≡, y, w]]]]]).
/* Define a relation which reverses a list. */
demos(nrev, [let, [nrev, [lambda, [l1, l2],
[lambda, [sc],
/* Either both lists are empty, or... */
[[conde, [[≡, l1, []],
[≡, l2, []]],
/* Or we can pull apart l2 */
[fresh, [h, t],
[≡, h, [first, l2]],
[≡, tl, [rest, l2]],
/* And reverse t to r */
[fresh, [r],
[nrev, t, r],
[append, r, [list, h], l2]]]],
sc]]]],
[run, 5, [q],
[nrev, [list], q]]]).
/* Should this work? */
demos(cons, [run, 5, [l, hd, tl],
[≡, [cons, hd, tl], l],
[≡, hd, 1],
[≡, tl, [list,2,3]]]).
/* Balancing trees */
/* ?- demos(balance, Code), eval(Code, R). */
demos(balance,
[map, [lambda, [[x]], [untree, x]], [run, 1, [a], [balanceo, [tree, [quote, [[[1,2],3],4]]], a]]]
).
eval(Exp, R) :-
prologue(P),
eval_exp([let, P, Exp], e{}, R).
@stevana

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@stevana stevana commented Oct 15, 2020

Which compiler are you using? It doesn't seem to work with SWI-Prolog (as suggested in the blog post).

@aphyr

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@aphyr aphyr commented Oct 15, 2020

SWI-Prolog.

@Ferada

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@Ferada Ferada commented Oct 16, 2020

There's a comma missing on line 1010.

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