How to write facts based on other facts

[vitrun]

The following code doesn't work as expected because two invocation of Line('B', 'C') result in different goal functions.

Line = Relation()
LengthOf = Relation()

facts(LengthOf, (Line('B', 'C'), 1))
l = var()
run(1, l, LengthOf(Line('B', 'C'), l))

Though possible, the workaround is not elegant:

g = Line('B', 'C')
facts(LengthOf, (g, 1))
l = var()
run(1, l, LengthOf(g, l))

What's the best practice to state facts based on other facts?

[brandonwillard]

Line(*args) should return a goal object, which is generally a callable that takes miniKanren state objects (i.e. "substitution"/unification mappings) and returns a stream of new states. Those goal objects are chained together using the ldisj and lconj operators (e.g. via the helper functions/aliases conde, lall, land, etc.) and evaluated by run.

Altogether, this means that the expression LengthOf(Line(*args), ...) will create a goal that relates a goal to something else; in other words, the result is some sort of "higher-order" relation on the machinery of miniKanren itself (i.e. goals). That's an interesting path to go down, but it might not be what you want.

More specifically, the goal generated by LengthOf(Line(*args), ...) will attempt to unify the two distinct goal instances created by the expressions Line("B", "C"), and that's where it probably fails. If you really do want this to work, you'll need to subclass Relation and define equality between your subclasses and/or specify how instances of those subclasses unify (e.g. create _unify and _reify implementations via logical-unification).