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Add occurs check for unification and tests for it. #1

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Add occurs check for unification and tests for it. #1

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11 changes: 9 additions & 2 deletions mk.lua
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Original file line number Diff line number Diff line change
Expand Up @@ -167,15 +167,22 @@ local function walk_all(v, s)
end
end

local function occurs_check(var, term)
if not is_table(term) then return false end
if #term == 0 then return false end
if is_var(term) then return var == term end
return occurs_check(var, car(term)) or occurs_check(var, cdr(term))
end

local function unify(k, v, s)
local k = walk(k, s)
local v = walk(v, s)
if k == v then
return s
elseif is_var(k) then
return cons(cons(k, v), s)
return (not occurs_check(k, v)) and cons(cons(k, v), s)
elseif is_var(v) then
return cons(cons(v, k), s)
return (not occurs_check(v, k)) and cons(cons(v, k), s)
elseif is_var(k) or is_var(v) then
return cons(cons(k, v), s)
elseif is_pair(v) and is_pair(k) then
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4 changes: 4 additions & 0 deletions test.lua
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Original file line number Diff line number Diff line change
Expand Up @@ -105,6 +105,10 @@ assert(equal(run(false, {a, b, c}, all(eq(a, 2), eq(a, b), not_eq(b, 3))), {{2,
assert(equal(run(false, {a, b, c}, all(eq(c, 2), eq(a, b), not_eq(b, 3))), { { "_.0 not eq: 3", "_.0 not eq: 3", 2 } }))
assert(equal(run(false, {a, b}, all(eq(a, b), not_eq(a, 3))), { { "_.0 not eq: 3", "_.0 not eq: 3" } }))

assert(equal(run(1, a, eq(a, list(a, a))), {}))
assert(equal(run(1, a, eq(a, list(a, b))), {}))
assert(equal(run(1, a, eq(a, list(b, a))), {}))

assert(equal(run(1, a, mergeo({1, {2, {3}}}, {4, {5, {6, {}}}}, a)), { list(1, 2, 3, 4, 5, 6) }))

assert(equal(run(1, a, mergeo({1}, {2}, a)), { {1, {2}} }))
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