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fix(library/class_instance_resolution): add hack for mk_subsingleton_instance API

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The comment in the source code explains why the hack is needed
This commit is contained in:
Leonardo de Moura 2015-10-18 16:13:36 -07:00
parent 1b92a8333e
commit 97407eb178
1 changed files with 24 additions and 0 deletions
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24
src/library/class_instance_resolution.cpp
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@ -92,6 +92,25 @@ static ci_type_inference_factory & get_ci_factory() {
return g_factory ? *g_factory : *g_default_factory;
}
/** \brief The following global thread local constant is a big hack for mk_subsingleton_instance.
When g_subsingleton_hack is true, the following type-class resolution problem fails
Given (A : Type{?u}), where ?u is a universe meta-variable created by an external module,
?Inst : subsingleton.{?u} A := subsingleton_prop ?M
This case generates the unification problem
subsingleton.{?u} A =?= subsingleton.{0} ?M
which can be solved by assigning (?u := 0) and (?M := A)
when the hack is enabled, the is_def_eq method in the type class module fails at the subproblem:
?u =?= 0
That is, when the hack is on, type-class resolution cannot succeed by instantiating an external universe
meta-variable with 0.
*/
LEAN_THREAD_VALUE(bool, g_subsingleton_hack, false);
struct cienv {
typedef rb_map<unsigned, level, unsigned_cmp> uassignment;
typedef rb_map<unsigned, expr, unsigned_cmp> eassignment;
@ -475,6 +494,10 @@ struct cienv {
// The unifier may invoke this module before universe metavariables in the class
// have been instantiated. So, we just ignore and assume they will be solved by
// the unifier.
// See comment at g_subsingleton_hack declaration.
if (g_subsingleton_hack && (is_zero(l1) || is_zero(l2)))
return false;
return true; // we ignore
}
@ -1303,6 +1326,7 @@ optional<expr> mk_hset_instance(type_checker & tc, io_state const & ios, list<ex
}
optional<expr> mk_subsingleton_instance(type_checker & tc, io_state const & ios, list<expr> const & ctx, expr const & type) {
flet<bool> set(g_subsingleton_hack, true);
level lvl = sort_level(tc.ensure_type(type).first);
expr subsingleton;
if (is_standard(tc.env()))