From 350eccaf085059846cf5558af5f9e02598d80822 Mon Sep 17 00:00:00 2001
From: Andreas Kloeckner <inform@tiker.net>
Date: Sun, 25 Mar 2012 16:57:54 -0400
Subject: [PATCH] Add better comments to unifier.
---
pymbolic/mapper/unifier.py | 29 ++++++++++++++++++++++++++---
1 file changed, 26 insertions(+), 3 deletions(-)
diff --git a/pymbolic/mapper/unifier.py b/pymbolic/mapper/unifier.py
index b159009..b5c14cc 100644
--- a/pymbolic/mapper/unifier.py
+++ b/pymbolic/mapper/unifier.py
@@ -72,16 +72,36 @@ def unify_many(unis1, uni2):
class UnifierBase(RecursiveMapper):
+ # The idea of the algorithm here is that the unifier accumulates a set of
+ # unification possibilities (:class:`UnificationRecord`) as it descends the
+ # expression tree. :func:`unify_many` above then checks if these possibilities
+ # are consistent with new incoming information (also encoded as a
+ # :class:`UnificationRecord`) and either augments or abandons them.
+
def __init__(self, lhs_mapping_candidates=None,
rhs_mapping_candidates=None,
force_var_match=True):
+ """
+ :arg lhs_mapping_candidates: list or set of variable names that may be
+ assigned in the left-hand ("first") expression
+ :arg rhs_mapping_candidates: list or set of variable names that may be
+ assigned in the right-hand ("second") expression
+ :arg force_var_match: In the (unimplemented) case of bidirectional
+ unification, only assign to variable names, don't make matches
+ between higher-level expressions.
+ """
+
self.lhs_mapping_candidates = lhs_mapping_candidates
self.rhs_mapping_candidates = rhs_mapping_candidates
self.force_var_match = force_var_match
def unification_record_from_equation(self, lhs, rhs):
if isinstance(lhs, (tuple, list)) or isinstance(rhs, (tuple, list)):
- # must match elementwise!
+ # Always force lists/tuples to agree elementwise, never
+ # generate a unification record between them directly.
+ # This pushes the matching process down to the elementwise
+ # level.
+
return None
lhs_is_var = isinstance(lhs, Variable)
@@ -110,7 +130,9 @@ class UnifierBase(RecursiveMapper):
def map_variable(self, expr, other, urecs):
new_uni_record = self.unification_record_from_equation(
expr, other)
+
if new_uni_record is None:
+ # Check if the variables match literally--that's ok, too.
if (isinstance(other, Variable)
and other.name == expr.name
and expr.name not in self.lhs_mapping_candidates):
@@ -210,8 +232,9 @@ class UnifierBase(RecursiveMapper):
class UnidirectionalUnifier(UnifierBase):
- """Only assigns variables encountered in the first expression to
- subexpression of the second.
+ """Finds assignments of variables encountered in the
+ first ("left") expression to subexpression of the second
+ ("right") expression.
"""
def treat_mismatch(self, expr, other, urecs):
--
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