/prolog.py

"""prolog.py

A simple prolog implementation to give me ideas about how to make the real
engine work.

"""

from itertools import count, izip
from copy import copy

# This is used to do things like standardizing apart variable names.  Every
# variable must have a unique name, but it also has a "preferred" name assigned
# by the user.  This generates the unique name.
def name_generator(prefix):
    """Generates unique names (using numbers as suffixes)"""
    for i in count():
        yield "%s%d" % (prefix, i)

global_varname_factory = name_generator('v')

class Predicate(object):
    def __init__( self, name, args=() ):
        self.name = name
        self.args = [x.copy() for x in args]

    def __str__( self ):
        if len(self.args) > 0:
            argstr = ", ".join(str(x) for x in self.args)
            return "%s(%s)" % (self.name, argstr)
        else:
            return "%s" % (self.name,)

    __repr__ = __str__

    def copy( self ):
        return self.__class__( self.name, self.args )

    def standardize_vars( self, factory, mapping ):
        for i, a in enumerate(self.args):
            if mapping.is_var(a):
                if a not in mapping:
                    mapping.add(a, Var(factory.next()))
                self.args[i] = mapping[a]
            else:
                a.standardize_vars(factory, mapping)

    def var_within( self, var, mapping ):
        """Returns True if the variable is somewhere in this predicate"""
        var = mapping[var]

        for a in self.args:
            if mapping.is_var(a):
                a = mapping[a]
                if a.name == var.name:
                    return True
            else:
                if a.var_within(var, mapping):
                    return True
        return False

    def unify( self, other, mapping ):
        """Attempts to do unification with the given predicate and given
        variable mapping.  Returns a boolean success value.

        The mapping will be changed when it comes back, so it should be copied
        before being passed in if rewindability is needed.
        """
        if mapping.is_var(other):
            other = mapping[other]

        # If it's still a variable after asking the map for it, then we defer
        # to that variable's unification routine.
        if mapping.is_var(other):
            return other.unify(self, mapping)

        # Other must be a predicate, so do the predicate matching logic: match
        # names and unify args.
        if self.name != other.name or len(self.args) != len(other.args):
            return False

        # Now attempt to do unification on each of the arguments.  This is
        # recursively defined.  Variables know how to do unification, for
        # example.
        for thisarg, otherarg in izip(self.args, other.args):
            if not thisarg.unify(otherarg, mapping):
                return False
        return True

    def substitute( self, mapping ):
        """Returns a copy of this predicate with all variables resolved"""
        pred = self.copy()
        for i, a in enumerate(pred.args):
            if mapping.is_var(a):
                a = mapping[a]
            # Vars just get inserted, predicates are recursively substituted
            if mapping.is_var(a):
                pred.args[i] = a
            else:
                pred.args[i] = a.substitute(mapping)
        return pred

class Var(object):
    def __init__( self, name ):
        self.name = name

    def __str__( self ):
        return "_%s" % self.name

    __repr__ = __str__

    def copy( self ):
        return self.__class__( self.name )

    def unify( self, other, mapping ):
        # Are we a variable when the map is queried?
        me = mapping[self]

        # If not a variable anymore, defer to the predicate routine
        if not mapping.is_var(me):
            return me.unify(other, mapping)

        # If we are trying to unify with a variable, attempt to get its value
        # out of the mapping first.
        if mapping.is_var(other):
            other = mapping[other]

        # If it's still a variable, then we just add it to the mapping and move
        # on with life; nothing to see here.
        if mapping.is_var(other):
            mapping.add(me, other)
            return True

        # Not a variable?  Must be a predicate!  We can typically just assign
        # these as well, but we first have to ensure that the variable does not
        # appear (recursively) in any of the predicate's arguments.
        if other.var_within(me, mapping):
            return False
        else:
            mapping.add(me, other)
            return True

class VarMap(object):
    """Contains variable assignment pairs"""
    def __init__( self ):
        self.vardict = {}

    def copy( self ):
        newmap = self.__class__()
        for k, (var, val) in self.vardict.iteritems():
            newmap.vardict[k] = (var.copy(), val.copy())
        return newmap

    def reversed( self ):
        map = self.__class__()
        for name, (var, val) in self.vardict.iteritems():
            map.add(val, var)
        return map

    def __str__( self ):
        items = []
        for k, (var, val) in self.vardict.iteritems():
            items.append("%s->%s" % (var, val))
        return "{" + ", ".join(items) + "}"

    __repr__ = __str__

    def is_var( self, value ):
        return isinstance(value, Var)

    def add( self, var, value ):
        """Adds a new variable mapping to the dictionary.

        Raises ValueError if a mapping already exists or the key is not a var.
        """
        if not self.is_var(var):
            raise ValueError("Tried to insert non-var %s into map" % (var,))
        if var.name in self.vardict:
            raise ValueError("%s is already in the mapping" % (var,))

        # No flattening is done at the moment
        self.vardict[var.name] = (var, value)

    def __contains__( self, var ):
        return var.name in self.vardict

    def __getitem__( self, var ):
        return self.deep_get(var)

    def shallow_get(self, var):
        if not self.is_var(var):
            raise ValueError("Got something other than a variable")

        var, val = self.vardict[var.name]
        return val

    def deep_get( self, var ):
        if not self.is_var(var):
            raise ValueError("Got something other than a variable")

        # You would think that the best thing to do in this case would be to
        # fail (the variable isn't in the mapping, right?), but it isn't.  If
        # we ask for a variable that isn't here, then we just return that
        # variable.  The purpose of the mapping is to tell us what stuff is
        # assigned to.  This variable is unassigned and therefore is just
        # itself.
        if var not in self:
            return var

        # If it's in there, then obviously get it and follow the thread.
        var, val = self.vardict[var.name]
        while self.is_var(val) and val in self:
            var, val = self.vardict[val.name]
        return val

class Rule(object):
    def __init__( self,
                  consequent,
                  antecedents = (),
                  varfactory=global_varname_factory ):

        self.varmap = VarMap()
        self.consequent = consequent.copy()
        self.consequent.standardize_vars(varfactory, self.varmap)

        self.antecedents = [a.copy() for a in antecedents]
        for a in self.antecedents:
            a.standardize_vars(varfactory, self.varmap)

    def __str__( self ):
        if len(self.antecedents) > 0:
            antstr = ", ".join(str(x) for x in self.antecedents)
            return "%s<=%s::%s" % (self.consequent, antstr, self.varmap)
        else:
            return "%s::%s" % (self.consequent, self.varmap)

    __repr__ = __str__

    def copy( self ):
        return self.__class__(self.consequent, self.antecedents)

    def try_to_satisfy( self ):
        if not self.pre_test():
            self.commands()
            if not self.post_test():
                return False
        return True

    def pre_test( self ):
        """Runs after all antecedents have been shown to be true for a rule.

        If the test returns True, then no further action is
        taken and the rule is deemed True.  If the test returns False, then the
        associated commands for the rule are run.
        """
        return True

    def post_test( self ):
        """Runs after this rule's commands have been executed.

        Its purpose is to verify that the commands succeeded.  Typically it
        just runs the pre_test again to make sure that the commands worked.
        """
        return self.pre_test()

    def commands( self ):
        """Runs if the pre_test comes back False.
        
        The intent is to *make* the test True if at all possible (e.g., if a
        file is not up to date, the commands are run to build it).
        """
        pass

class Prolog(object):
    def __init__( self ):
        # Contains all of the rules
        self.rules = []

        # Keyed on the name of the consequent predicate, to make searching
        # faster.
        self.rules_dict = {}

    def add_rule( self, rule ):
        self.rules.append(rule)
        if rule.consequent.name not in self.rules_dict:
            self.rules_dict[rule.consequent.name] = []
        self.rules_dict[rule.consequent.name].append(rule)

    def answer_iter( self, queries, varmap=None ):
        """Finds matches for an entire list of queries by finding answers for
        the first one and for each answer, passing the *rest* of the list into
        this function.  When the list is empty, simply return the varmap
        because an empty list is vacuously true.
        """

        if varmap is None:
            varmap = VarMap()

        if not queries:
            yield varmap, []
            return

        query = queries[0]
        rest = queries[1:]

        # First, we determine whether the query (the first in the list) matches
        # any rules.  For each matching rule, there are a number of ways in
        # which the entire antecedent list for that rule can be made true, and
        # we have to try them all.  But that's okay, because we can just call
        # ourselve to get an iterator of all valid mappings for the entire list
        # (recursion is fun, right?)
        if query.name not in self.rules_dict:
            return

        for rule in self.rules_dict[query.name]:
            rulemap = varmap.copy()

            if query.unify(rule.consequent, rulemap):
                # We found a matching rule.  Now try all of the ways that the
                # antecedents can be made true.  For each of them, we call this
                # function again with the *rest* of the query list and yield
                # all resulting maps.  Neat!
                for antmap, antrules in self.answer_iter(
                        rule.antecedents, rulemap):
                    for finalmap, finalrules in self.answer_iter(rest, antmap):
                        # It is not quite enough in this system to have true
                        # antecedents and therefore assume a true consequent.
                        # If the following test succeeds, though, we can
                        # proceed.
                        if rule.try_to_satisfy():
                            yield finalmap, [rule] + finalrules + antrules

if __name__ == '__main__':
    prolog = Prolog()

    # Create some facts:

    prolog.add_rule(Rule(Predicate('exists', [
                        Predicate('file',
                            [Predicate('myfile'), Predicate('.cc')])
                        ])))
    prolog.add_rule(Rule(Predicate('exists', [
                        Predicate('file',[Predicate('myfile'), Predicate('.y')])
                        ])))
    prolog.add_rule(Rule(Predicate('buildable', [
                        Predicate('file',[Var('base'), Predicate('.o')])
                        ]),
                [Predicate('buildable', [
                        Predicate('file',[Var('base'), Predicate('.c')])
                        ])
                ]))
    prolog.add_rule(Rule(Predicate('buildable', [
                        Predicate('file',[Var('base'), Predicate('.c')])
                        ]),
                [Predicate('exists', [
                        Predicate('file',[Var('base'), Predicate('.y')])
                        ])
                ]))
    prolog.add_rule(Rule(Predicate('buildable', [
                        Predicate('file',[Var('base'), Predicate('.o')])
                        ]),
                [Predicate('exists', [
                        Predicate('file',[Var('base'), Predicate('.cc')])
                        ])
                ]))

    q = Predicate('buildable',
            [Predicate('file',[Predicate('myfile'), Predicate('.o')])])


    print "RULES"
    for rule in prolog.rules:
        print rule

    print
    print "QUERY"
    print q

    print
    print "ANSWERS"
    for answer in prolog.answer_iter( [q] ):
        print answer

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