DeCarabas/lrparsers
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(And a small bug fix.)
This commit is contained in:
John Doty 2016-12-09 06:43:53 -08:00
parent cf10891c19
commit 95b01a627d
1 changed files with 15 additions and 2 deletions
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17
parser.py
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@ -311,6 +311,10 @@ class GenerateSLR1(GenerateLR0):
def gen_first_symbol(self, symbol, visited):
"""Compute the first set for a single symbol.
The first set is the set of tokens that can appear as the first token
for a given symbol. (Obviously, if the symbol is itself a token, then
this is trivial.)
'visited' is a set of already visited symbols, to stop infinite
recursion on left-recursive grammars. That means that sometimes this
function can return an empty tuple. Don't confuse that with a tuple
@ -341,7 +345,16 @@ class GenerateSLR1(GenerateLR0):
def gen_first(self, symbols, visited=None):
"""Compute the first set for a sequence of symbols.
An epsilon in the set is indicated by 'None'.
The first set is the set of tokens that can appear as the first token
for this sequence of symbols. The interesting wrinkle in computing the
first set for a sequence of symbols is that we keep computing the first
sets so long as Epsilon appears in the set. i.e., if we are computing
for ['A', 'B', 'C'] and the first set of 'A' contains epsilon, then the
first set for the *sequence* also contains the first set of ['B', 'C'],
since 'A' could be missing entirely.
An epsilon in the result is indicated by 'None'. There will always be
at least one element in the result.
The 'visited' parameter, if not None, is a set of symbols that are
already in the process of being evaluated, to deal with left-recursive
@ -355,7 +368,7 @@ class GenerateSLR1(GenerateLR0):
result = self.gen_first_symbol(symbols[0], visited)
if None in result:
result = tuple(set(s for s in result if s is not None))
result = result + self.gen_first(symbols[1:])
result = result + self.gen_first(symbols[1:], visited)
return result
def gen_follow(self, symbol):