Re: Why LL(1) Parsers do not support left recursion?

SM Ryan <wyrmwif@tsoft.org>
18 Jul 2006 01:18:40 -0400

          From comp.compilers

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[26 later articles]
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From: SM Ryan <wyrmwif@tsoft.org>
Newsgroups: comp.compilers
Date: 18 Jul 2006 01:18:40 -0400
Organization: Quick STOP Groceries
References: 06-07-024
Keywords: parse, LL(1)
Posted-Date: 18 Jul 2006 01:18:40 EDT

"DevInstinct" <martin_lapierre@videotron.ca> wrote:
# Hi, first of all, I'm not an expert in the theory of computation.
#
# I've read about LL(1) parsers and I have seen that they do not support
# left recursion, because it is said that it would lead to infinite
# recursivity.


LL(k) grammars make parsing decisions based on the first k symbols.


For
S ::= Sb | a
FIRST(Sb,1) = FIRST(S,1) + FIRST(a,1) = {a}
FIRST(a,1) = {a}


So both alternatives have the same FIRST(1) sets, and you can't decide
which alternative to use. It's not an LL(1) grammar.


--
SM Ryan http://www.rawbw.com/~wyrmwif/
I'm not even supposed to be here today.


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