Re: Recursive descent and left recursion

mfinney@lynchburg.net wrote:
>> I have noticed the occassional post here, as well as assertions in
>> various texts, that left recursion is not usable with recursive
>> descent (and LR parsers in general).
                                  ^^
cfc@world.std.com (Chris F Clark) writes:
>I thought certainly someone else would catch this mis-impression, but
>since no one has mentioned it. LR parsers allow left-recursion as
>well as right recursion and any other recursion. It is LL parsers
>which don't like left recursion. In fact, that is the main reason for
>using LR (or LALR) parsing over LL.


All true... but now you go on to add a mis-impression of your own,
because you confuse LR parsing with operator precedence parsing:


>You can write your expression
>grammars "naturally" with out inventing extra non-terminals to handle
>precedence levels.


Yes, but only if your LR parsing tool/technique allows operator
precedence grammars -- and you can do that with LL parsing too, if you
have an LL parsing tool/technique that supports operator precedence
grammars. Certainly it's not that hard to write an operator
precedence recursive descent parser.


Perhaps you meant "... without having to left-factor your grammar"?


--
Fergus Henderson <fjh@cs.mu.oz.au>
WWW: <http://www.cs.mu.oz.au/~fjh>
PGP: finger fjh@128.250.37.3
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