Can anyone give me a simple example of LL parsing versus LR parsing?
4 Answers
514
votes
At a high level, the difference between LL parsing and LR parsing is that LL parsers begin at the start symbol and try to apply productions to arrive at the target string, whereas LR parsers begin at the target string and try to arrive back at the start symbol.
An LL parse is a left-to-right, leftmost derivation. That is, we consider the input symbols from the left to the right and attempt to construct a leftmost derivation. This is done by beginning at the start symbol and repeatedly expanding out the leftmost nonterminal until we arrive at the target string. An LR parse is a left-to-right, rightmost derivation, meaning that we scan from the left to right and attempt to construct a rightmost derivation. The parser continuously picks a substring of the input and attempts to reverse it back to a nonterminal.
During an LL parse, the parser continuously chooses between two actions:
- Predict: Based on the leftmost nonterminal and some number of lookahead tokens, choose which production ought to be applied to get closer to the input string.
- Match: Match the leftmost guessed terminal symbol with the leftmost unconsumed symbol of input.
As an example, given this grammar:
- S → E
- E → T + E
- E → T
- T →
int
Then given the string int + int + int, an LL(2) parser (which uses two tokens of lookahead) would parse the string as follows:
Production Input Action
---------------------------------------------------------
S int + int + int Predict S -> E
E int + int + int Predict E -> T + E
T + E int + int + int Predict T -> int
int + E int + int + int Match int
+ E + int + int Match +
E int + int Predict E -> T + E
T + E int + int Predict T -> int
int + E int + int Match int
+ E + int Match +
E int Predict E -> T
T int Predict T -> int
int int Match int
Accept
Notice that in each step we look at the leftmost symbol in our production. If it's a terminal, we match it, and if it's a nonterminal, we predict what it's going to be by choosing one of the rules.
In an LR parser, there are two actions:
- Shift: Add the next token of input to a buffer for consideration.
- Reduce: Reduce a collection of terminals and nonterminals in this buffer back to some nonterminal by reversing a production.
As an example, an LR(1) parser (with one token of lookahead) might parse that same string as follows:
Workspace Input Action
---------------------------------------------------------
int + int + int Shift
int + int + int Reduce T -> int
T + int + int Shift
T + int + int Shift
T + int + int Reduce T -> int
T + T + int Shift
T + T + int Shift
T + T + int Reduce T -> int
T + T + T Reduce E -> T
T + T + E Reduce E -> T + E
T + E Reduce E -> T + E
E Reduce S -> E
S Accept
The two parsing algorithms you mentioned (LL and LR) are known to have different characteristics. LL parsers tend to be easier to write by hand, but they are less powerful than LR parsers and accept a much smaller set of grammars than LR parsers do. LR parsers come in many flavors (LR(0), SLR(1), LALR(1), LR(1), IELR(1), GLR(0), etc.) and are far more powerful. They also tend to have much more complex and are almost always generated by tools like yacc or bison. LL parsers also come in many flavors (including LL(*), which is used by the ANTLR tool), though in practice LL(1) is the most-widely used.
As a shameless plug, if you'd like to learn more about LL and LR parsing, I just finished teaching a compilers course and have some handouts and lecture slides on parsing on the course website. I'd be glad to elaborate on any of them if you think it would be useful.
61
votes
Josh Haberman in his article LL and LR Parsing Demystified claims that LL parsing directly corresponds with the Polish Notation, whereas LR corresponds to Reverse Polish Notation. The difference between PN and RPN is the order of traversing the binary tree of the equation:
+ 1 * 2 3 // Polish (prefix) expression; pre-order traversal.
1 2 3 * + // Reverse Polish (postfix) expression; post-order traversal.
According to Haberman, this illustrates the main difference between LL and LR parsers:
The primary difference between how LL and LR parsers operate is that an LL parser outputs a pre-order traversal of the parse tree and an LR parser outputs a post-order traversal.
For the in-depth explanation, examples and conclusions check out Haberman's article.
12
votes
10
votes
LL parsing is handicapped, when compared to LR. Here is a grammar that is a nightmare for an LL parser generator:
Goal -> (FunctionDef | FunctionDecl)* <eof>
FunctionDef -> TypeSpec FuncName '(' [Arg/','+] ')' '{' '}'
FunctionDecl -> TypeSpec FuncName '(' [Arg/','+] ')' ';'
TypeSpec -> int
-> char '*' '*'
-> long
-> short
FuncName -> IDENTIFIER
Arg -> TypeSpec ArgName
ArgName -> IDENTIFIER
A FunctionDef looks exactly like a FunctionDecl until the ';' or '{' is encountered.
An LL parser cannot handle two rules at the same time, so it must chose either FunctionDef or FunctionDecl. But to know which is correct it has to lookahead for a ';' or '{'. At grammar analysis time, the lookahead (k) appears to be infinite. At parsing time it is finite, but could be large.
An LR parser does not have to lookahead, because it can handle two rules at the same time. So LALR(1) parser generators can handle this grammar with ease.
Given the input code:
int main (int na, char** arg);
int main (int na, char** arg)
{
}
An LR parser can parse the
int main (int na, char** arg)
without caring what rule is being recognized until it encounters a ';' or a '{'.
An LL parser gets hung up at the 'int' because it needs to know which rule is being recognized. Therefore it must lookahead for a ';' or '{'.
The other nightmare for LL parsers is left recursion in a grammar. Left recursion is a normal thing in grammars, no problem for an LR parser generator, but LL can't handle it.
So you have to write your grammars in an unnatural way with LL.
