15 Facts About LR parser

An LR parser reads input text from left to right without backing up, and produces a rightmost derivation in reverse: it does a bottom-up parse – not a top-down LL parse or ad-hoc parse.

An LL LR parser has to decide or guess what it is seeing much sooner, when it has only seen the leftmost input symbol of that pattern.

At every point in this pass, the LR parser has accumulated a list of subtrees or phrases of the input text that have been already parsed.

Those subtrees are not yet joined together because the LR parser has not yet reached the right end of the syntax pattern that will combine them.

LR parser stacks add the state information in purple, summarizing the black phrases to their left on the stack and what syntax possibilities to expect next.

The LR parser's program code is a simple generic loop that is the same for all grammars and languages.

The LR parser has to check the lookahead symbol to tell what to do.

Individual table cells must not hold multiple, alternative actions, otherwise the LR parser would be nondeterministic with guesswork and backtracking.

LR parser begins with a nearly empty parse stack containing just the start state 0, and with the lookahead holding the input stream's first scanned symbol.

The LR parser generator adds additional rules or items for all the possible next steps in building up the expected Products:.

In words, that means if the LR parser has seen a single Products, it might be done, or it might still have even more things to multiply together.

The LR parser enumerates every possible single-symbol insertion, deletion, or substitution at the error point.

LR parser generator decides what should happen for each combination of parser state and lookahead symbol.

Some LR parser generators create separate tailored program code for each state, rather than a parse table.

Finally, the LR parser reads a '$' from the input stream, which means that according to the action table the LR parser accepts the input string.