Error Recovery in LL(1) parser

. Error occurs when a parse table entry M[A,a] is empty

. Skip symbols in the input until a token in a selected set (synch) appears

. Place symbols in follow(A) in synch set. Skip tokens until an element in follow(A) is seen. Pop(A) and continue parsing

. Add symbol in first(A) in synch set. Then it may be possible to resume parsing according to A if a symbol in first(A) appears in input.

Let us consider error recovery in an LL(1) parser by panic-mode recovery method. An error occurs when the terminal on top of the stack does not match the next input symbol or when non-terminal A is on top of the stack, a is the next input symbol, and the parsing table entry M[A, a ] is empty. Panic-mode error recovery is based on the idea of skipping symbols on the input until a token in a selected set of synchronizing tokens appears. Its effectiveness depends on the choice of synchronizing set. The sets should be chosen so that the parser recovers quickly from errors that are likely to occur in practice. Some heuristics are as follows:

1. As a starting point, we can place all symbols in FOLLOW(A) into the synchronizing set for non-terminal A. If we skip tokens until an element of FOLLOW(A) is seen and pop A from the stack, it is likely that parsing can continue.

2. If we add symbols in FIRST(A) to the synchronizing set for non-terminal A, then it may be possible to resume parsing according to A if a symbol in FIRST(A) appears in the input.