SLR (simple LR) is like LR(0) but reduces on (X::=p.) only if additionally:

• t follow(X) where is current token

Otherwise, same as LR(0) Parser Actions

Here is then the algorithm:

Let K be parser state at the end of stack and t current token:

• if there is a transition for non-empty
  • shift: push t onto stack
  • if t is EOF, then accept
• reduce: if item K and t follow(X) then
  • pop p from stack
  • push X onto stack
• if none possible, report syntax error
• if both possible, shift-reduce conflict: grammar not SLR

Note: SLR uses lookahead in a simpler way than (LR(1)), but it is useful

Example 1: Parsing our example using the automaton we constructed

Example 2: Consider the ambiguous grammar

D' ::= E eof
E ::= E - E
E ::= ID

How does the automaton look like? Compute

goto(goto(goto(I0,E),-),E)

where I0 is the initial state.