An SLR parser will typically have more conflict states than an LALR parser

LALR parser

In computer science, an LALR parser is a type of LR parser based on a finite-state-automata concept. The data structure used by an LALR parser is a pushdown automaton...

. For real-world computer languages, an SLR parser is usually not adequate, but for student projects in a compiler class it is a good learning tool.

A grammar that has no conflicts reported by an SLR parser generator is an SLR grammar

SLR Grammar

In computer science, an SLR grammar is a class of formal grammar that can be parsed by a simple LR parser.- Definitions :A rule of the form B → y. within a state of a SLR automaton is said to be irreducible or in a reduce state because it has been completely expanded and is incapable of undergoing...

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Algorithm

The SLR parsing algorithm

Initialize the stack with S
Read input symbol
while (true)
if Action(top(stack), input) = S
NS <- Goto(top(stack),input)
push NS
Read next symbol
else if Action(top(stack), input) = Rk
output k
pop |RHS| of production k from stack
NS <- Goto(top(stack), LHS_k)
push NS
else if Action(top(stack),input) = A
output valid, return
else
output invalid, return

Example

A grammar that can be parsed by an SLR parser but not by an LR(0) parser is the following:

(0) S → E

(1) E → 1 E

(2) E → 1

Constructing the action and goto table as is done for LR(0) parsers would give the following item sets and tables:

Item set 0

S → • E

+ E → • 1 E

+ E → • 1

Item set 1

E → 1 • E

E → 1 •

+ E → • 1 E

+ E → • 1

Item set 2

S → E •

Item set 3

E → 1 E •

The action and goto tables:

	action		goto
state	1	$	E
0	s1		2
1	s1/r2	r2	3
2		acc
3	r1	r1

As can be observed there is a shift-reduce conflict for state 1 and terminal '1'. However, the follow set of E is { $ } so the reduce actions r1 and r2 are only valid in the column for $. The result is the following conflict-less action and goto table:

	action		goto
state	1	$	E
0	s1		2
1	s1	r2	3
2		acc
3		r1