[citation needed] Many computer languages don't readily fit the restrictions of SLR, as is.
[citation needed] SLR and LALR were both developed by Frank DeRemer as the first practical uses of Donald Knuth's LR parser theory.
The one difference between SLR and LALR is how their generators calculate the lookahead sets of input symbols that should appear next, whenever some completed production rule is found and reduced.
SLR generators calculate that lookahead by an easy approximation method based directly on the grammar, ignoring the details of individual parser states and transitions.
[citation needed] LALR generators calculate lookahead sets by a more precise method based on exploring the graph of parser states and their transitions.
It customizes the handling of each grammar occurrence of some nonterminal S. See article LALR parser for further details of this calculation.