Configuration

Defaulting and lookahead prefetch can be controlled both globally and locally on production level

Global options

Several options exist for controlling lookahead use:

Parser.Reduction.Defaulting

This option turns defaulting globally on and off. Possible settings are:

• On – states are defaulted wherever possible
• Off – no defaulting is done. Actions in all states are executed conditionally depending on next input symbol
• Auto – Alpag decides if defaulting should be used depending on size of grammar. For small grammars, where parser size table savings are small defaulting will be left off.

Parser.Reduction.LookaheadFetch

Controls if lookahead token should be always fetched regardless if defaulting is enabled or not.

• Always – lookahead is fetched always even if current state id defaulted. This results in stable prefetch behavior.
• WhenRequired – symbols are fetched when necessary.

Parser.Reduction.PreviewCheck

When set to Yes parser simulates reductions until non-defaulted state is found. Then it verifies next input symbol using action fable for non-defaulted state. This option enables early error reporting for defaulted states.

Parser.Reduction.ExplicitDefaultingPurpose

User can control defaulting behavior on per-production level using options set for individual productions. User's declarations can be interpreted as either advisory hints or mandatory requirements. This option sets interpretation of these declarations:

• Optimization – explicit defaulting declarations were placed for optimization reasons and can be ignored if necessary
• Functional – explicit defaulting declarations were placed for functional reasons. If some defaulting declaration cannot be applied, parser generation fails

Parser.Reduction.LookaheadContextDefaulting

Grammar productions can be declared with explicit lookahead context. This option controls defaulting when reducing productions with lookahead context. Possible values are:

• Never – defaulting is not used. When productions have explicit lookahead contexts their terminal states are never defaulted. Productions with lookahead contexts are reduced only if next token matches this lookahead context.
• WhenNonDiscerning – defaulting is allowed in these end-of-production states, where next token is not necessary to tell between productions with different lookahead contexts

Local defaulting control

It is possible to control defaulting on per-production level. Following options can be placed at the end of grammar production:

Declares that state at the end of production should not be defaulted

Declares that state at the end of production should be defaulted (no lookahead symbol should be fetched).

It is possible to make conflicting declarations for different productions. If Parser.Reduction.ExplicitDefaultingPurpose is set to Functional colliding declarations produce an error.