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7.1.4.1 Switching Spur Update Mode ¶

Only the adaptive top-down parser atd-parser supports specifiers for spur update mode switching. The following spur update modes are available:

Enabled

The parser updates spur when leading out PCFG productions with terminal symbols or nonterminal symbols at the right-hand side. The specifier

#su

(without the following ‘0’) sets this mode. This is default spur update mode.

Disabled

The parser does not update spur when leading out PCFG productions with terminal symbols at the right-hand side. The parser still updates spur when leading out PCFG productions with nonterminal symbols at the right-hand side. The specifier

#su0

sets this mode.

The specifiers #su and #su0 have the scope of a grouping: they do not affect spur update mode outside of an expression ‘(’ … ‘)’ containing the specifiers.

The specifier #su or #su0 in a production right-hand side changes current spur update mode, that is, spur update mode for subsequent elements in a sequence or grouping until the next specifier #su or #su0 in the sequence or grouping. If a sequence element is a grouping, current spur update mode propagates to the grouping, that is, it becomes initial spur update mode for the grouping.

Example

To disable updating spur on processing a put-back sequence, put #su0 before the put-back sequence and optionally put #su after it:

#su0 .~ | . . .

The specifier #su or #su0 can follow a nonterminal symbol at a production left-hand side for setting initial spur update mode for the right-hand side. As default spur update mode is “enabled”, it only makes sense to put the specifier #su0 in the left-hand side for switching initial spur update mode to “disabled”.