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Loyc.LLParserGenerator.LLParserGenerator.ComputeNext Class Reference

Gathers a list of all one-token transitions starting from a single position. Also gathers any and-predicates that must be traversed before completing a transition. More...


Source file:
Inheritance diagram for Loyc.LLParserGenerator.LLParserGenerator.ComputeNext:
Loyc.LLParserGenerator.PredVisitor

Remarks

Gathers a list of all one-token transitions starting from a single position. Also gathers any and-predicates that must be traversed before completing a transition.

For example, given

rule X 'x' Y '0'..'9' 'x' };
rule Y.('y' | Z)? };
rule Z ('z' | '0'..'9' '0'..'9'*) };

If the dot (.) represents the current position, then this class computes the possible Transitions, which are as follows:

Transition.Set Transition.Position
'y' rule Y ('y' | Z)?.}; (EndOfRule)
'0'..'9' rule X 'x' Y '0'..'9'.'x' }; (TerminalPred)
'z' rule Z ('z' | '0'..'9' '0'..'9'*).}; (EndOfRule)
'0'..'9' rule Z ('z' | '0'..'9'.'0'..'9'*) }; (Alts)

Notice that there can be duplicate sets–different destinations for the same input character. This means that there is an LL(1) ambiguity. The ambiguity may (or may not, depending on the situation) be resolved by looking ahead further (it is the responsibility of PredictionAnalysisVisitor.ComputePredictionTree to do so).

This class is derived from GetCanonical just to inherit some code from it.

What to do with and-predicates? It's a tricky question. And-predicates are not used nearly as often as normal terminals and nonterminals, yet they can produce the most complicated prediction code. Consider Alts such as:

( ( &{a} {f();} | &{b} {g();} ) &{c}
( &{a} 'a' | &{x} 'b' | &{x} 'c')
| &{x} ( 'a' | &{y} 'b' 'c' )
)

It's enough to make your head explode. IIRC, PredictionAnalysisVisitor deals with such complications–all ComputeNext does is gather a list of AndPreds.

Public Member Functions

void Do (KthSet result, GrammarPos position)
 
void Visit (Pred pred, GrammarPos newPos=null)
 
override void Visit (Seq seq)
 
override void Visit (Gate gate)
 
override void Visit (TerminalPred term)
 
override void Visit (RuleRef rref)
 
override void Visit (Alts alts)
 
override void Visit (AndPred and)
 
override void Visit (ActionPred and)
 
override void Visit (EndOfRule end)
 
- Public Member Functions inherited from Loyc.LLParserGenerator.PredVisitor
void Visit (Pred pred)
 
virtual void VisitOther (Pred pred)
 
void VisitChildrenOf (Seq pred)
 
void VisitChildrenOf (AndPred pred)
 
void VisitChildrenOf (Gate pred)
 
void VisitChildrenOf (Alts pred, bool includeError)