Non-circularity

Grammar tests > Non-circularity

No non-terminal may be derived from itself, neither directly nor indirectly. The following production is an example for a rule, which hurts this principle:

Circular1 ::= Circular2 | "T"

Circular2 ::= Circular1 | "T"

If you parse this rule, following error message will occur:

Circular derivation: "Circular1" . "Circular2"

Circular derivation: "Circular2" . "Circular1"

Remark:

Following rules however are yielding the error message, no being derivable to terminals.

Circular1 ::= Circular2

Circular2 ::= Circular1

The test of derivability is done before the circularity is found and the second test will not be performed.

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