This topic explains the restrictions placed on grammar definitions.
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When a Grammar instance is complete and used to generate the lexical and syntax analyzers, it must be in a valid state. There are certain things which are not allowed because they are either logical contradictions or because they lack sufficient information needed to create the analyzers. Here are the conditions which make a Grammar invalid.
The StartSymbol is not set – without specifying the start symbol the Syntax Parsing Engine does not know what non-terminal symbol should be at the root of the syntax trees.
It is not very useful if a LexerState.Symbols collection is empty. If there are no terminal symbols in a lexer state, it would only create tokens associated with the Grammar.UnrecognizedSymbol and it can never be exited. Therefore, each lexer state must contain at least one terminal symbol.
A NonTerminalSymbol.Rule is null – All non-terminal symbols need to have at least one production and so they need to have at least one rule in their rule tree. If a non-terminal symbol is meant to match nothing (like the omitted argument in Visual Basic), then the Rule should be set to an instance of EmptySyntaxRule.
A rule in a non-terminal symbol’s rule tree is invalid based on one of the following conditions:
An AlternationSyntaxRule or ConcatenationSyntaxRule has fewer than two child rules – These rules are meant to contain multiple rules. One of many options must be used for an alternation rule and multiple rules must be concatenated together for a concatenation rule. It doesn’t make sense for these rules to contain zero or one child rule.
An ExceptionSyntaxRule has a null Exception or Rule property.
An ExceptionSyntaxRule.Exception property is a rule tree which contains one or more recursively defined symbols – recursion is not allowed in syntactic exceptions. This includes repetition symbols (which require recursively defined productions internally). So the following production would be invalid in a grammar
RestrictedBlock → Block-{Statement}
This pseudo-production uses a minus sign “-” to indicate a syntactic exception and curly braces to represent a repetition.
An ExceptionSyntaxRule has an Exception which disallows everything from the Rule – For example in this production
Nothing → Statement-Statement
what this basically says is “A ‘Nothing’ can be a ‘Statement’ as long as that ‘Statement’ is not a ‘Statement’”. This is a logical contradiction because ‘Nothing’ can never be found in a document. This type of direct contradiction probably wouldn’t be done accidentally or on purpose, but it is possible to create a contradiction without realizing it. Here is a slightly more complicated example of a contradiction
Letter → a|b|c|…|z
AThruM → a|b|c|…|m
NThruZ → n|o|p|…|z
RestrictedLetter → Letter-(AThruM|NThruZ)
"RestrictedLetter" now has a contradiction and will cause the grammar to be invalid.
A FactorSyntaxRule has a null Rule property
An OptionalSyntaxRule has a null Rule property
A RepetitionSyntaxRule has a null Rule property
A SymbolReferenceSyntaxRule has a Symbol which doesn’t belong to the Grammar
A SymbolReferenceSyntaxRule references the Grammar.EndOfStreamSymbol and the owning non-terminal symbol is not the Grammar.StartSymbol – Only the start symbol can expect the symbol indicating the end of a document. It doesn’t make sense for another symbol to use it in one of its productions because they are by definition somewhere in the interior of the document. There are even some restrictions to how the start symbol can use it. The start symbol can only reference the end of stream symbol if it has only one production of the following form
StartSymbol → SomeOtherNonTerminalSymbol EndOfStreamSymbol
If it has multiple productions, more than two symbols in the body, references itself in its production body, or requires the EndOfStreamSymbol to be first in the body, the rule is invalid and so is the grammar. If a grammar writer would like their start symbol to refer to more than two symbols, use recursion, or have multiple productions, they can easily do so by not referencing the EndOfStreamSymbol:
StartSymbol →
StartSymbol → Symbol1 StartSymbol Symbol2
And then internally, the grammar will create a “resolved” start symbol like this
ResolvedStartSymbol → StartSymbol EndOfStreamSymbol
A SymbolReferenceSyntaxRule references the resolved start symbol for the grammar – If the grammar writer has designed the start symbol to be well formed, meaning it owns a single ConcatenationSyntaxRule which requires a reference to another non-terminal symbol followed by the EndOfStreamSymbol, the Syntax Parsing Engine will not create a resolved start symbol for the grammar. Their start symbol will be used as the resolved start symbol. In that case, no other non-terminal symbol can reference the start symbol in their production bodies. The resolved start symbol can never be part of a recursive definition, either directly or indirectly.
One way to analyze the syntax of a document is to derive the start symbol into a sequence of terminal symbols corresponding to the sequence of tokens from the lexical analyzer. A derivation starts with the start symbol and is a set of steps where each non-terminal symbol is replaced by one of its production bodies. In the new sequence, with the production body in place where the head was, if there are still non-terminal symbols, another is replaced by a production body until only terminal symbols are remaining. However it is possible to define a start symbol that can never be derived fully and the sequence of derivation steps is therefore infinite. Consider the following grammar:
StartSymbol → GroupedContent
GroupedContent → (Parens | Brackets)
Parens → OpenParenToken GroupedContent CloseParenToken
Brackets → OpenBracketToken GroupedContent CloseBracketToken
There is no way for the start symbol to ever derive to a sequence of only terminal symbols. Every substitution for the "Parens" or "Brackets" non-terminal symbol leaves another "GroupedContent" non-terminal symbol in the sequence of symbols.
The following topics provide additional information related to this topic.