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(bison.info)Semantics

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Defining Language Semantics
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   The grammar rules for a language determine only the syntax.  The
semantics are determined by the semantic values associated with various
tokens and groupings, and by the actions taken when various groupings
are recognized.

   For example, the calculator calculates properly because the value
associated with each expression is the proper number; it adds properly
because the action for the grouping `X + Y' is to add the numbers
associated with X and Y.

Value Type

Specifying one data type for all semantic values.

Multiple Types

Specifying several alternative data types.

Actions

An action is the semantic definition of a grammar rule.

Action Types

Specifying data types for actions to operate on.

Mid-Rule Actions

Most actions go at the end of a rule.

This says when, why and how to use the exceptional action in the middle of a rule.