6.4. Grammar
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## 6.4. Grammar

The grammar section comes after the directives, separated from them by a double-percent (`%%`) symbol. This section contains a number of productions, each of which defines a single non-terminal. Each production has the following syntax:

    <non-terminal> [ :: { <type> } ]
            :  <id> ... {[%] <expression> }
          [ |  <id> ... {[%] <expression> }
            ... ]

The first line gives the non-terminal to be defined by the production and optionally its type (type signatures for productions are discussed in Section 2.4, “Type Signatures”).

Each production has at least one, and possibly many right-hand sides. Each right-hand side consists of zero or more symbols (terminals or non-terminals) and a Haskell expression enclosed in braces.

The expression represents the semantic value of the non-terminal, and may refer to the semantic values of the symbols in the right-hand side using the meta-variables `$1 ... $n`. It is an error to refer to `$i` when `i` is larger than the number of symbols on the right hand side of the current rule. The symbol `$` may be inserted literally in the Haskell expression using the sequence `\$` (this isn't necessary inside a string or character literal).

Additionally, the sequence `$>` can be used to represent the value of the rightmost symbol.

A semantic value of the form `{% ... }` is a monadic action, and is only valid when the grammar file contains a `%monad` directive (Section 6.3.5, “Monad Directive”). Monadic actions are discussed in Section 2.5, “Monadic Parsers”.

Remember that all the expressions for a production must have the same type.

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