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><H1
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><A
NAME="SEC-GRAMMAR"
>4.4. Grammar</A
></H1
><P
>The grammar section comes after the directives, separated
from them by a double-percent (<TT
CLASS="LITERAL"
>%%</TT
>) symbol.
This section contains a number of
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>productions</I
></SPAN
>, each of which defines a single
non-terminal. Each production has the following syntax:</P
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><PRE
CLASS="PROGRAMLISTING"
><non-terminal> [ :: { <type> } ]
: <id> ... {[%] <expression> }
[ | <id> ... {[%] <expression> }
... ]</PRE
></TD
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><P
>The first line gives the non-terminal to be defined by the
production and optionally its type (type signatures for
productions are discussed in <A
HREF="sec-type-signatures.html"
>Section 2.4</A
>).</P
><P
>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.</P
><P
>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
<TT
CLASS="LITERAL"
>$1 ... $n</TT
>. It is an error to
refer to <TT
CLASS="LITERAL"
>$i</TT
> when <TT
CLASS="LITERAL"
>i</TT
>
is larger than the number of symbols on the right hand side of
the current rule. The symbol <TT
CLASS="LITERAL"
>$</TT
> may be
inserted literally in the Haskell expression using the sequence
<TT
CLASS="LITERAL"
>\$</TT
> (this isn't necessary inside a
string or character literal).</P
><P
>A semantic value of the form <TT
CLASS="LITERAL"
>{% ... }</TT
> is a
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>monadic action</I
></SPAN
>, and is only valid when the grammar
file contains a <TT
CLASS="LITERAL"
>%monad</TT
> directive (<A
HREF="sec-directives.html#SEC-MONAD-DECL"
>Section 4.3.4</A
>). Monadic actions are discussed in
<A
HREF="sec-monads.html"
>Section 2.5</A
>.</P
><P
>Remember that all the expressions for a production must
have the same type.</P
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