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>The Error Token</TITLE
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>Chapter 2. Using <SPAN
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><A
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>2.6. The Error Token</A
></H1
><P
><SPAN
CLASS="APPLICATION"
>Happy</SPAN
> supports a limited form of error
recovery, using the special symbol <TT
CLASS="LITERAL"
>error</TT
> in a grammar
file. When <SPAN
CLASS="APPLICATION"
>Happy</SPAN
> finds a parse error during
parsing, it automatically inserts the <TT
CLASS="LITERAL"
>error</TT
> symbol; if
your grammar deals with <TT
CLASS="LITERAL"
>error</TT
> explicitly, then it can
detect the error and carry on.</P
><P
>For example, the <SPAN
CLASS="APPLICATION"
>Happy</SPAN
> grammar for Haskell
uses error recovery to implement Haskell layout. The grammar
has a rule that looks like this:</P
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><PRE
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>close : '}' { () }
| error { () }</PRE
></TD
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><P
>This says that a close brace in a layout-indented context
may be either a curly brace (inserted by the lexical analyser),
or a parse error. </P
><P
>This rule is used to parse expressions like <TT
CLASS="LITERAL"
>let x
= e in e'</TT
>: the layout system inserts an open brace before
<TT
CLASS="LITERAL"
>x</TT
>, and the occurrence of the <TT
CLASS="LITERAL"
>in</TT
> symbol
generates a parse error, which is interpreted as a close brace
by the above rule.</P
><P
>Note for <TT
CLASS="LITERAL"
>yacc</TT
> users: this form of error recovery
is strictly more limited than that provided by <TT
CLASS="LITERAL"
>yacc</TT
>.
During a parse error condition, <TT
CLASS="LITERAL"
>yacc</TT
> attempts to
discard states and tokens in order to get back into a state
where parsing may continue; <SPAN
CLASS="APPLICATION"
>Happy</SPAN
> doesn't do this.
The reason is that normal <TT
CLASS="LITERAL"
>yacc</TT
> error recovery is
notoriously hard to describe, and the semantics depend heavily
on the workings of a shift-reduce parser. Furthermore,
different implementations of <TT
CLASS="LITERAL"
>yacc</TT
> appear to implement
error recovery differently. <SPAN
CLASS="APPLICATION"
>Happy</SPAN
>'s limited error
recovery on the other hand is well-defined, as is just
sufficient to implement the Haskell layout rule (which is why it
was added in the first place).</P
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