2.6. The Error Token
Prev | Chapter 2. Using Happy | Next

## 2.6. The Error Token

Happy supports a limited form of error recovery, using the special symbol `error` in a grammar file. When Happy finds a parse error during parsing, it automatically inserts the `error` symbol; if your grammar deals with `error` explicitly, then it can detect the error and carry on.

For example, the Happy grammar for Haskell uses error recovery to implement Haskell layout. The grammar has a rule that looks like this:

    close : '}'                  { () }
          | error		     { () }

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.

This rule is used to parse expressions like `let x = e in e'`: the layout system inserts an open brace before `x`, and the occurrence of the `in` symbol generates a parse error, which is interpreted as a close brace by the above rule.

Note for `yacc` users: this form of error recovery is strictly more limited than that provided by `yacc`. During a parse error condition, `yacc` attempts to discard states and tokens in order to get back into a state where parsing may continue; Happy doesn't do this. The reason is that normal `yacc` error recovery is notoriously hard to describe, and the semantics depend heavily on the workings of a shift-reduce parser. Furthermore, different implementations of `yacc` appear to implement error recovery differently. Happy'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).

Prev | Up | Next
2.5. Monadic Parsers | Home | 2.7. Generating Multiple Parsers From a Single Grammar