How to provide better error messages?

[csells]

If I have a parser like so:

  Parser<String> id() => ((letter() | char('_')) &
          (letter() | digit() | anyOf('_- ()')).star().flatten())
      .map((vals) => vals.listOf<String>().join());

and I feed it input like 'foo.1', I'll get an error like this:

"\n" expected at 1:4

but the error I want to supply is:

Illegal character '.' found in id.

How can I get better error messages in my parsers?

[renggli]

The problem in your example is that foo is a perfectly valid identifier as part of your specification and the id parser succeeds. The error message "\n" expected at 1:4 is produced somewhere else in your grammar.

A common way to fix this is to use a negative lookahead. With it you can assert that your ID is not followed by a dot, i.e.

Parser<String> id() => (((letter() | char('_')) &
            (letter() | digit() | anyOf('_- ()')).star() &
            char('.').not('end of id expected')))
        .flatten();

For the input foo.1 this gives end of id expected at 1:4.

Note: I also moved the flatten operator around everything, then you don't need the custom flattening with the map operator.