Re: [Caml-list] Matching start of input in lexer created with ocamllex

["Janne Hellsten" <jjhellst@gmail.com>]

On 4/6/07, David Allsopp <dra-news@metastack.com> wrote:
> > I'd like to match the beginning of input (or beginning of line) in my
> > lexer.  Is there an easy way to do that?
> Ocamllex doesn't have a notion for beginning of line. Three possible
> solutions:
>
> 1. You can simulate it with a bool ref parameter to your lexer that gets set
> to true by each rule to indicate that you're no longer at the beginning of a
> line - the "!for" rule than raises Failure if it matches when this ref is
> true. Slightly tedious for code maintenance...
> 2. You use two lexers - one with the "!for" rule and one without and call
> one lexer from the other (not very nice, because ocamllex doesn't support
> code reuse between lexers so you'll be duplicating a lot of code).
> 3. Pre-process the input to ocamllex to include a special character that
> cannot appear in your text and place that at the beginning of the line (e.g.
> one of the control characters in 0..31).

Thanks, my program was already running its input through a
preprocessor and I thus chose to use solution #3.  I had already
considered solution #2 but came to the same conclusion: it leads to
code duplication.

Janne