[TxMt] language grammar question

[Allan Odgaard]

On 23/11/2005, at 14:26, Sylvain Benilan wrote:

> My question is about a language description design. I tried (but  
> failed) to design the grammar such anything that is a valid CLAIRE  
> construct is described by a specific pattern and anything else  
> falls into the scope of 'invalid.illegal.something'. For instance,  
> in CLAIRE, a variable scope is introduced with a 'let' construct :
>
> 	let x := 1, y := 2 in (x + y) // valid
> 	let x := 1, y := 2 *BAD* in (x + y) // invalid : *BAD* illegal
>
> The rule that describe the 'let' construct would look like :
>
> 	let <#var-def-list> in <#any>
>
> Which can't be described with a begin/end pattern, unless I can  
> reference a repository rule from whitin the begin/end pattern. Can  
> we do that ?

It is possible to include a repository rule as one of the sub-rules  
inside begin/end. But what you want to do is not straightforward. The  
Property List (Old-Style) does rather strict matching, e.g. it'll  
allow: ( foo, bar ) as an array, but not: ( foo bar ) or ( foo, , bar ).

The way this is done is by matching everything that is valid in the  
current context, and then have a catch-all rule last, which matches  
the leftovers as invalid.

So for example for the let … in part, we could make a rule like this:

    begin = "\blet\b";
    end = "\bin\b";
    patterns = (
       { include = "#variable-declaration"; }, // should match  
optional trailing comma
       { match = "\S"; name = "invalid.illegal...."; } // we should  
never reach this!
    );

(\S is everything but whitespace)

To be really strict though, this can quickly get rather complex. Also  
keep in mind, that often syntax highlight lose a bit of value, if it  
doesn't color anything until the complete statement is correct, so  
being 100% strict is not always an advantage.