Re: Sometimes keywords

On 03/30/2016 09:37 AM, Gracjan Polak wrote:
> Hi,
>
> I'm working on Haskell grammar and there is a language construct when an
> identifier is a keyword only in specific context, for example 'family'
> is ordinary identifier unless it follows 'type', for example:
>
> type family X a b = .... (a keyword here)
>
> let family = Flistones in .... (a variable here)
>
> How do I create this context sensitivity in Wisent/Semantic?

Hi Gracjan,

If you are using wisent for your grammar, and the lexer framework for 
lexical analysis, then things are a bit tricky.  The symbol table system 
in the lexer just blindly converts symbols into keywords if it is a 
textual match.

Likewise, the 'symbol' lexical token is anything that isn't a keyword.

I suppose you could create a 'haskell-symbol' rule that is either a 
regular symbol, or the FAMILY identifier, and then convert FAMILY back 
into a string "family", but I suspect there may be complications there.

If you want to handle this all in the lexer, you could (and this leaves 
me feeling a little conflicted) make a copy of 
'semantic-lex-symbol-or-keyword' and augment it.

In your new lex analyzer you would need to:

1) Track the last symbol found.
2) if last symbol is keyword is NOT 'type' then make family (if found) 
be a symbol.

You would then add family as a %keyword in your grammar.

I suspect all this extra string value checking would slow things down a 
bit, but perhaps it will be ok.

I don't have any other better ideas or ready-made support for this concept.

Eric




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