Felix syntax grammars now support groups. For example:
nlist := lpar name (,name)* rpar =># "`(cons ,_2 ,(map car _3))";
First, note that ( and ) must be recognised as tokens with
the special predefined nonterminals lpar, rpar, because now
the symbols ( and ) have a new meaning.
Now, see that ( and ) define a new nonterminal:
__grp_42 := , name =># "`(,_2)";
whose name is replaced in the original production:
nlist := lpar name __grp_42* rpar =># "`(cons ,_2 ,(map car _3))";
Now, examine closely the generated user action: it is always
a list of the nonterminals of the group. Take care noting
that special symbols like 'lpar' are nonterminals!
For this reason __grp_42 returns a list of a list of a single name,
instead of a list of names, so we map 'car' over that list to make
a list of names instead (car is Schemish for 'hd', the first element
of a list).
[Flame: I thought scheme was supposed to be good at lists,
but Ocaml is miles better: lack of pattern matching in Scheme
is a serious deficiency .. I'm temped to add it to Ocs Scheme.]
--
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net
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