What does the syntax condition `!` (labelled "except") do?

The Rascal grammar contains a production for syntax rules that's not documented:

| except: Sym symbol "!" NonterminalLabel

It acts syntactically like a follow condition and is in a section commented as "conditional". I see that it's used in the Rascal grammar itself. The NonterminalLabel is for individual production rules (not a production with all its alternates). So what does this condition do?

Solution

if ‘E ! add’ occurs in a rule then it simply means E but the rule labeled "add" for E will be as-if not existing. This restriction goes only one level deep, so recursive E's will have the "add" rule again.

For example:

syntax E 
  = id: Id
  | app: E "(" {E!comma ","}* ")"
  > left mul: E "*" E
  > left add: E "+" E
  > right comma: E "," E
  ;

The E in {E ","}* of the function application rule is restricted to not be the comma expression, in order to avoid syntactic ambiguity.

The benefit here is that you do not have to introduce yet another non-terminal which also logically represents expressions.
The pitfall is that the ! operator is a hard restriction and thus may make the accepted language actually smaller: use of ! may lead to unexpected parse errors if not used carefully.

Side note: The ! operator is the primitive constraint type used to model the priority (>) and associative (left, right) semantics as well, but those are transitive and checked for safety, etc. This on the other hand is a brutal removal of a definition and thus may change the accepted sentences of your language.