Q: how to restrict a rule

[cppden]

Is there a way to restrict matching in the following case:

struct param_a : seq< istring<'a','='>, digit > {};
struct param_b : seq< istring<'b','='>, alpha > {};
struct param_other : seq< alpha, one<'='>, alnum > {};
struct params : sor< param_a, param_b, param_other > {};

Namely, I don't want grammar to accept neither 'a=c' nor 'b=3' but the last choice allows it. Please note that this is simplified example and in reality there's about 10 choices with lengthy name part (instead of 'a' and 'b'). Therefore it's really hard and clumsy to exclude in param_other case all previous cases. I wonder if there's more elegant way to restrict the rule?
Thanks a lot!

[ColinH]

What we usually do in this situation is to use if_must<> instead of seq<> in param_a and param_b. (That way there will be no backtracking once you get past the "distinguishing" part of the sor<>-branch.) If, as per the other question, you are trying to eliminate the use of must<>, you could use the new minus<> rule to exclude the first cases from param_other, but that is slow and ugly. (Probably very much more slow and ugly than using must<> and catching the exceptions.)

[d-frey]

You could use nested if_then_else rules, e.g.

if_then_else< istring<'a','='>, digit,
if_then_else< istring<'b','='>, alpha,
param_other > >

which is strangely formatted when it comes to nesting and which leaves the question of where to attach the actions. But from a pure parsing POV, it will do exactly what you need.

I think we could also easily build a better interface, maybe a "sequenced switch" with an interface like

sswitch< istring<'a','='>, digit,
         istring<'b','='>, alpha,
         param_other >

and maybe allow specializing on sswitch_case<istring<'a','='>> for the actions and passing the whole matching input of the case. @ColinH What do you think?

[cppden]

Thanks a lot! If-then-else looks fine to me, and I hope actions can be attached to digit and alpha since their values are all what's needed.
Regarding sequenced switch it seems less readable/explicit at 1st glance... And personally I prefer minimal interface. :) More alternatives, more fuss.
Thank you very much for the help and great lib!
And pardon me for responding much slower than you reply to my questions. ;)

[d-frey]

You could attach actions to digit or alpha, but that will most probably lead to a lot of problems as the rest of your grammar might also contain those (even param_other in your example). The solution is, as usual, to create anchor points explicitly:

struct intro_a : istring<'a','='> {};
struct value_a : digit {};

struct intro_b : istring<'b','='> {};
struct value_b : alpha {};

struct other : seq< alpha, one<'='>, alnum > {};

struct params : if_then_else< intro_a, value_a,
                if_then_else< intro_b, value_b,
                other > > {};

Then just attach actions to value_a, etc.

Also: Phew! Good to know I'm not the only one who prefers minimalistic interfaces :)