Header `lexy/dsl/branch.hpp`

The operator>> overload for rules and else_ branch rule.

lexy/dsl/branch.hpp

template <typename T>
concept branch-rule = rule<T> && …;

A branch rule is a rule that has an associated condition. The parsing algorithm can efficiently check whether the condition would match at the current reader position. As such, they are used to make decisions: once a branch condition matches, the branch is taken without backtracking.

Branch rule `lexy::dsl::operator>>`

lexy/dsl/branch.hpp

namespace lexy::dsl
{
    struct branch {}; // models branch-rule

    constexpr branch operator>>(branch-rule auto condition, rule auto then);
}

operator>> (branch) turns a rule into a branch rule by giving it another branch rule as condition.

Parsing: Parses condition and then in sequence .
Branch parsing: Tries to parse condition and backtracks if that backtracks. Then parses then.
Errors: All errors raised by condition and then. The rule then fails if they have failed.
Values: All values produced by condition followed by all values of then.

If the result of operator>> is used in the beginning of a sequence with a rule, i.e. as (condition >> rule1) + rule2, this is equivalent to condition >> rule1 + rule2, i.e. the original branch is extended. rule1 + (condition >> rule2), on the other hand, is equivalent to rule1 + condition + rule2.

Example 1. Only parse a fraction if preceded by a dot

struct production
{
    static constexpr auto rule = [] {
        auto integer  = dsl::digits<>.no_leading_zero();
        auto fraction = dsl::digits<>;

        return integer + dsl::if_(dsl::period >> fraction);
    }();
};

Note	A branch rule is only parsed as a branch if its required. If you just use it by itself, the branch condition does not matter. Use `lexy::dsl::if_` to conditionally execute a branch.

Caution

If then is also a branch rule, it will still only use condition as the condition.

Branch rule `lexy::dsl::else_`

lexy/dsl/branch.hpp

namespace lexy::dsl
{
    struct else // note: not a rule itself
    {
        friend constexpr branch-rule operator>>(else, rule auto rule);
    };

    constexpr else else_;
}

else_ is a tag that can be used as the condition for operator>> to indicate that the rule should always be taken.

(Branch) Parsing: Parses rule without every backtracking.
Errors: All errors raised by parsing rule.
Values: All values produced by rule.

Example 2. If it is not a known greeting, require that the empty is empty

struct production
{
    static constexpr auto rule
        // Input should be empty if greeting isn't known.
        = LEXY_LIT("Hello") | LEXY_LIT("Hi") | dsl::else_ >> dsl::eof;
};

Branch rule lexy::dsl::operator>>

Branch rule lexy::dsl::else_

See also

Branch rule `lexy::dsl::operator>>`

Branch rule `lexy::dsl::else_`