# 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`

.

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`

.