Header lexy/dsl/context_identifier.hpp

Rule DSL lexy::dsl::context_identifier

lexy/dsl/context_identifier.hpp
namespace lexy
{
    struct different_identifier {};
}

namespace lexy::dsl
{
    struct context_identifier-dsl // note: not a rule itself
    {
        constexpr rule auto create() const;

        constexpr branch-rule auto capture() const;

        constexpr see-below rematch() const;
    };

    template <typename Id>
    constexpr context_identifier-dsl context_identifier(identifier-dsl identifier);
}

context_identifier is not a rule, but a DSL for specifying rules that manipulate an lexy::dsl::identifier  variable of the current context.

Example 1. Parse the same word twice
struct production
{
    static constexpr auto rule = [] {
        // Declare an identifier variable - it is not created yet!
        auto word     = dsl::identifier(dsl::ascii::alpha);
        auto word_var = dsl::context_identifier<production>(word);

        // Parse a word and capture it in the variable.
        auto first_word = word_var.capture();
        // Parse another word and compare it agains the variable.
        auto second_word = word_var.rematch();

        // Create the empty variable, then parse the two words.
        return word_var.create() + first_word + dsl::lit_c<','> + second_word;
    }();
};

Rule .create()

lexy/dsl/context_identifier.hpp
constexpr rule auto create() const;

.create() returns a rule that creates the identifier variable.

Parsing

Matches everything, without consuming anything. As a side effect, it creates an identifier with name Id inside the current context. If a context variable (of any) type with that name already exists, it is shadowed. This identifier is initialized to the empty string; it’s underlying type is lexy::lexeme .

Errors

None.

Values

None.

Rule .capture()

lexy/dsl/context_identifier.hpp
constexpr branch-rule auto capture() const;

.capture() returns a branch rule that parses an identifier and stores it in the variable.

Requires

The identifier with the name Id has been created in the current context, i.e. .create() has been parsed earlier.

(Branch) Parsing

Parses identifier. As a side effect, it sets the identifier with the name Id to contain the lexy::lexeme  also produced by the lexy::dsl::identifier  rule.

Errors

All errors raised by parsing identifier. The rule then fails if identifier has failed.

Values

The value produced by parsing identifier, which is the same as the one stored in the variable.

Rule .rematch()

lexy/dsl/context_identifier.hpp
struct rematch-dsl // models branch-rule
{
    template <typename Tag>
    static constexpr rematch-dsl auto error;
};

constexpr rematch-dsl rematch() const;

.rematch() returns a branch rule that matches the stored identifier again..

Requires

The identifier with the name Id has been created in the current context, i.e. .create() has been parsed earlier.

Parsing

Parses identifier and checks it against the one stored in the variable.

Branch parsing

Parses identifier and backtracks when that fails. Also backtracks when the new identifier does not match the old one.

Errors

  • All errors raised by parsing identifier. The rule then fails if identifier has failed.

  • A generic error with the specified Tag or lexy::different_identifier if the new identifier does not match the identifier stored in the variable with name Id of the current context. Its range covers everything consumed by identifier and the rule then fails.

Values

None; discards the value produced by identifier.

See also