README.md

Compile Time C++ Parser Combinators

Demonstration of compile time parser combinators as a C++ template metaprogram. Allows parsing arbitrary strings at compile time to produce a type result.

Example shows the parsing of Apple's autolayout visual format strings at compile time to check their validity. This could easily be extended to produce an type representing the contents of the visual format string.

Syntax of parsers is very readable:

struct orientation : choice<character<'H'>, character<'V'>> { };

struct superview : character<'|'> { };

struct relation : choice<
    string<'=', '='>,
    string<'<', '='>,
    string<'>', '='>> { };

struct number : many1<anyDigit> { };

struct viewName : identifier { };

struct priority : choice<identifier, number> { };

struct constant : choice<identifier, number> { };

struct objectOfPredicate : choice<constant, viewName> { };

struct predicate : seq<
    optional<None, relation>,
    objectOfPredicate,
    optional<None, next<character<'@'>, priority>>> { };

struct predicateList : sepBy1<character<','>, predicate> { };

struct predicateListWithParens :
    between<character<'('>, character<')'>,
        predicateList> { };

struct connection : choice<
    between<character<'-'>, character<'-'>,
        predicateList>,
    character<'-'>,
    always<None>> { };

struct view :
    between<character<'['>, character<']'>,
        next<
            viewName,
            optional<List<>, predicateListWithParens>>> { };

struct visualFormatString : seq<
    optional<None, then<orientation, character<':'>>>,
    optional<None, next<superview, connection>>,
    view,
    many<next<connection, view>>,
    optional<None, then<connection, superview>>> { };