## Concept The Fortran language recognizer here can be classified as an LL recursive descent parser. It is composed from a *parser combinator* library that defines a few fundamental parsers and a few ways to compose them into more powerful parsers. For our purposes here, a *parser* is any object that attempts to recognize an instance of some syntax from an input stream. It may succeed or fail. On success, it may return some semantic value to its caller. In C++ terms, a parser is any instance of a class that 1. has a `constexpr` default constructor, 1. defines a type named `resultType`, and 1. provides a function (`const` member or `static`) that accepts a pointer to a `ParseState` as its argument and returns a `std::optional` as a result, with the presence or absence of a value in the `std::optional<>` signifying success or failure, respectively. ``` std::optional Parse(ParseState *) const; ``` The `resultType` of a parser is typically the class type of some particular node type in the parse tree. `ParseState` is a class that encapsulates a position in the source stream, collects messages, and holds a few state flags that determine tokenization (e.g., are we in a character literal?). Instances of `ParseState` are independent and complete -- they are cheap to duplicate whenever necessary to implement backtracking. The `constexpr` default constructor of a parser is important. The functions (below) that operate on instances of parsers are themselves all `constexpr`. This use of compile-time expressions allows the entirety of a recursive descent parser for a language to be constructed at compilation time through the use of templates. ### Fundamental Predefined Parsers These objects and functions are (or return) the fundamental parsers: * `ok` is a trivial parser that always succeeds without advancing. * `pure(x)` returns a trivial parser that always succeeds without advancing, returning some value `x`. * `fail(msg)` denotes a trivial parser that always fails, emitting the given message as a side effect. The template parameter is the type of the value that the parser never returns. * `cut` is a trivial parser that always fails silently. * `guard(pred)` returns a parser that succeeds if and only if the predicate expression evaluates to true. * `nextChar` returns the next character, and fails at EOF. ### Combinators These functions and operators combine existing parsers to generate new parsers. They are `constexpr`, so they should be viewed as type-safe macros. * `!p` succeeds if p fails, and fails if p succeeds. * `p >> q` fails if p does, otherwise running q and returning its value when it succeeds. * `p / q` fails if p does, otherwise running q and returning p's value if q succeeds. * `p || q` succeeds if p does, otherwise running q. The two parsers must have the same type, and the value returned by the first succeeding parser is the value of the combination. * `lookAhead(p)` succeeds if p does, but doesn't modify any state. * `attempt(p)` succeeds if p does, safely preserving state on failure. * `many(p)` recognizes a greedy sequence of zero or more nonempty successes of p, and returns `std::list<>` of their values. It always succeeds. * `some(p)` recognized a greedy sequence of one or more successes of p. It fails if p immediately fails. * `skipMany(p)` is the same as `many(p)`, but it discards the results. * `maybe(p)` tries to match p, returning an `std::optional` value. It always succeeds. * `defaulted(p)` matches p, and when p fails it returns a default-constructed instance of p's resultType. It always succeeds. * `nonemptySeparated(p, q)` repeatedly matches "p q p q p q ... p", returning a `std::list<>` of only the values of the p's. It fails if p immediately fails. * `extension(p)` parses p if strict standard compliance is disabled, or with a warning if nonstandard usage warnings are enabled. * `deprecated(p)` parses p if strict standard compliance is disabled, with a warning if deprecated usage warnings are enabled. * `inContext(..., p)` runs p within an error message context. Note that ``` a >> b >> c / d / e ``` matches a sequence of five parsers, but returns only the result that was obtained by matching `c`. ### Applicatives The following *applicative* combinators combine parsers and modify or collect the values that they return. * `construct{}(p1, p2, ...)` matches zero or more parsers in succession, collecting their results and then passing them with move semantics to a constructor for the type T if they all succeed. * `applyFunction(f, p1, p2, ...)` matches one or more parsers in succession, collecting their results and passing them as rvalue reference arguments to some function, returning its result. * `applyLambda([](&&x){}, p1, p2, ...)` is the same thing, but for lambdas and other function objects. * `applyMem(mf, p1, p2, ...)` is the same thing, but invokes a member function of the result of the first parser for updates in place. ### Non-Advancing State Inquiries and Updates These are non-advancing state inquiry and update parsers: * `getColumn` returns the 1-based column position. * `inFixedForm` succeeds in fixed form Fortran source. * `setInFixedForm` sets the fixed form flag, returning its prior value. * `columns` returns the 1-based column number after which source is clipped. * `setColumns(c)` sets the column limit and returns its prior value. ### Token Parsers Last, we have these basic parsers on which the actual grammar of the Fortran is built. All of the following parsers consume characters acquired from `nextChar`. * `spaces` always succeeds after consuming any spaces or tabs * `digit` matches one cooked decimal digit (0-9) * `letter` matches one cooked letter (A-Z) * `CharMatch<'c'>{}` matches one specific cooked character. * `"..."_tok` match the content of the string, skipping spaces before and after, and with multiple spaces accepted for any internal space. (Note that the `_tok` suffix is optional when the parser appears before the combinator `>>` or after `/`.) * `parenthesized(p)` is shorthand for `"(" >> p / ")"`. * `bracketed(p)` is shorthand for `"[" >> p / "]"`. * `nonEmptyListOf(p)` matches a comma-separated list of one or more instances of p. * `optionalListOf(p)` is the same thing, but can be empty, and always succeeds. ### Debugging Parser Last, a string literal `"..."_debug` denotes a parser that emits the string to `std::cerr` and succeeds. It is useful for tracing while debugging a parser but should obviously not be committed for production code.