llvm-project/flang/documentation/ParserCombinators.md

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,
2. defines a type named resultType, and
3. provides a function (const member or static) that accepts a pointer to a ParseState as its argument and returns a std::optional<resultType> as a result, with the presence or absence of a value in the std::optional<> signifying success or failure, respectively.

std::optional<resultType> 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<T>(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.
• rawNextChar returns the next raw character, and fails at EOF.
• cookedNextChar returns the next character after preprocessing, skipping Fortran line continuations and comments; it also 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<T> 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<T>{}(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.
• inCharLiteral succeeds under withinCharLiteral.
• inFortran succeeds unless in a preprocessing directive.
• inFixedForm succeeds in fixed-form 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.

Monadic Combination

When parsing depends on the result values of earlier parses, the monadic bind combinator is available. Please try to avoid using it, as it makes automatic analysis of the grammar difficult. It has the syntax p >>= f, and it constructs a parser that matches p, yielding some value x on success, then matches the parser returned from the function call f(x).

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 cookedNextChar.

• 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 / "]".
• withinCharLiteral(p) applies the parser p, tokenizing for CHARACTER/Hollerith literals.
• 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, the parser "..."_debug emits the string to the standard error and succeeds. It is useful for tracing while debugging a parser but should obviously not be committed for production code.