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Parse

A set of parsing tools for Go inspired by Sprache.

Build up complex parsers from small, simple functions that chomp away at the input.

Input

The input moves along as the parser succeeds.

input := parse.NewInput("ABCD")
item, ok, err := parse.String("A").Parse(input)
// Input is now at index 1.
item, ok, err := parse.String("B").Parse(input)
// Input is now at index 2.
item, ok, err := parse.String("XYZ").Parse(input)
// Input index didn't change and ok=false.
item, ok, err := parse.String("CD").Parse(input)
// Input is now at index 4.

Design

A parser must match the parse.Parser interface, or be created by the use of the parser.Func helper. These 3 parsers are equivalent.

parse.String("<")
parse.Func(func(in *parse.Input) (item string, ok bool, err error) {
	item, _ = in.Peek(1)
	ok = item == "<"
	return
})
type lessThanParser struct{}

func (ltp lessThanParser) Parse(in *parse.Input) (item string, ok bool, err error) {
	item, _ = in.Peek(1)
	ok = item == "<"
	return
}

Functions

Parser functions provide a way of matching patterns in a given input. They are designed to be able to be composed together to make more complex operations.

The examples directory contains several examples of composing the primitive functions.

  • Any
    • Parse any of the provided parse functions, or roll back.
  • AnyRune
    • Parse any rune.
  • AtLeast
    • Parse the provided function at least the number of times specified, or roll back.
  • AtMost
    • Parse the provided function at least once, and at most the number of times specified, or roll back.
  • Letter
    • Parse any letter in the Unicode Letter range or roll back.
  • Many
    • Parse the provided parse function a number of times or roll back.
  • Optional
    • Attempt to parse, but don't roll back if a match isn't found.
  • Or
    • Return the first successful result of the provided parse functions, or roll back.
  • Rune
    • Parse the specified rune (character) or fallback.
  • RuneIn
    • Parse a rune from the input stream if it's in the specified string, or roll back.
  • RuneInRanges
    • Parse a rune from the input stream if it's in the specified Unicode ranges, or roll back.
  • RuneNotIn
    • Parse a rune from the input stream if it's not in the specified string, or roll back.
  • RuneWhere
    • Parse a rune from the input stream if the predicate function passed in succeeds, or roll back.
  • String
    • Parse a string from the input stream if it exactly matches the provided string, or roll back.
  • StringUntil
    • Parse a string from the input stream until the specified until parser is matched.
  • Then
    • Return the results of the first and second parser passed through the combiner function which converts the two results into a single output (a map / reduce operation), or roll back if either doesn't match.
  • Times
    • Parse using the specified function a set number of times or roll back.
  • Until
    • Parse from the input stream until the specified until parser is matched.
  • ZeroToNine
    • Parse a rune from the input stream if it's within the set of 1234567890.

More complex parsers

More complex parsers will need to store the start position, and rollback by using the input's Seek function if the current parser does not match the input.

func ExampleParser() {
	type GotoStatement struct {
		Line int64
	}
	gotoParser := parse.Func(func(in *parse.Input) (item GotoStatement, ok bool, err error) {
		start := in.Index()

		if _, ok, err = parse.String("GOTO ").Parse(in); err != nil || !ok {
			// Rollback, and return.
			in.Seek(start)
			return
		}

		// Read until the next newline or the EOF.
		until := parse.Any(parse.NewLine, parse.EOF[string]())
		var lineNumber string
		if lineNumber, ok, err = parse.StringUntil(until).Parse(in); err != nil || !ok {
			err = parse.Error("Syntax error: GOTO is missing line number", in.Position())
			return
		}
		// We must have a valid line number now, or there is a syntax error.
		item.Line, err = strconv.ParseInt(lineNumber, 10, 64)
		if err != nil {
			return item, false, parse.Error("Syntax error: GOTO has invalid line number", in.Position())
		}

		// Chomp the newline we read up to.
		until.Parse(in)

		return item, true, nil
	})

	inputs := []string{
		"GOTO 10",
		"GOTO abc",
		"FOR i = 0",
	}
	for _, input := range inputs {
		stmt, ok, err := gotoParser.Parse(parse.NewInput(input))
		fmt.Printf("%+v, ok=%v, err=%v\n", stmt, ok, err)
	}
	// Output:
	// {Line:10}, ok=true, err=<nil>
	// {Line:0}, ok=false, err=Syntax error: GOTO has invalid line number: line 0, col 8
	// {Line:0}, ok=false, err=<nil>
}