pars

package module
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 Go to latest Published: Dec 13, 2019 License: BSD-2-Clause Imports: 10 Imported by: 1

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pars - Simple parser combinators for Go

For documentation please read the godoc.

Documentation

Overview

Package pars is a simple framework for parser combinators. It is designed to be easy to use, yet powerful enough to allow solving real world problems.

Parsers can be arranged into flexible stacks of more elemental parsers. Parser results can be transformed via transformers to easily convert their results into a more fitting format or to implement additional conditions that must be fulfilled for a successful parse.

Complex parsers can be debugged by wrapping them with a logger.

pars parsers can read from a string or from an io.Reader, so streaming parsing is possible if you need it.

Index

Examples

Constants

This section is empty.

Variables

This section is empty.

Functions

func AssignInto added in v2.1.0 

func AssignInto(dest interface{}) func(interface{})

AssignInto returns a setter for Into parsers. The setter will set a given field to the parsers result value on success and to the zero value on failure.

This function or the generated setter might panic if the types do not fit.

Example 
data := "My name is 'Ark'   "

var name string
parser := Seq(String("My name is"), Into(SwallowWhitespace(RuneDelimitedString('\'', '\'')), AssignInto(&name)), EOF)

_, err := ParseString(data, parser)
if err != nil {
	fmt.Println("Error while parsing:", err)
	return
}

fmt.Printf("Hello, %v!\n", name)

Output:

Hello, Ark!

func AssignIntoSlice added in v2.1.0 

func AssignIntoSlice(dest interface{}) func(interface{})

AssignIntoSlice returns a setter for Into parsers. The setter will set a given slice pointer to the parsers result value on success and to nil on failure.

This function or the generated setter might panic if the types do not fit.

Example 
data := "123,456,789"

var numbers []int

parser := Seq(Into(Sep(Int(), Char(',')), AssignIntoSlice(&numbers)), EOF)

_, err := ParseString(data, parser)
if err != nil {
	fmt.Println("Error while parsing:", err)
	return
}

fmt.Printf("Type safe numbers from parsing:\n%#v\n", numbers)

var sum int
for _, number := range numbers {
	sum += number
}

fmt.Println("Sum:", sum)

Output:

Type safe numbers from parsing:
[]int{123, 456, 789}
Sum: 1368

func ParseFromReader 

func ParseFromReader(ior io.Reader, p Parser) (interface{}, error)

ParseFromReader parses from an io.Reader.

func ParseString 

func ParseString(s string, p Parser) (interface{}, error)

ParseString is a helper function to directly use a parser on a string.

Types

type Clause 

type Clause []Parser

Clause is the most simple DispatchClause. It is just a slice of parsers without any transformations.

func (Clause) Parsers 

func (c Clause) Parsers() []Parser

Parsers returns the parser slice for this clause.

func (Clause) TransformError 

func (c Clause) TransformError(err error) error

TransformError returns the given error unchanged.

func (Clause) TransformResult 

func (c Clause) TransformResult(val []interface{}) interface{}

TransformResult returns the only value if the slice of values has only one element. Otherwise it returns the slice of values unchanged.

type ClonableDispatchClause added in v2.1.0 

type ClonableDispatchClause interface {
	DispatchClause
	//Clone creates a functionally equivalent clause with unique state.
	Clone() ClonableDispatchClause
}

ClonableDispatchClause is the interface of stateful DispatchClause types that need clones with unique state.

type DescribeClause 

type DescribeClause struct {
	DispatchClause
	Description string
}

DescribeClause extends the error message of a clause so that a custom description is part of the message.

func (DescribeClause) Clone added in v2.1.0 

func (d DescribeClause) Clone() DispatchClause

Clone calls clone of the inner clause if implemented.

func (DescribeClause) TransformError 

func (d DescribeClause) TransformError(err error) error

TransformError extends the error message of a clause so that a custom description is part of the message.

type DispatchClause 

type DispatchClause interface {
	//Parsers returns the parsers of the clause.
	Parsers() []Parser
	//TransformResult allows the DispatchClause to combine the results of its parsers to a single result.
	TransformResult([]interface{}) interface{}
	//TransformError allows the DispatchClause to replace or extend the error returned on a failed match.
	TransformError(error) error
}

DispatchClause is the interface of a clause used by Dispatch.

type InsteadOfClause added in v2.1.0 

type InsteadOfClause struct {
	DispatchClause
	Result interface{}
}

InsteadOfClause extends a clause so that it returns a fixed value as if it was wrapped in an InsteadOf.

func (InsteadOfClause) Clone added in v2.1.0 

func (i InsteadOfClause) Clone() DispatchClause

Clone calls clone of the inner clause if implemented.

func (InsteadOfClause) TransformResult added in v2.1.0 

func (i InsteadOfClause) TransformResult(_ []interface{}) interface{}

TransformResult returns the Result as if the clause was wrapped in an InsteadOf.

type InsteadOfDerefClause added in v2.1.0 

type InsteadOfDerefClause struct {
	DispatchClause
	Result interface{}
}

InsteadOfDerefClause extends a clause so that it returns a value that will be dereferenced as if the clause was wrapped in an InsteadOfDeref.

func (InsteadOfDerefClause) Clone added in v2.1.0 

func (i InsteadOfDerefClause) Clone() DispatchClause

Clone calls clone of the inner clause if implemented.

func (InsteadOfDerefClause) TransformResult added in v2.1.0 

func (i InsteadOfDerefClause) TransformResult(_ []interface{}) interface{}

TransformResult returns the dereferenced Result as if the clause was wrapped in an InsteadOfDeref.

type Logger 

type Logger interface {
	Println(...interface{})
}

Logger is anything that lines can be printed to.

type Parser 

type Parser interface {
	//Parse is used for the actual parsing. It reads from the reader and returns the result or an error value.
	//
	//Each parser must remember enough from the call to this method to undo the reading in case of a parsing error that occurs later.
	//
	//When Parse returns with an error, Parse must make sure that all read bytes are unread so that another parser could try to parse them.
	Parse(*Reader) (interface{}, error)
	//Unread puts read bytes back to the reader so that they can be read again by other parsers.
	Unread(*Reader)
	//Clone creates a parser that works the same as the receiver. This allows to create a single parser as a blueprint for other parsers.
	//
	//Internal state from reading operations should not be cloned.
	Clone() Parser
}

Parser contains the methods that each parser in this framework has to provide. 

var EOF Parser = eof{}

EOF is a parser that never yields a value but that succeeds if and only if the source reached EOF

func AnyByte 

func AnyByte() Parser

AnyByte returns a parser that reads exactly one byte from the source.

func AnyRune 

func AnyRune() Parser

AnyRune returns a parser that parses a single valid rune. If no such rune can be read, ErrRuneExpected is returned.

func BigInt 

func BigInt() Parser

BigInt returns a parser that parses an integer. The parsed integer is returned as a math/big.Int.

func Byte 

func Byte(b byte) Parser

Byte returns a parser used to read a single known byte. A different byte is treated as a parsing error.

func Char 

func Char(r rune) Parser

Char returns a parser used to read a single known rune. A different rune is treated as a parsing error.

func CharPred 

func CharPred(pred func(rune) bool) Parser

CharPred returns a parser that parses a single rune as long as it fulfills the given predicate.

func DelimitedString 

func DelimitedString(beginDelimiter, endDelimiter string) Parser

DelimitedString returns a parser that parses a string between two given delimiter strings and returns the value between.

func DiscardLeft 

func DiscardLeft(left, right Parser) Parser

DiscardLeft returns a parser that calls two other parsers but only returns the result of the second parser. Both parsers must succeed.

Example 
data := "$123"

dollarParser := DiscardLeft(Char('$'), Int())

result, err := ParseString(data, dollarParser)
if err != nil {
	fmt.Println("Error while parsing:", err)
	return
}

fmt.Printf("%v: %T\n", result, result)

Output:

123: int

func DiscardRight 

func DiscardRight(left, right Parser) Parser

DiscardRight returns a parser that calls two other parsers but only returns the result of the first parser. Both parsers must succeed.

func Dispatch 

func Dispatch(clauses ...DispatchClause) Parser

Dispatch returns a parser that is like a combination of Seq and Or with limited backtracking.

A Dispatch contains multiple clauses consisting of parsers. Dispatch parses by trying the clauses one by one. The first matching clause is used, later clauses are not tried. Each clause can contain multiple parsers. Clauses are special because they limit the backtracking: If the first parser of a clause matches, that clause is selected even if a later parser of that clause fails. If no clause matches, the error from the last clause is returned.

The motivation for limited backtracking is in better error reporting. When an Or parser fails, all you know is that not a single parser succeeded. When a Dispatch parser fails after a clause was selected, you know which subclause was supposed to be parsed and can return a fitting error message.

func DispatchSome added in v2.1.0 

func DispatchSome(clauses ...DispatchClause) Parser

DispatchSome is a parser that combines Dispatch and Some. Like Dispatch, it tries to find a clause where the first parser succeeds and then commits to that clause. If the whole clause succeeds, the TransformResult method is called as usual. If a committed clause fails, the whole parser fails and returns the corresponding error (transformed by the TransformError method).

Unlike Dispatch, after a successful parse another round starts with the first clause again. DispatchSome parses until either an error occurs or no single clause matched. Different from Dispatch, if no clause matches, it is not an error but marks the end of the loop.

A successful DispatchSome returns a slice of all single results from each Dispatch round. This slice may be empty.

func Error 

func Error(err error) Parser

Error returns a parser that always fails with the given error

func ErrorTransformer 

func ErrorTransformer(parser Parser, transformer func(error) (interface{}, error)) Parser

ErrorTransformer wraps a parser so that an error result is transformed according to the given function. If the wrapped parser was successful, the result is not changed.

func Except 

func Except(parser, except Parser) Parser

Except returns a parser that wraps another parser so that it fails if a third, excepted parser would succeed.

func Float 

func Float() Parser

Float returns a parser that parses a floating point number. The supported format is an optional minus sign followed by digits optionally followed by a decimal point and more digits.

func InsteadOf added in v2.1.0 

func InsteadOf(parser Parser, value interface{}) Parser

InsteadOf wraps a parser so that a different given value is used as a result on success.

func InsteadOfDeref added in v2.1.0 

func InsteadOfDeref(parser Parser, value interface{}) Parser

InsteadOfDeref wraps a parser so that a different given value is used as a result on success. The given value has to be a pointer to something (not nil) and will be dereferenced.

func Int 

func Int() Parser

Int returns a parser that parses an integer. The parsed integer is converted via strconv.Atoi.

func Into added in v2.1.0 

func Into(parser Parser, setter func(interface{})) Parser

Into wraps a parser so that a successful parse calls the given setter. If the wrapped parser fails, the setter is not called. If the returned parser succeeds at first, but is unread later, the setter will be called again with nil as the value.

It is recommended that the variable the setter writes into is not read from other parsers (e.g. via Into or Transformer) as it might become confusing to understand which parser has seen which setter result at which time.

Also, Into should be used as outmost as possible. A parser like 'Some(Into(AnyRune(), setter))' will have its setter called many times, so each value will be overwritten by the next one. On the other hand, you can use this for a setter that appends all values into a slice instead of just setting a single variable. Just have a clear idea of what your setter will do.

func JoinString 

func JoinString(parser Parser) Parser

JoinString wraps a parser that returns a slice of runes or strings so that it returns a single string instead. Runes and strings can be mixed in the same slice. The slice also can contain other slices of runes and strings, recursively.

The returned parser WILL PANIC if the wrapped parser returns something that is not a slice of runes or strings!

func Many 

func Many(parser Parser) Parser

Many returns a parser that matches a given parser one or more times. Not matching at all is an error.

func Optional 

func Optional(parser Parser) Parser

Optional returns a parser that reads exactly one result according to a given other parser. If it fails, the error is discarded and nil is returned.

func Or 

func Or(parsers ...Parser) Parser

Or returns a parser that matches the first of a given set of parsers. A later parser will not be tried if an earlier match was found. The returned parser uses the error message of the last parser verbatim.

Example 
data := "124"

parser := Or(String("123"), String("124"))

result, err := ParseString(data, parser)
if err != nil {
	fmt.Println("Error while parsing:", err)
	return
}

fmt.Printf("%v: %T\n", result, result)

Output:

124: string

func Recursive 

func Recursive(factory func() Parser) Parser

Recursive allows to recursively define a parser in terms of itself.

func RuneDelimitedString added in v2.1.0 

func RuneDelimitedString(beginDelimiter, endDelimiter rune) Parser

RuneDelimitedString returns a parser that parses a string between two given delimiter runes and returns the value between.

func RunesUntil 

func RunesUntil(endCondition Parser) Parser

RunesUntil returns a parser that parses runes as long as the given endCondition parser does not match.

func Sep 

func Sep(item, separator Parser) Parser

Sep returns a parser that parses a sequence of items according to a first parser that are separated by matches of a second parser.

func Seq 

func Seq(parsers ...Parser) Parser

Seq returns a parser that matches all of its given parsers in order or none of them.

Example 
data := "$123"

dollarParser := Seq(Char('$'), Int())

result, err := ParseString(data, dollarParser)
if err != nil {
	fmt.Println("Error while parsing:", err)
	return
}

values := result.([]interface{})
fmt.Printf("%c: %T\n", values[0], values[0])
fmt.Printf("%v: %T\n", values[1], values[1])

Output:

$: int32
123: int

func Some 

func Some(parser Parser) Parser

Some returns a parser that matches a given parser zero or more times. Not matching at all is not an error.

func SplicingSeq 

func SplicingSeq(parsers ...Parser) Parser

SplicingSeq returns a parser that works like a Seq but joins slices returned by its subparsers into a single slice.

func String 

func String(expected string) Parser

String returns a parser for a single known string. Different strings are treated as a parsing error.

func StringCI 

func StringCI(expected string) Parser

StringCI returns a case-insensitive parser for a single known string. Different strings are treated as a parsing error.

func SwallowLeadingWhitespace 

func SwallowLeadingWhitespace(parser Parser) Parser

SwallowLeadingWhitespace wraps a parser so that it removes leading whitespace.

func SwallowTrailingWhitespace 

func SwallowTrailingWhitespace(parser Parser) Parser

SwallowTrailingWhitespace wraps a parser so that it removes trailing whitespace.

func SwallowWhitespace 

func SwallowWhitespace(parser Parser) Parser

SwallowWhitespace wraps a parser so that it removes leading and trailing whitespace.

func Transformer 

func Transformer(parser Parser, transformer func(interface{}) (interface{}, error)) Parser

Transformer wraps a parser so that the result is transformed according to the given function. If the transformer returns an error, the parsing is handled as failed.

Example 
package main

import (
	"bitbucket.org/ragnara/pars/v2"
	"fmt"
)

// Celsius contains a temperature in degree celsius.
type Celsius int

func (c Celsius) String() string {
	return fmt.Sprintf("%v°C", int(c))
}

// TemperatureParser is a parser for temperature strings returning Celsius instances.
type TemperatureParser struct {
	pars.Parser
}

// NewTemperatureParser creates a new TemperatureParser instance.
func NewTemperatureParser() TemperatureParser {
	//Define the format
	simpleParser := pars.Seq(pars.Int(), pars.Or(pars.String("°C"), pars.String("°F")))
	//Add an conversion
	transformedParser := pars.Transformer(simpleParser, transformParsedTemperatureToCelsius)

	return TemperatureParser{Parser: transformedParser}
}

// Parse returns the Celsius instance for a temperature string containing an integer followed by either "°C" or "°F". Fahrenheit strings are converted to celsius.
// For other strings, an error is returned.
func (t TemperatureParser) Parse(s string) (Celsius, error) {
	val, err := pars.ParseString(s, t.Parser)
	if err != nil {
		return Celsius(0), err
	}
	return val.(Celsius), nil
}

// MustParse parses exactly like Parse but panics if an invalid string was found. It should not be used on user input!
func (t TemperatureParser) MustParse(s string) Celsius {
	val, err := t.Parse(s)
	if err != nil {
		panic(err)
	}
	return val
}

func transformParsedTemperatureToCelsius(parserResult interface{}) (interface{}, error) {
	values := parserResult.([]interface{})
	degrees := values[0].(int)
	unit := values[1].(string)

	switch unit {
	case "°C":
		return Celsius(degrees), nil
	case "°F":
		return Celsius((degrees - 32) * 5 / 9), nil
	default:
		panic("Impossible unit: " + unit)
	}
}

func main() {
	sample1 := "32°C"
	sample2 := "104°F"
	sample3 := "128K"

	fmt.Println("Sample1:", NewTemperatureParser().MustParse(sample1))
	fmt.Println("Sample2:", NewTemperatureParser().MustParse(sample2))

	val, err := NewTemperatureParser().Parse(sample3)
	fmt.Println("Sample3:", val)
	fmt.Println("Sample3 error:", err.Error())

}

Output:

Sample1: 32°C
Sample2: 40°C
Sample3: 0°C
Sample3 error: Could not find expected sequence item 1: Could not parse expected string "°F": EOF

func WithLogging 

func WithLogging(parser Parser, logger Logger) Parser

WithLogging wraps a parser so that calls to it are logged to a given logger.

func WithStdLogging 

func WithStdLogging(parser Parser, prefix string) Parser

WithStdLogging wraps a parser so that calls to it are logged to a logger logging to StdErr with a given prefix.

type Reader 

type Reader struct {
	// contains filtered or unexported fields
}

Reader is an io.Reader that can Unread as many bytes as necessary.

func NewReader 

func NewReader(r io.Reader) *Reader

NewReader creates a new Reader from an io.Reader.

func (*Reader) Read 

func (br *Reader) Read(p []byte) (n int, err error)

Read reads a slice of bytes.

func (*Reader) Unread 

func (br *Reader) Unread(p []byte)

Unread unreads a slice of bytes so that they will be read again by Read.

type Scanner 

type Scanner struct {
	// contains filtered or unexported fields
}

Scanner provides a convenient interface to use a single parser multiple times on the same reader.

Successive calls to Scan will parse the input and allow the results to be accessed one at a time.

Scanner stops at the first error.

Example 
data := "this is a text of words"
reader := NewReader(strings.NewReader(data))

wordParser := SwallowTrailingWhitespace(JoinString(RunesUntil(CharPred(unicode.IsSpace))))

scanner := NewScanner(reader, wordParser)

for scanner.Scan() {
	fmt.Println(scanner.ResultString())
}
fmt.Println(scanner.Err())

Output:

this
is
a
text
of
words
<nil>

func NewScanner 

func NewScanner(r *Reader, p Parser) Scanner

NewScanner returns a new scanner using a given Reader and Parser.

func (Scanner) Err 

func (s Scanner) Err() error

Err returns the last encountered error that is not io.EOF. It returns nil otherwise.

func (Scanner) Result 

func (s Scanner) Result() interface{}

Result returns the most recently parsed value from a call to Scan.

func (Scanner) ResultString 

func (s Scanner) ResultString() string

ResultString returns the most recently parsed value from a call to Scan, cast to a String.

This will panic if the last result is not a string!

func (*Scanner) Scan 

func (s *Scanner) Scan() bool

Scan invokes the parser on the reader and makes the results available via Result and Err.

Scan returns true if the parsing succeeded and returns false otherwise.

type StringJoiningClause 

type StringJoiningClause struct {
	DispatchClause
}

StringJoiningClause extends a clause that consists of parsers that return runes or strings so that it returnes a single string instead. Slices are handled recursively.

StringJoiningClause WILL PANIC if any of the parsers return something other than a rune or a string or a slice of these types.

func (StringJoiningClause) Clone added in v2.1.0 

func (s StringJoiningClause) Clone() DispatchClause

Clone calls clone of the inner clause if implemented.

func (StringJoiningClause) TransformResult 

func (s StringJoiningClause) TransformResult(vals []interface{}) interface{}

TransformResult joins runes and strings together like JoinString.

Source Files

View all Source files

Directories

Path Synopsis
examples
pars-calc
pars-calc is a small cli calculator that takes floats or calculations of floats consisting of additions, substractions, multiplications or divisions on StdIn, parses them via the parser implemented in parser.go into something easily evaluable, and prints the result of the calculation.
 pars-calc is a small cli calculator that takes floats or calculations of floats consisting of additions, substractions, multiplications or divisions on StdIn, parses them via the parser implemented in parser.go into something easily evaluable, and prints the result of the calculation.

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