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 ¶
- func ParseFromReader(ior io.Reader, p Parser) (interface{}, error)
- func ParseString(s string, p Parser) (interface{}, error)
- type Clause
- type DescribeClause
- type DispatchClause
- type Logger
- type Parser
- func AnyByte() Parser
- func AnyRune() Parser
- func BigInt() Parser
- func Byte(b byte) Parser
- func Char(r rune) Parser
- func CharPred(pred func(rune) bool) Parser
- func DelimitedString(beginDelimiter, endDelimiter string) Parser
- func DiscardLeft(left, right Parser) Parser
- func DiscardRight(left, right Parser) Parser
- func Dispatch(clauses ...DispatchClause) Parser
- func Error(err error) Parser
- func ErrorTransformer(parser Parser, transformer func(error) (interface{}, error)) Parser
- func Except(parser, except Parser) Parser
- func Float() Parser
- func Int() Parser
- func JoinString(parser Parser) Parser
- func Many(parser Parser) Parser
- func Optional(parser Parser) Parser
- func Or(parsers ...Parser) Parser
- func Recursive(factory func() Parser) Parser
- func RunesUntil(endCondition Parser) Parser
- func Sep(item, separator Parser) Parser
- func Seq(parsers ...Parser) Parser
- func Some(parser Parser) Parser
- func SplicingSeq(parsers ...Parser) Parser
- func String(expected string) Parser
- func StringCI(expected string) Parser
- func SwallowLeadingWhitespace(parser Parser) Parser
- func SwallowTrailingWhitespace(parser Parser) Parser
- func SwallowWhitespace(parser Parser) Parser
- func Transformer(parser Parser, transformer func(interface{}) (interface{}, error)) Parser
- func WithLogging(parser Parser, logger Logger) Parser
- func WithStdLogging(parser Parser, prefix string) Parser
- type Reader
- type Scanner
- type StringJoiningClause
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
func ParseFromReader ¶
ParseFromReader parses from an io.Reader.
func ParseString ¶
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) TransformError ¶
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 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) 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 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 ¶
Byte returns a parser used to read a single known byte. A different byte is treated as a parsing error.
func Char ¶
Char returns a parser used to read a single known rune. A different rune is treated as a parsing error.
func CharPred ¶
CharPred returns a parser that parses a single rune as long as it fulfills the given predicate.
func DelimitedString ¶
DelimitedString returns a parser that parses a string between two given delimiter strings and returns the value between.
func DiscardLeft ¶
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 ¶
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 ErrorTransformer ¶
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 ¶
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 Int ¶
func Int() Parser
Int returns a parser that parses an integer. The parsed integer is converted via strconv.Atoi.
func JoinString ¶
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 ¶
Many returns a parser that matches a given parser one or more times. Not matching at all is an error.
func Optional ¶
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 ¶
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 RunesUntil ¶
RunesUntil returns a parser that parses runes as long as the given endCondition parser does not match.
func Sep ¶
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 ¶
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 ¶
Some returns a parser that matches a given parser zero or more times. Not matching at all is not an error.
func SplicingSeq ¶
SplicingSeq returns a parser that works like a Seq but joins slices returned by its subparsers into a single slice.
func String ¶
String returns a parser for a single known string. Different strings are treated as a parsing error.
func StringCI ¶
StringCI returns a case-insensitive parser for a single known string. Different strings are treated as a parsing error.
func SwallowLeadingWhitespace ¶
SwallowLeadingWhitespace wraps a parser so that it removes leading whitespace.
func SwallowTrailingWhitespace ¶
SwallowTrailingWhitespace wraps a parser so that it removes trailing whitespace.
func SwallowWhitespace ¶
SwallowWhitespace wraps a parser so that it removes leading and trailing whitespace.
func Transformer ¶
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 ¶
WithLogging wraps a parser so that calls to it are logged to a given logger.
func WithStdLogging ¶
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.
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 ¶
NewScanner returns a new scanner using a given Reader and Parser.
func (Scanner) Err ¶
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 ¶
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!
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) TransformResult ¶
func (s StringJoiningClause) TransformResult(vals []interface{}) interface{}
TransformResult joins runes and strings together like JoinString.
Source Files ¶
Directories ¶
Path | Synopsis |
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examples
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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.
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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. |