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Creating parser combinators. Related blog post: http://fsharpforfunandprofit.com/posts/understanding-parser-combinators-3/
(*
ParserLibrary.fsx
Final version of a parser library.
Related blog post: http://fsharpforfunandprofit.com/posts/understanding-parser-combinators-3/
*)
module TextInput =
open System
type Position = {
line : int
column : int
}
/// define an initial position
let initialPos = {line=0; column=0}
/// increment the column number
let incrCol pos =
{pos with column=pos.column + 1}
/// increment the line number and set the column to 0
let incrLine pos =
{line=pos.line + 1; column=0}
/// Define the current input state
type InputState = {
lines : string[]
position : Position
}
// return the current line
let currentLine inputState =
let linePos = inputState.position.line
if linePos < inputState.lines.Length then
inputState.lines.[linePos]
else
"end of file"
/// Create a new InputState from a string
let fromStr str =
if String.IsNullOrEmpty(str) then
{lines=[||]; position=initialPos}
else
let separators = [| "\r\n"; "\n" |]
let lines = str.Split(separators, StringSplitOptions.None)
{lines=lines; position=initialPos}
/// Get the next character from the input, if any
/// else return None. Also return the updated InputState
/// Signature: InputState -> InputState * char option
let nextChar input =
let linePos = input.position.line
let colPos = input.position.column
// three cases
// 1) if line >= maxLine ->
// return EOF
// 2) if col less than line length ->
// return char at colPos, increment colPos
// 3) if col at line length ->
// return NewLine, increment linePos
if linePos >= input.lines.Length then
input, None
else
let currentLine = currentLine input
if colPos < currentLine.Length then
let char = currentLine.[colPos]
let newPos = incrCol input.position
let newState = {input with position=newPos}
newState, Some char
else
// end of line, so return LF and move to next line
let char = '\n'
let newPos = incrLine input.position
let newState = {input with position=newPos}
newState, Some char
// ===========================================
// Parser code
// ===========================================
open System
// Aliases for input, etc
type Input = TextInput.InputState // type alias
type ParserLabel = string
type ParserError = string
/// Stores information about the parser position for error messages
type ParserPosition = {
currentLine : string
line : int
column : int
}
// Result type
type Result<'a> =
| Success of 'a
| Failure of ParserLabel * ParserError * ParserPosition
/// A Parser structure has a parsing function & label
type Parser<'a> = {
parseFn : (Input -> Result<'a * Input>)
label: ParserLabel
}
/// Run the parser on a InputState
let runOnInput parser input =
// call inner function with input
parser.parseFn input
/// Run the parser on a string
let run parser inputStr =
// call inner function with input
runOnInput parser (TextInput.fromStr inputStr)
// =============================================
// Error messages
// =============================================
let parserPositionFromInputState (inputState:Input) = {
currentLine = TextInput.currentLine inputState
line = inputState.position.line
column = inputState.position.column
}
let printResult result =
match result with
| Success (value,input) ->
printfn "%A" value
| Failure (label,error,parserPos) ->
let errorLine = parserPos.currentLine
let colPos = parserPos.column
let linePos = parserPos.line
let failureCaret = sprintf "%*s^%s" colPos "" error
// examples of formatting
// sprintf "%*s^%s" 0 "" "test"
// sprintf "%*s^%s" 10 "" "test"
printfn "Line:%i Col:%i Error parsing %s\n%s\n%s" linePos colPos label errorLine failureCaret
// =============================================
// Label related
// =============================================
/// get the label from a parser
let getLabel parser =
// get label
parser.label
/// update the label in the parser
let setLabel parser newLabel =
// change the inner function to use the new label
let newInnerFn input =
let result = parser.parseFn input
match result with
| Success s ->
// if Success, do nothing
Success s
| Failure (oldLabel,err,pos) ->
// if Failure, return new label
Failure (newLabel,err,pos)
// return the Parser
{parseFn=newInnerFn; label=newLabel}
/// infix version of setLabel
let ( <?> ) = setLabel
// =============================================
// Standard combinators
// =============================================
/// Match an input token if the predicate is satisfied
let satisfy predicate label =
let innerFn input =
let remainingInput,charOpt = TextInput.nextChar input
match charOpt with
| None ->
let err = "No more input"
let pos = parserPositionFromInputState input
Failure (label,err,pos)
| Some first ->
if predicate first then
Success (first,remainingInput)
else
let err = sprintf "Unexpected '%c'" first
let pos = parserPositionFromInputState input
Failure (label,err,pos)
// return the parser
{parseFn=innerFn;label=label}
/// "bindP" takes a parser-producing function f, and a parser p
/// and passes the output of p into f, to create a new parser
let bindP f p =
let label = "unknown"
let innerFn input =
let result1 = runOnInput p input
match result1 with
| Failure (label,err,pos) ->
// return error from parser1
Failure (label,err,pos)
| Success (value1,remainingInput) ->
// apply f to get a new parser
let p2 = f value1
// run parser with remaining input
runOnInput p2 remainingInput
{parseFn=innerFn; label=label}
/// Infix version of bindP
let ( >>= ) p f = bindP f p
/// Lift a value to a Parser
let returnP x =
let label = sprintf "%A" x
let innerFn input =
// ignore the input and return x
Success (x,input)
// return the inner function
{parseFn=innerFn; label=label}
/// apply a function to the value inside a parser
let mapP f =
bindP (f >> returnP)
/// infix version of mapP
let ( <!> ) = mapP
/// "piping" version of mapP
let ( |>> ) x f = mapP f x
/// apply a wrapped function to a wrapped value
let applyP fP xP =
fP >>= (fun f ->
xP >>= (fun x ->
returnP (f x) ))
/// infix version of apply
let ( <*> ) = applyP
/// lift a two parameter function to Parser World
let lift2 f xP yP =
returnP f <*> xP <*> yP
/// Combine two parsers as "A andThen B"
let andThen p1 p2 =
let label = sprintf "%s andThen %s" (getLabel p1) (getLabel p2)
p1 >>= (fun p1Result ->
p2 >>= (fun p2Result ->
returnP (p1Result,p2Result) ))
<?> label
/// Infix version of andThen
let ( .>>. ) = andThen
/// Combine two parsers as "A orElse B"
let orElse p1 p2 =
let label = sprintf "%s orElse %s" (getLabel p1) (getLabel p2)
let innerFn input =
// run parser1 with the input
let result1 = runOnInput p1 input
// test the result for Failure/Success
match result1 with
| Success result ->
// if success, return the original result
result1
| Failure _ ->
// if failed, run parser2 with the input
let result2 = runOnInput p2 input
// return parser2's result
result2
// return the inner function
{parseFn=innerFn; label=label}
/// Infix version of orElse
let ( <|> ) = orElse
/// Choose any of a list of parsers
let choice listOfParsers =
List.reduce ( <|> ) listOfParsers
let rec sequence parserList =
// define the "cons" function, which is a two parameter function
let cons head tail = head::tail
// lift it to Parser World
let consP = lift2 cons
// process the list of parsers recursively
match parserList with
| [] ->
returnP []
| head::tail ->
consP head (sequence tail)
/// (helper) match zero or more occurences of the specified parser
let rec parseZeroOrMore parser input =
// run parser with the input
let firstResult = runOnInput parser input
// test the result for Failure/Success
match firstResult with
| Failure (_,_,_) ->
// if parse fails, return empty list
([],input)
| Success (firstValue,inputAfterFirstParse) ->
// if parse succeeds, call recursively
// to get the subsequent values
let (subsequentValues,remainingInput) =
parseZeroOrMore parser inputAfterFirstParse
let values = firstValue::subsequentValues
(values,remainingInput)
/// matches zero or more occurences of the specified parser
let many parser =
let label = sprintf "many %s" (getLabel parser)
let rec innerFn input =
// parse the input -- wrap in Success as it always succeeds
Success (parseZeroOrMore parser input)
{parseFn=innerFn; label=label}
/// matches one or more occurences of the specified parser
let many1 p =
let label = sprintf "many1 %s" (getLabel p)
p >>= (fun head ->
many p >>= (fun tail ->
returnP (head::tail) ))
<?> label
/// Parses an optional occurrence of p and returns an option value.
let opt p =
let label = sprintf "opt %s" (getLabel p)
let some = p |>> Some
let none = returnP None
(some <|> none) <?> label
/// Keep only the result of the left side parser
let (.>>) p1 p2 =
// create a pair
p1 .>>. p2
// then only keep the first value
|> mapP (fun (a,b) -> a)
/// Keep only the result of the right side parser
let (>>.) p1 p2 =
// create a pair
p1 .>>. p2
// then only keep the second value
|> mapP (fun (a,b) -> b)
/// Keep only the result of the middle parser
let between p1 p2 p3 =
p1 >>. p2 .>> p3
/// Parses one or more occurrences of p separated by sep
let sepBy1 p sep =
let sepThenP = sep >>. p
p .>>. many sepThenP
|>> fun (p,pList) -> p::pList
/// Parses zero or more occurrences of p separated by sep
let sepBy p sep =
sepBy1 p sep <|> returnP []
// =============================================
// Standard parsers
// =============================================
// ------------------------------
// char and string parsing
// ------------------------------
/// parse a char
let pchar charToMatch =
// label is just the character
let label = sprintf "%c" charToMatch
let predicate ch = (ch = charToMatch)
satisfy predicate label
/// Choose any of a list of characters
let anyOf listOfChars =
let label = sprintf "anyOf %A" listOfChars
listOfChars
|> List.map pchar // convert into parsers
|> choice
<?> label
/// Convert a list of chars to a string
let charListToStr charList =
String(List.toArray charList)
/// Parses a sequence of zero or more chars with the char parser cp.
/// It returns the parsed chars as a string.
let manyChars cp =
many cp
|>> charListToStr
/// Parses a sequence of one or more chars with the char parser cp.
/// It returns the parsed chars as a string.
let manyChars1 cp =
many1 cp
|>> charListToStr
/// parse a specific string
let pstring str =
// label is just the string
let label = str
str
// convert to list of char
|> List.ofSeq
// map each char to a pchar
|> List.map pchar
// convert to Parser<char list>
|> sequence
// convert Parser<char list> to Parser<string>
|> mapP charListToStr
<?> label
// ------------------------------
// whitespace parsing
// ------------------------------
/// parse a whitespace char
let whitespaceChar =
let predicate = Char.IsWhiteSpace
let label = "whitespace"
satisfy predicate label
/// parse zero or more whitespace char
let spaces = many whitespaceChar
/// parse one or more whitespace char
let spaces1 = many1 whitespaceChar
// ------------------------------
// number parsing
// ------------------------------
/// parse a digit
let digitChar =
let predicate = Char.IsDigit
let label = "digit"
satisfy predicate label
// parse an integer
let pint =
let label = "integer"
// helper
let resultToInt (sign,digits) =
let i = digits |> int // ignore int overflow for now
match sign with
| Some ch -> -i // negate the int
| None -> i
// define parser for one or more digits
let digits = manyChars1 digitChar
// an "int" is optional sign + one or more digits
opt (pchar '-') .>>. digits
|> mapP resultToInt
<?> label
// parse a float
let pfloat =
let label = "float"
// helper
let resultToFloat (((sign,digits1),point),digits2) =
let fl = sprintf "%s.%s" digits1 digits2 |> float
match sign with
| Some ch -> -fl // negate the float
| None -> fl
// define parser for one or more digits
let digits = manyChars1 digitChar
// a float is sign, digits, point, digits (ignore exponents for now)
opt (pchar '-') .>>. digits .>>. pchar '.' .>>. digits
|> mapP resultToFloat
<?> label
(*
understanding_parser_combinators-3.fsx
Adding more informative errors to the parser library.
Related blog post: http://fsharpforfunandprofit.com/posts/understanding-parser-combinators-3/
*)
// =============================================
// Section 1 -- Labeled parsers
// =============================================
(*
// The problem:
let parseDigit = anyOf ['0'..'9']
run parseDigit "|ABC" // Failure "Expecting '9'. Got '|'"
// We want error to say: Failure "Expecting digit. Got '|'"
*)
module LabelledParser =
open System
// Aliases
type ParserLabel = string
type ParserError = string
// Result type
type Result<'a> =
| Success of 'a
| Failure of ParserLabel * ParserError
(*
// before
type Parser<'a> = Parser of (string -> Result<'a * string>)
*)
/// A Parser structure has a parsing function & label
type Parser<'a> = {
parseFn : (string -> Result<'a * string>)
label: ParserLabel
}
let printResult result =
match result with
| Success (value,input) ->
printfn "%A" value
| Failure (label,error) ->
printfn "Error parsing %s\n%s" label error
/// parse a char
let pchar charToMatch =
let label = sprintf "%c" charToMatch
let innerFn input =
if String.IsNullOrEmpty(input) then
Failure (label,"No more input")
else
let first = input.[0]
if first = charToMatch then
let remainingInput = input.[1..]
Success (charToMatch,remainingInput)
else
let err = sprintf "Unexpected '%c'" first
Failure (label,err)
// return the parser
{parseFn=innerFn;label=label}
/// Run a parser with some input
let run parser input =
// get inner function
let innerFn = parser.parseFn
// call inner function with input
innerFn input
/// get the label from a parser
let getLabel parser =
// get label
parser.label
/// update the label in the parser
let setLabel parser newLabel =
// change the inner function to use the new label
let newInnerFn input =
let result = parser.parseFn input
match result with
| Success s ->
// if Success, do nothing
Success s
| Failure (oldLabel,err) ->
// if Failure, return new label
Failure (newLabel,err) // <====== use newLabel here
// return the Parser
{parseFn=newInnerFn; label=newLabel} // <====== use newLabel here
/// infix version of setLabel
let ( <?> ) = setLabel
/// "bindP" takes a parser-producing function f, and a parser p
/// and passes the output of p into f, to create a new parser
let bindP f p =
let label = "unknown" // <====== "label" is new!
let innerFn input =
let result1 = run p input
match result1 with
| Failure (label,err) -> // <====== "label" is new!
// return error from parser1
Failure (label,err)
| Success (value1,remainingInput) ->
// apply f to get a new parser
let p2 = f value1
// run parser with remaining input
run p2 remainingInput
{parseFn=innerFn; label=label} // <====== "parseFn" and "label" are new!
/// Infix version of bindP
let ( >>= ) p f = bindP f p
/// Lift a value to a Parser
let returnP x =
let label = sprintf "%A" x
let innerFn input =
// ignore the input and return x
Success (x,input)
// return the inner function
{parseFn=innerFn; label=label}
/// apply a function to the value inside a parser
let mapP f =
bindP (f >> returnP)
/// infix version of mapP
let ( <!> ) = mapP
/// "piping" version of mapP
let ( |>> ) x f = mapP f x
/// apply a wrapped function to a wrapped value
let applyP fP xP =
fP >>= (fun f ->
xP >>= (fun x ->
returnP (f x) ))
/// infix version of apply
let ( <*> ) = applyP
/// lift a two parameter function to Parser World
let lift2 f xP yP =
returnP f <*> xP <*> yP
/// Combine two parsers as "A andThen B"
let andThen p1 p2 =
let label = sprintf "%s andThen %s" (getLabel p1) (getLabel p2)
p1 >>= (fun p1Result ->
p2 >>= (fun p2Result ->
returnP (p1Result,p2Result) ))
<?> label // <====== provide a custom label
/// Infix version of andThen
let ( .>>. ) = andThen
// combine two parsers as "A orElse B"
let orElse parser1 parser2 =
// construct a new label
let label = sprintf "%s orElse %s" (getLabel parser1) (getLabel parser2)
let innerFn input =
// run parser1 with the input
let result1 = run parser1 input
// test the result for Failure/Success
match result1 with
| Success result ->
// if success, return the result
result1
| Failure (_,err) ->
// if failed, run parser2 with the input
let result2 = run parser2 input
// return parser2's result
match result2 with
| Success _ ->
// if success, return the result
result2
| Failure (_,err) ->
// if failed, return the error with overall label
Failure (label,err)
// return the Parser
{parseFn=innerFn; label=label}
// infix version of orElse
let ( <|> ) = orElse
/// choose any of a list of parsers
let choice listOfParsers =
List.reduce ( <|> ) listOfParsers
/// choose any of a list of characters
let anyOf listOfChars =
let label = sprintf "any of %A" listOfChars
listOfChars
|> List.map pchar // convert into parsers
|> choice
<?> label // <====== provide a custom label
module Label_Test =
let parseAB =
pchar 'A' .>>. pchar 'B'
run parseAB "A|C"
|> printResult
// Error parsing A andThen B
// Unexpected '|'
let parseAB_WithLabel =
pchar 'A' .>>. pchar 'B'
<?> "AB"
run parseAB_WithLabel "A|C"
|> printResult
// Error parsing AB
// Unexpected '|'
let parseDigit =
anyOf ['0'..'9']
run parseDigit "|ABC"
|> printResult
// Error parsing any of ['0'; '1'; '2'; '3'; '4'; '5'; '6'; '7'; '8'; '9']
// Unexpected '|'
let parseDigit_WithLabel =
anyOf ['0'..'9']
<?> "digit"
run parseDigit_WithLabel "|ABC"
|> printResult
// Error parsing digit
// Unexpected '|'
type Keyword = IF | FOR
let keyword_IF =
pchar 'i' .>>. pchar 'f' |>> (fun _ -> IF)
let keyword_FOR =
pchar 'f' .>>. pchar 'o' .>>. pchar 'r' |>> (fun _ -> FOR)
let keyword =
choice [keyword_IF; keyword_FOR]
<?> "keyword"
run keyword "if x then"
|> printResult
// IF
run keyword "while x then"
|> printResult
// Error parsing keyword
// Unexpected 'w'
// =============================================
// Section 2 - replacing "pchar" with "satisfy"
// =============================================
module ReplacePcharWithSatisfy =
open System
open LabelledParser
/// Match an input token if the predicate is satisfied
let satisfy predicate label =
let innerFn input =
if String.IsNullOrEmpty(input) then
Failure (label,"No more input")
else
let first = input.[0]
if predicate first then // <====== use predicate here
let remainingInput = input.[1..]
Success (first,remainingInput)
else
let err = sprintf "Unexpected '%c'" first
Failure (label,err)
// return the parser
{parseFn=innerFn;label=label}
/// parse a char
let pchar charToMatch =
let predicate ch = (ch = charToMatch)
let label = sprintf "%c" charToMatch
satisfy predicate label
/// parse a digit
let digitChar =
let predicate = Char.IsDigit
let label = "digit"
satisfy predicate label
/// parse a whitespace char
let whitespaceChar =
let predicate = Char.IsWhiteSpace
let label = "whitespace"
satisfy predicate label
// =============================================
// Section 3. Adding positional context to error messages
// =============================================
module TextInput =
open System
type Position = {
line : int
column : int
}
/// define an initial position
let initialPos = {line=0; column=0}
/// increment the column number
let incrCol pos =
{pos with column=pos.column + 1}
/// increment the line number and set the column to 0
let incrLine pos =
{line=pos.line + 1; column=0}
/// Define the current input state
type InputState = {
lines : string[]
position : Position
}
// return the current line
let currentLine inputState =
let linePos = inputState.position.line
if linePos < inputState.lines.Length then
inputState.lines.[linePos]
else
"end of file"
/// Create a new InputState from a string
let fromStr str =
if String.IsNullOrEmpty(str) then
{lines=[||]; position=initialPos}
else
let separators = [| "\r\n"; "\n" |]
let lines = str.Split(separators, StringSplitOptions.None)
{lines=lines; position=initialPos}
/// Get the next character from the input, if any
/// else return None. Also return the updated InputState
/// Signature: InputState -> InputState * char option
let nextChar input =
let linePos = input.position.line
let colPos = input.position.column
// three cases
// 1) if line >= maxLine ->
// return EOF
// 2) if col less than line length ->
// return char at colPos, increment colPos
// 3) if col at line length ->
// return NewLine, increment linePos
if linePos >= input.lines.Length then
input, None
else
let currentLine = currentLine input
if colPos < currentLine.Length then
let char = currentLine.[colPos]
let newPos = incrCol input.position
let newState = {input with position=newPos}
newState, Some char
else
// end of line, so return LF and move to next line
let char = '\n'
let newPos = incrLine input.position
let newState = {input with position=newPos}
newState, Some char
// ---------
// test
// ---------
module Input_Test =
let rec readAllChars input =
[
let remainingInput,charOpt = nextChar input
match charOpt with
| None ->
// end of input
()
| Some ch ->
// return first character
yield ch
// return the remaining characters
yield! readAllChars remainingInput
]
fromStr "" |> readAllChars // []
fromStr "a" |> readAllChars // ['a'; '\n']
fromStr "ab" |> readAllChars // ['a'; 'b'; '\n']
fromStr "a\nb" |> readAllChars // ['a'; '\n'; 'b'; '\n']
module ParserWithPositionalErrors =
open System
// Aliases for input, etc
type Input = TextInput.InputState // type alias
type ParserLabel = string
type ParserError = string
/// Stores information about the parser position for error messages
type ParserPosition = {
currentLine : string
line : int
column : int
}
// Result type
type Result<'a> =
| Success of 'a
| Failure of ParserLabel * ParserError * ParserPosition
/// A Parser structure has a parsing function & label
type Parser<'a> = {
parseFn : (Input -> Result<'a * Input>)
label: ParserLabel
}
let parserPositionFromInputState (inputState:Input) = {
currentLine = TextInput.currentLine inputState
line = inputState.position.line
column = inputState.position.column
}
let printResult result =
match result with
| Success (value,input) ->
printfn "%A" value
| Failure (label,error,parserPos) ->
let errorLine = parserPos.currentLine
let colPos = parserPos.column
let linePos = parserPos.line
let failureCaret = sprintf "%*s^%s" colPos "" error
// examples of formatting
// sprintf "%*s^%s" 0 "" "test"
// sprintf "%*s^%s" 10 "" "test"
printfn "Line:%i Col:%i Error parsing %s\n%s\n%s" linePos colPos label errorLine failureCaret
(*
let exampleError =
Failure ("identifier", "unexpected |",
{currentLine = "123 ab|cd"; line=1; column=6})
printResult exampleError
// Line:1 Col:6 Error parsing identifier
// 123 ab|cd
// ^unexpected |
*)
/// Run the parser on a InputState
let runOnInput parser input =
// call inner function with input
parser.parseFn input
/// Run the parser on a string
let run parser inputStr =
// call inner function with input
runOnInput parser (TextInput.fromStr inputStr)
/// get the label from a parser
let getLabel parser =
// get label
parser.label
/// update the label in the parser
let setLabel parser newLabel =
// change the inner function to use the new label
let newInnerFn input =
let result = parser.parseFn input
match result with
| Success s ->
// if Success, do nothing
Success s
| Failure (oldLabel,err,pos) ->
// if Failure, return new label
Failure (newLabel,err,pos)
// return the Parser
{parseFn=newInnerFn; label=newLabel}
/// infix version of setLabel
let ( <?> ) = setLabel
/// Match an input token if the predicate is satisfied
let satisfy predicate label =
let innerFn input =
let remainingInput,charOpt = TextInput.nextChar input
match charOpt with
| None ->
let err = "No more input"
let pos = parserPositionFromInputState input
//Failure (label,err) // <====== old version
Failure (label,err,pos) // <====== new version
| Some first ->
if predicate first then
Success (first,remainingInput)
else
let err = sprintf "Unexpected '%c'" first
let pos = parserPositionFromInputState input
//Failure (label,err) // <====== old version
Failure (label,err,pos) // <====== new version
// return the parser
{parseFn=innerFn;label=label}
/// "bindP" takes a parser-producing function f, and a parser p
/// and passes the output of p into f, to create a new parser
let bindP f p =
let label = "unknown"
let innerFn input =
let result1 = runOnInput p input
match result1 with
| Failure (label,err,pos) -> // <====== new with pos
// return error from parser1
Failure (label,err,pos)
| Success (value1,remainingInput) ->
// apply f to get a new parser
let p2 = f value1
// run parser with remaining input
runOnInput p2 remainingInput
{parseFn=innerFn; label=label}
/// Infix version of bindP
let ( >>= ) p f = bindP f p
/// Lift a value to a Parser
let returnP x =
let label = sprintf "%A" x
let innerFn input =
// ignore the input and return x
Success (x,input)
// return the inner function
{parseFn=innerFn; label=label}
/// apply a function to the value inside a parser
let mapP f =
bindP (f >> returnP)
/// infix version of mapP
let ( <!> ) = mapP
/// "piping" version of mapP
let ( |>> ) x f = mapP f x
/// apply a wrapped function to a wrapped value
let applyP fP xP =
fP >>= (fun f ->
xP >>= (fun x ->
returnP (f x) ))
/// infix version of apply
let ( <*> ) = applyP
/// lift a two parameter function to Parser World
let lift2 f xP yP =
returnP f <*> xP <*> yP
/// Combine two parsers as "A andThen B"
let andThen p1 p2 =
let label = sprintf "%s andThen %s" (getLabel p1) (getLabel p2)
p1 >>= (fun p1Result ->
p2 >>= (fun p2Result ->
returnP (p1Result,p2Result) ))
<?> label
/// Infix version of andThen
let ( .>>. ) = andThen
/// Combine two parsers as "A orElse B"
let orElse p1 p2 =
let label = sprintf "%s orElse %s" (getLabel p1) (getLabel p2)
let innerFn input =
// run parser1 with the input
let result1 = runOnInput p1 input
// test the result for Failure/Success
match result1 with
| Success result ->
// if success, return the original result
result1
| Failure _ ->
// if failed, run parser2 with the input
let result2 = runOnInput p2 input
// return parser2's result
result2
// return the inner function
{parseFn=innerFn; label=label}
/// Infix version of orElse
let ( <|> ) = orElse
/// Choose any of a list of parsers
let choice listOfParsers =
List.reduce ( <|> ) listOfParsers
let rec sequence parserList =
// define the "cons" function, which is a two parameter function
let cons head tail = head::tail
// lift it to Parser World
let consP = lift2 cons
// process the list of parsers recursively
match parserList with
| [] ->
returnP []
| head::tail ->
consP head (sequence tail)
/// (helper) match zero or more occurences of the specified parser
let rec parseZeroOrMore parser input =
// run parser with the input
let firstResult = runOnInput parser input
// test the result for Failure/Success
match firstResult with
| Failure (_,_,_) ->
// if parse fails, return empty list
([],input)
| Success (firstValue,inputAfterFirstParse) ->
// if parse succeeds, call recursively
// to get the subsequent values
let (subsequentValues,remainingInput) =
parseZeroOrMore parser inputAfterFirstParse
let values = firstValue::subsequentValues
(values,remainingInput)
/// matches zero or more occurences of the specified parser
let many parser =
let label = sprintf "many %s" (getLabel parser)
let rec innerFn input =
// parse the input -- wrap in Success as it always succeeds
Success (parseZeroOrMore parser input)
{parseFn=innerFn; label=label}
/// matches one or more occurences of the specified parser
let many1 p =
let label = sprintf "many1 %s" (getLabel p)
p >>= (fun head ->
many p >>= (fun tail ->
returnP (head::tail) ))
<?> label
/// Parses an optional occurrence of p and returns an option value.
let opt p =
let label = sprintf "opt %s" (getLabel p)
let some = p |>> Some
let none = returnP None
(some <|> none) <?> label
/// Keep only the result of the left side parser
let (.>>) p1 p2 =
// create a pair
p1 .>>. p2
// then only keep the first value
|> mapP (fun (a,b) -> a)
/// Keep only the result of the right side parser
let (>>.) p1 p2 =
// create a pair
p1 .>>. p2
// then only keep the second value
|> mapP (fun (a,b) -> b)
/// Keep only the result of the middle parser
let between p1 p2 p3 =
p1 >>. p2 .>> p3
/// Parses one or more occurrences of p separated by sep
let sepBy1 p sep =
let sepThenP = sep >>. p
p .>>. many sepThenP
|>> fun (p,pList) -> p::pList
/// Parses zero or more occurrences of p separated by sep
let sepBy p sep =
sepBy1 p sep <|> returnP []
module PositionalError_Test =
/// parse a char
let pchar charToMatch =
let predicate ch = (ch = charToMatch)
let label = sprintf "%c" charToMatch
satisfy predicate label
let parseAB =
pchar 'A' .>>. pchar 'B'
<?> "AB"
run parseAB "A|C"
|> printResult
// Line:0 Col:1 Error parsing AB
// A|C
// ^Unexpected '|'
// =============================================
// Section 4. Adding some standard parsers to the library
// =============================================
module StandardParsers =
open System
open ParserWithPositionalErrors
// ------------------------------
// char and string parsing
// ------------------------------
/// parse a char
let pchar charToMatch =
// label is just the character
let label = sprintf "%c" charToMatch
let predicate ch = (ch = charToMatch)
satisfy predicate label
/// Choose any of a list of characters
let anyOf listOfChars =
let label = sprintf "anyOf %A" listOfChars
listOfChars
|> List.map pchar // convert into parsers
|> choice
<?> label
/// Convert a list of chars to a string
let charListToStr charList =
String(List.toArray charList)
/// Parses a sequence of zero or more chars with the char parser cp.
/// It returns the parsed chars as a string.
let manyChars cp =
many cp
|>> charListToStr
/// Parses a sequence of one or more chars with the char parser cp.
/// It returns the parsed chars as a string.
let manyChars1 cp =
many1 cp
|>> charListToStr
/// parse a specific string
let pstring str =
// label is just the string
let label = str
str
// convert to list of char
|> List.ofSeq
// map each char to a pchar
|> List.map pchar
// convert to Parser<char list>
|> sequence
// convert Parser<char list> to Parser<string>
|> mapP charListToStr
<?> label
module StringParsers_Test =
run (pstring "AB") "ABC"
|> printResult
// Success
// "AB"
run (pstring "AB") "A|C"
|> printResult
// Line:0 Col:1 Error parsing AB
// A|C
// ^Unexpected '|'
// ------------------------------
// whitespace parsing
// ------------------------------
/// parse a whitespace char
let whitespaceChar =
let predicate = Char.IsWhiteSpace
let label = "whitespace"
satisfy predicate label
/// parse zero or more whitespace char
let spaces = many whitespaceChar
/// parse one or more whitespace char
let spaces1 = many1 whitespaceChar
module WhitespaceParsers_Test =
run spaces " ABC"
|> printResult
// [' ']
run spaces "A"
|> printResult
// []
run spaces1 " ABC"
|> printResult
// [' ']
run spaces1 "A"
|> printResult
// Line:0 Col:0 Error parsing many1 whitespace
// A
// ^Unexpected 'A'
// ------------------------------
// number parsing
// ------------------------------
/// parse a digit
let digitChar =
let predicate = Char.IsDigit
let label = "digit"
satisfy predicate label
// parse an integer
let pint =
let label = "integer"
// helper
let resultToInt (sign,digits) =
let i = digits |> int // ignore int overflow for now
match sign with
| Some ch -> -i // negate the int
| None -> i
// define parser for one or more digits
let digits = manyChars1 digitChar
// an "int" is optional sign + one or more digits
opt (pchar '-') .>>. digits
|> mapP resultToInt
<?> label
// parse a float
let pfloat =
let label = "float"
// helper
let resultToFloat (((sign,digits1),point),digits2) =
let fl = sprintf "%s.%s" digits1 digits2 |> float
match sign with
| Some ch -> -fl // negate the float
| None -> fl
// define parser for one or more digits
let digits = manyChars1 digitChar
// a float is sign, digits, point, digits (ignore exponents for now)
opt (pchar '-') .>>. digits .>>. pchar '.' .>>. digits
|> mapP resultToFloat
<?> label
module NumericParsers_Test =
run pint "-123Z"
|> printResult
// -123
run pint "-Z123"
|> printResult
// Line:0 Col:1 Error parsing integer
// -Z123
// ^Unexpected 'Z'
run pfloat "-123.45Z"
|> printResult
// -123.45
run pfloat "-123Z45"
|> printResult
// Line:0 Col:4 Error parsing float
// -123Z45
// ^Unexpected 'Z'
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