Indentation has the last word on error messages (UserState) - FParsec - f#

Question

I have a small indentation management module with FParsec (found here); it works wonderfully well, but the only concern is that, when an error is encountered in the stream to be parsed, most of the time, FParsec returns the error message from the indentation manager, i.e. the UserState (correct me if I'm wrong on this point); which is problematic because it makes the errors very blurry, and all the same... How can I display indentation errors only when they are necessary?
Here is the module used for indentation:
module IndentParser
open FParsec
type Indentation =
| Fail
| Any
| Greater of Position
| Exact of Position
| AtLeast of Position
| StartIndent of Position
with
member this.Position = match this with
| Any | Fail -> None
| Greater p -> Some p
| Exact p -> Some p
| AtLeast p -> Some p
| StartIndent p -> Some p
type IndentState<'T> = { Indent : Indentation; UserState : 'T }
type CharStream<'T> = FParsec.CharStream<IndentState<'T>>
type IndentParser<'T, 'UserState> = Parser<'T, IndentState<'UserState>>
let indentState u = {Indent = Any; UserState = u}
let runParser p u s = runParserOnString p (indentState u) "" s
let runParserOnFile p u path = runParserOnFile p (indentState u) path System.Text.Encoding.UTF8
let getIndentation : IndentParser<_,_> =
fun stream -> match stream.UserState with
| {Indent = i} -> Reply i
let getUserState : IndentParser<_,_> =
fun stream -> match stream.UserState with
| {UserState = u} -> Reply u
let putIndentation newi : IndentParser<unit, _> =
fun stream ->
stream.UserState <- {stream.UserState with Indent = newi}
Reply(Unchecked.defaultof<unit>)
let failf fmt = fail << sprintf fmt
let acceptable i (pos : Position) =
match i with
| Any _ -> true
| Fail -> false
| Greater bp -> bp.Column < pos.Column
| Exact ep -> ep.Column = pos.Column
| AtLeast ap -> ap.Column <= pos.Column
| StartIndent _ -> true
let nestableIn i o =
match i, o with
| Greater i, Greater o -> o.Column < i.Column
| Greater i, Exact o -> o.Column < i.Column
| Exact i, Exact o -> o.Column = i.Column
| Exact i, Greater o -> o.Column <= i.Column
| _, _ -> true
let tokeniser p = parse {
let! pos = getPosition
let! i = getIndentation
if acceptable i pos then return! p
else return! fail "incorrect indentation"
}
let nestP i o p = parse {
do! putIndentation i
let! x = p
do! notFollowedBy (tokeniser anyChar) <?> (sprintf "unterminated %A" i)
do! putIndentation o
return x
}
let nest indentor p = parse {
let! outerI = getIndentation
let! curPos = getPosition
let innerI = indentor curPos
if nestableIn innerI outerI
then return! nestP innerI outerI p
else return! nestP Fail outerI p
}
let nestWithPos indentor pos p = parse {
let! outerI = getIndentation
let innerI = indentor pos
if nestableIn innerI outerI
then return! nestP innerI outerI p
else return! nestP Fail outerI p
}
let neglectIndent p = parse {
let! o = getIndentation
do! putIndentation Any
let! x = p
do! putIndentation o
return x
}
let checkIndent<'u> : IndentParser<unit, 'u> = tokeniser (preturn ())
let indented<'a,'u> i (p : Parser<'a,_>) : IndentParser<_, 'u> = parse {
do! putIndentation i
do! spaces
return! tokeniser p
}
/// Allows to check if the position of the parser currently being analyzed (`p`)
/// is on the same line as the defined position (`pos`).
let exact<'a,'u> pos p: IndentParser<'a, 'u> = indented (Exact pos) p
/// Allows to check if the position of the parser currently being analyzed (`p`)
/// is further away than the defined position (`pos`).
let greater<'a,'u> pos p: IndentParser<'a, 'u> = indented (Greater pos) p
/// Allows to check if the position of the parser currently being analyzed (`p`)
/// is on the same OR line further than the defined position (`pos`).
let atLeast<'a,'u> pos p: IndentParser<'a, 'u> = indented (AtLeast pos) p
/// Simply check if the parser (`p`) exists, regardless of its position in the text to be analyzed.
let any<'a,'u> pos p: IndentParser<'a, 'u> = indented Any p
let newline<'u> : IndentParser<unit, 'u> = many (skipAnyOf " \t" <?> "whitespace") >>. newline |>> ignore
let rec blockOf p = parse {
do! spaces
let! pos = getPosition
let! x = exact pos p
let! xs = attempt (exact pos <| blockOf p) <|> preturn []
return x::xs
}
and here is an example of the problem encountered:
open FParsec
open IndentParser
// ---------- AST ----------
type Statement
= Let of string * Expr
and Expr
= Tuple of Expr list
| Literal of Literal
and Literal
= Int of int
| Float of float
| Char of char
// ---------- Parser ----------
let inline pstr's s = stringReturn s s <?> sprintf "`%s`" s
let inline pstr'u s = stringReturn s () <?> sprintf "`%s`" s
let identifier = manySatisfy (fun c -> isLetter c || c = ''')
let comment = pstr'u "//" >>. skipRestOfLine true <?> ""
let numberFormat =
NumberLiteralOptions.AllowBinary
||| NumberLiteralOptions.AllowMinusSign
||| NumberLiteralOptions.AllowHexadecimal
||| NumberLiteralOptions.AllowOctal
||| NumberLiteralOptions.AllowPlusSign
||| NumberLiteralOptions.AllowFraction
let number<'u> : IndentParser<Literal, 'u> =
(numberLiteral numberFormat "number" |>> fun nl ->
if nl.IsInteger then Int(int nl.String)
else Float(float nl.String))
let char<'u> : IndentParser<Literal, 'u> =
((between (pstr'u "'") (pstr'u "'")
(satisfy (fun c -> c <> '\'')) <?> "char literal") |>> Char)
let rec let'parser =
parse { let! pos = getPosition
do! exact pos (pstr'u "let" <?> "let statement")
let! name = greater pos identifier <?> "identifier"
do! greater pos (pstr'u "=" <?> "value assignment")
let! value = greater pos expression
return Let(name, value) }
and tuple'parser =
parse { let! pos = getPosition
do! exact pos (pstr'u "(" <?> "tuple")
let! uplets = greater pos (sepBy1 expression (pstr'u ","))
do! greater pos (pstr'u ")" <?> "right parenthese")
return Tuple uplets }
and literal'parser = attempt number <|> char |>> Literal
and expression =
spaces >>? (attempt tuple'parser <|> literal'parser)
and statement = spaces >>? let'parser .>>? spaces .>>? (attempt comment <|> (spaces >>% ()))
// ---------- Test ----------
System.Console.Clear()
let res = runParser (spaces >>? blockOf statement .>>? (spaces .>>? eof)) () #"
let foo = (0, 1) // it works well
let bar = 887 // it works well
let oof = 'x' // it works well
let rab = // it fail with 'incorrect indentation' (without this comment)
let ofo = (0, 2, // it fail with 'incorrect indentation' (without this comment)
"
printfn "%A" res
It's really annoying...
Would someone explain to me how to solve this problem?