FParsec: how to parse date in fparsec (newbie)

问题

I am using the Bill Casarin post on how to parse delimited files with fparsec, I am dumbing the logic down to get an understanding of how the code works. I am parsing a multi row delimited document into Cell list list structure (for now) where a Cell is a string or a float. I am a complete newbie on this.

I am having issues parsing the floats - in a typical case (a cell delimitted by tabs, containing a numeric) it works. However when a cell happens to be a string that starts with a number - it falls apart.

How do I modify pFloatCell to either parse (although the way through the tab) as a float or nothing?

Thank you

type Cell = 
    | String of string 
    | Float of float
.
.
.
let pStringCell delim = 
    manyChars (nonQuotedCellChar delim)
    |>> String

// this is my issue. pfloat parses the string one 
// char at a time, and once it starts off with a number 
// it is down that path, and errors out
let pFloatCell delim = 
    FParsec.CharParsers.pfloat
    |>> Float

let pCell delim = 
    (pFloatCell delim) <|> (pStringCell delim)
.
.
.
let ParseTab s  =
  let delim = "\t"
  let res = run (csv delim) s in
    match res with
     | Success (rows, _, _) -> { IsSuccess = true; ErrorMsg = "Ok"; Result = stripEmpty rows }
     | Failure (s, _, _) -> { IsSuccess = false; ErrorMsg = s; Result = [[]] }
.
.
.
let test() =

    let parsed = ParseTab data

oops late for me last night. I meant to post the data. This first one works

let data = 
    "s10 Mar 2011 18:28:11 GMT\n"

while this returns an error:

let data = 
    "10 Mar 2011 18:28:11 GMT\n"

returns, both with and witout ChaosP's recommendation:

ErrorMsg = "Error in Ln: 1 Col: 3\r\n10 Mar 2011 18:28:11 GMT\r\n ^\r\nExpecting: end of file, newline or '\t'\r\n"

It looks as though the attempt is working fine. in the second case it is only grabbing up to the 10 - and the code for pfloat looks only up to the first whitespace. I need to convice pfloat that it needs to look all the way up to the next tab or newline regardless of whether there is a space before it; write my own version of pfloat by performing a Double.Parse - but I would rather rely on the library.

回答1:

Since it seems the text you'll be parsing is a bit ambiguous you'll need to modify your pCell parser.

let sep delim =
     skipString delim <|> skipAnyOf "\r\n" <|> eof

let pCell delim = 
    attempt (pFloatCell delim .>> sep delim) <|> (pStringCell delim .>> sep delim)

This also means you'll need to modify whichever parser uses pCell.

let pCells delim =
    many pCell delim

Note

The .>> operator is actually quite simple. Think of it like the leap-frog operator. The value of the left hand side is returned after applying the right hand side and ignoring the result.

Parser<'a, 'b> -> Parser<'c, 'b> -> Parser<'a, 'b>

来源：https://stackoverflow.com/questions/5630012/fparsec-how-to-parse-date-in-fparsec-newbie

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parsec

fparsec