How to parse seq of words separated by double spaces using fparsec?

Given the input:

alpha beta gamma  one two three

How could I parse this into the below?

[["alpha"; "beta"; "gamma"]; ["one"; "two"; "three"]]

I can write this when there is a better separator (e.g.__), as then

sepBy (sepBy word (pchar ' ')) (pstring "__")

works, but in the case of double space, the pchar in the first sepBy consumes the first space and then the parser fails.

The FParsec manual says that in sepBy p sep, if sep succeds and the subsequent p fails (without changing the state), the entire sepBy fails, too. Hence, your goal is:

1. to make the separator fail if it encounters more than a single space char;
2. to backtrack so that the "inner" sepBy loop closed happily and passed control to the "outer" sepBy loop.

Here's how to do the both:

// this is your word parser; it can be different of course,
// I just made it as simple as possible;
let pWord = many1Satisfy isAsciiLetter

// this is the Inner separator to separate individual words
let pSepInner =
    pchar ' '
    .>> notFollowedBy (pchar ' ') // guard rule to prevent 2nd space
    |> attempt                    // a wrapper that fails NON-fatally

// this is the Outer separator
let pSepOuter =
    pchar ' '
    |> many1                      // loop over 1+ spaces

// this is the parser that would return String list list
let pMain =
    pWord
    |> sepBy <| pSepInner         // the Inner loop
    |> sepBy <| pSepOuter         // the Outer loop

Use:

run pMain "alpha beta gamma  one two three"
Success: [["alpha"; "beta"; "gamma"]; ["one"; "two"; "three"]]