FParsec: Alternative or Choice

We left off with a working solution for parsing either “po/k”, “x4/0”, and “s12” into our choice type of Operation. We can always go back and look at our original solution here. We really want to be able to parse a bunch of operations separated by a comma.

Parsing Many

When we want to parse a large blob of operations separated by a comma we can use the sepBy function.

sepBy : Parser<‘a, ‘u> -> Parser<‘b, ‘u> -> Parser<‘a list, ‘u>
We can see by the function signature, it takes two parsers as arguments. The first parser determines the result. The second parser determines the separator. Our operations are separated by a comma so we’ll use pchar ‘,’ parser. Let’s combine the sepBy with our previous function parseOperation and can now parse an entire blob of operations.

"po/k,x4/0,s12,x7/6" |> run (sepBy parseOperation (pchar ','))

Alternative or Choice

Revisiting the parserOperation function of

let parseOperation = pspin <|> pexchange <|> ppartner
we could have used the choice : seq> -> Parser<‘a,‘u> function. choice takes a collection of parsers and evaluates them in order to find a match. This can help with readability when having multiple alternative parsers. Instead of weaving the &lt|&gt operator between them. We can re-write our function as:

let parseOperation = choice [ pspin; pexchange; ppartner ]


We’ve replaced our alternative operator <|> with the choice function for readability. We’ve also introduced the sepBy function to parse a block of comma separated operations. The simplest way to test our parsers is by using the run function as it does not require or use any state.

Full Code

Shane Charles
Software Developer