Question

I’m trying to write a string processing function in F#, which looks like this:

let rec Process html =   match html with   | '-' :: '-' :: '>' :: tail -> ('→' |> List.of_seq) @ Process tail   | head :: tail -> head :: Process tail   | [] -> [] 

My pattern matching expression against several elements is a bit ugly (the whole '-' :: '-' :: '>' thing). Is there any way to make it better? Also, is what I’m doing efficient if I were to process large texts? Or is there another way?

Clarification: what I mean is, e.g., being able to write something like this:

match html with | '-->' :: tail ->  

Answer 1

I agree with others that using a list of characters for doing serious string manipulation is probably not ideal. However, if you’d like to continue to use this approach, one way to get something close to what you’re asking for is to define an active pattern. For instance:

let rec (|Prefix|_|) s l =   if s = '' then     Some(Prefix l)   else     match l with     | c::(Prefix (s.Substring(1)) xs) when c = s.[0] -> Some(Prefix xs)     | _ -> None 

Then you can use it like:

let rec Process html =     match html with     | Prefix '-->' tail -> ('→' |> List.of_seq) @ Process tail     | head :: tail -> head :: Process tail     | [] -> []