Pattern matching on the beginning of a string in f#

Question

I am trying to match the beginning of strings in f#. Not sure if I have to treat them as a list of characters or what. Any suggestions would be appreciated.

Here is a psuedo code version of what I am trying to do

let text = "The brown fox.."

match text with
| "The"::_ -> true
| "If"::_ -> true
| _ -> false

So, I want to look at the beginning of the string and match. Note I am not matching on a list of strings just wrote the above as an idea of the essence of what I am trying to do.

Answer 1

Yes you have to treat them as a list of characters if you want to use a match expression.

Simply transform the string with:

let text = "The brown fox.." |> Seq.toList

Then you can use a match expression but you will have to use chars (the type of elements in the list) for each letter:

match text with
| 'T'::'h'::'e'::_ -> true
| 'I'::'f'::_ -> true
| _ -> false

As Brian suggest Parameterized Active Patterns are much nicer, there a some useful patterns here (go the end of the page).

Answer 2

Parameterized active patterns to the rescue!

let (|Prefix|_|) (p:string) (s:string) =
    if s.StartsWith(p) then
        Some(s.Substring(p.Length))
    else
        None

match "Hello world" with
| Prefix "The" rest -> printfn "Started with 'The', rest is %s" rest
| Prefix "Hello" rest -> printfn "Started with 'Hello', rest is %s" rest
| _ -> printfn "neither"