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Check whether item exists in a list by using pattern matching
f#

Hi,

I've just started learning F# and am trying to port a piece of C# code to F#. :) I have written the following code snippet which actually works as I would have expected but I'm sure you can express it more compactly or more efficiently.

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let checkSubGroup subGroup =
    let count =
    subGroup
    |> Seq.map (fun (c : char) ->                           
        match c with
        | c when c >= '1' && c <= '8'      -> int c - 48
        | c when List.exists (fun e -> e = c)
            ['p'; 'b'; 'n'; 'r'; 'q'; 'k';
             'P'; 'B'; 'N'; 'R'; 'Q'; 'K'] -> 1
        | _                                -> 99)
        |> Seq.sum
    count = 8

Is there any way I can match c against a list of characters because that would simplify the code enormously? I tried this but fsi keeps telling me "character range matches" are deprecated and "'a list is incompatible with char":

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match c with
    | ['1' .. '8']                    -> int c - 48
    | ['p'; 'b'; 'n'; 'r'; 'q'; 'k';
        'P'; 'B'; 'N'; 'R'; 'Q'; 'K'] -> 1
    | _                               -> 99)

Thanks in advance! :)

.

Hi,
matching character agains a list or a range isn't directly supported, but you can write active patterns to allow this. Active patterns are a way for implementing your own "tests" for pattern matching:

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// tests whether the character 'c' is in the range 'cfrom' .. 'cto'
let (|CharRange|_|) (cfrom, cto) c = 
  if c >= cfrom && c <= cto then Some(c) else None
//  tests whtether the character 'c' is in the specified list 'clist'
let (|CharList|_|) clist c =
  if List.exists (fun e -> e = c) clist then Some(c) else None

Now you can write the pattern match using these patterns like this (note that the list of characters or a specified range is input argument for the active pattern - the 'c' value is the result of the pattern and this is the value returned from the pattern if it returns 'Some(c)' ):

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match 'a' with
| CharRange('1','8') c -> int c - 48
| CharList ['p'; 'b'; 'n' ] c -> 1
| _ -> 99

Hope this helps!
T.

By on 11/4/2008 3:26 AM ()

Thanks! Works perfectly. Better than nothing. :)

By on 11/5/2008 11:17 AM ()

Hi,

Note that the CharRange function given by Tomas is generic: you can use it with any type that supports comparison (basically, anything but functions). So I suggest you to rename the function. :)

For your letters, you don't have to use a list here:

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match 'a' with
| 'p' | 'b' | 'n' -> 1

Laurent.

By on 11/5/2008 11:48 AM ()

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