F#: Pattern matching on field literals

F# gave me the following error when working with some C# code:

error FS0729: This field is not a literal and cannot be used in a pattern

I’m not entirely sure it’s a good idea, but I managed to work around this using a partial active pattern.

Here’s the gist of the C# code I was working with:

namespace Workshop.SomeCSharpLib {
    public class Thingamabobs {
        private Thingamabobs(string s) { }
        // ... 
        public static readonly Thingamabobs Foo = new Thingamabobs("foo");
        public static readonly Thingamabobs Bar = new Thingamabobs("bar");
        public static readonly Thingamabobs Clunk = new Thingamabobs("clunk");
        public static readonly Thingamabobs Zap = new Thingamabobs("zap");
    }
}

Thingamabobs represent a sort of enum with an associated value - the kind of thing we’d typically use a discriminated union for in F#.

Trying to convert this to my own type using pattern matching resulted in the FS0729 error:

type Things = Foo | Bar | Clunk | Zap

let convertThingamabob =
    function
    | Thingamabobs.Foo   -> Some Foo // *
    | Thingamabobs.Bar   -> Some Bar
    | Thingamabobs.Clunk -> Some Clunk
    | Thingamabobs.Zap   -> Some Zap
    | _                  -> None

    // * error FS0729: This field is not a literal and cannot be used in a pattern

I couldn’t find much information on this error, but I gather I need to explicitly compare the argument to the field value using if ... else if ... else, something like:

   fun x -> if x = Thingamabobs.Foo then Some Foo
            else if x = ...

I think it looks neater as a pattern match, so worked around this using a partial active pattern to do the comparison:

// Partial active pattern. Match if field equals value.
let (|Field|_|) field x = if field = x then Some () else None

type Things = Foo | Bar | Clunk | Zap

let convertThingamabob =
    function
    | Field Thingamabobs.Foo   -> Some Foo
    | Field Thingamabobs.Bar   -> Some Bar
    | Field Thingamabobs.Clunk -> Some Clunk
    | Field Thingamabobs.Zap   -> Some Zap
    | _                        -> None

I’m not sure if there are any drawbacks to this approach, so if you can think of any please let me know.

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