Pattern matching based on the function signature

Question

In F# can you pattern match on a function signature. I want to decorate a number of functions with a function that measures the execution of the function and calls out to statsd. The current function I have is:

let WrapFunctionWithPrefix(metrics:Metric.Client.IRecorder, functionToWrap, prefix) =
    let metricsIdentifier = (sprintf "%s.%s" prefix Environment.MachineName)
    using (metrics.StartTimer(metricsIdentifier)) ( fun metrics -> functionToWrap)

As you can see above, the prefix will vary, and in our application this will vary per function definition. So rather than having to pass in the measure prefix every time I want to do something like the following:

let WrapFunction metrics afunc = 
    match afunc with
    | :? (int -> int) -> WrapFunctionWithPrefix(metrics, afunc, "My function 1")
    | :? (string -> string) -> WrapFunctionWithPrefix(metrics, afunc, "My function 2")
    | _ -> failwith "Unknown function def"

Is there any way of pattern matching based on the function signature in F#?

Any help appreciated.

Billy

score 7 · Accepted Answer · answered Apr 14 '15 at 13:51

7

Would it be possible to declare the cases as a DU?

type MyFunctions =
| Intish of int -> int
| Stringish of string -> string

answered Apr 14 '15 at 13:51

Daniel Fabian

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Cheers, went down this route and worked. Slightly more elegant! – bstack Apr 14 '15 at 15:05

score 3 · Answer 2 · answered Apr 14 '15 at 13:10

let WrapFunction metrics afunc = 
    match box afunc with
    | :? (int -> int) -> WrapFunctionWithPrefix(metrics, afunc, "My function 1")
    | :? (string -> string) -> WrapFunctionWithPrefix(metrics, afunc, "My function 2")
    | _ -> failwith "Unknown function def"

will work for your pattern match. You normally end up having to box unknown types before trying to cast them, as :? doesn't like being used on value types.

I'm not totally sure how your using statement will interact with the function you return though. I think it will dispose metrics and return the function immediately, which is probably not what you want.

Also note that you get no type safety at all on your ``afunc`` parameter. If someone wants to pass in something that isn't a function, the compiler won't stop them. — mavnn, Apr 14 '15 at 13:18

Pattern matching based on the function signature

2 Answers2

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