Question

I am trying to implement a generic timer function in OCaml which will take as input a function of arbitrary arity and return type ‘r and returns a function with:

• the same arity and types of input parameters
  , and
• return type float * 'r where the float would be a metric of the time spent in the function (e.g. reported by Sys.time())

The problem is I can’t implement it in such a way that it can handle functions of any arity. E.g. the following code:

let timer f =              
   let timerf x y =                                 
      let t0 = Sys.time ()                                         
      in let result = f x y                                                 
      in let diff = Sys.time() -. t0                                     
      in diff, result                                    
   in timerf    

works only with functions of input arity 2. It is not obvious to me how to generalize it to handle functions of any arity. I was hoping the partial function applications would somehow magically solve the conundrum but I can’t get it to work.

Answer 1

I understand your intention of making a timer function with arbitrary arity. But you cannot do it in an easy way in OCaml.

Moreover, a timer function with only one param is enough for use in practice:

let timer f x =
   let t0 = Sys.time()                                         
   in let result = f x                                              
   in let diff = Sys.time() -. t0                                     
   in diff, result

Since any function g with any arity can be passed to timer easily by:

let diff, result = timer (fun () -> g x1 x2 x3 ... xN) ()

or better by using partial application (as suggested by @Andreas):

let diff, result = timer (g x1 x2 x3 ... xN-1) xN