Functions with generic parameter types

Question

I am trying to figure out how to define a function that works on multiple types of parameters (e.g. int and int64). As I understand it, function overloading is not possible in F# (certainly the compiler complains). Take for example the following function.

let sqrt_int = function
    | n:int   -> int (sqrt (float n))
    | n:int64 -> int64 (sqrt (float n))

The compiler of course complains that the syntax is invalid (type constraints in pattern matching are not supported it seems), though I think this illustrates what I would like to achieve: a function that operates on several parameter types and returns a value of the according type. I have a feeling that this is possible in F# using some combination of generic types/type inference/pattern matching, but the syntax has eluded me. I've also tried using the :? operator (dynamic type tests) and when clauses in the pattern matching block, but this still produces all sorts errors.

As I am rather new to the language, I may very well be trying to do something impossible here, so please let me know if there is alternative solution.

Answer 1

Here's another way using runtime type checks...

let sqrt_int<'a> (x:'a) : 'a = // '
    match box x with
    | :? int as i -> downcast (i |> float |> sqrt |> int |> box)
    | :? int64 as i -> downcast (i |> float |> sqrt |> int64 |> box)
    | _ -> failwith "boo"

let a = sqrt_int 9
let b = sqrt_int 100L
let c = sqrt_int "foo" // boom

Answer 2

Overloading is typically the bugaboo of type-inferenced languages (at least when, like F#, the type system isn't powerful enough to contain type-classes). There are a number of choices you have in F#:

• Use overloading on methods (members of a type), in which case overloading works much like as in other .Net languages (you can ad-hoc overload members, provided calls can be distinguished by the number/type of parameters)
• Use "inline", "^", and static member constraints for ad-hoc overloading on functions (this is what most of the various math operators that need to work on int/float/etc.; the syntax here is weird, this is little-used apart from the F# library)
• Simulate type classes by passing an extra dictionary-of-operations parameter (this is what INumeric does in one of the F# PowerPack libraries to generalize various Math algorithms for arbitrary user-defined types)
• Fall back to dynamic typing (pass in an 'obj' parameter, do a dynamic type test, throw a runtime exception for bad type)

For your particular example, I would probably just use method overloading:

type MathOps =
    static member sqrt_int(x:int) = x |> float |> sqrt |> int
    static member sqrt_int(x:int64) = x |> float |> sqrt |> int64

let x = MathOps.sqrt_int 9
let y = MathOps.sqrt_int 100L

Answer 3

Yes, this can be done. Take a look at this hubFS thread.

In this case, the solution would be:

let inline retype (x:'a) : 'b = (# "" x : 'b #)
let inline sqrt_int (n:'a) = retype (sqrt (float n)) : 'a

Caveat: no compile-time type checking. I.e. sqrt_int "blabla" compiles fine but you'll get a FormatException at runtime.

Answer 4

Not to take away from the correct answers already provided, but you can in fact use type constraints in pattern matching. The syntax is:

| :? type ->

Or if you want to combine type checking and casting:

| :? type as foo ->

Answer 5

This works:

type T = T with
    static member ($) (T, n:int  ) = int   (sqrt (float n))
    static member ($) (T, n:int64) = int64 (sqrt (float n))

let inline sqrt_int (x:'t) :'t = T $ x

It uses static constraints and overloading, which makes a compile-time lookup on the type of the argument.

The static constraints are automatically generated in presence of an operator (operator $ in this case) but it can always be written by hand:

type T = T with
    static member Sqr (T, n:int  ) = int   (sqrt (float n))
    static member Sqr (T, n:int64) = int64 (sqrt (float n))

let inline sqrt_int (x:'N) :'N = ((^T or ^N) : (static member Sqr: ^T * ^N -> _) T, x)

More about this here.