Back to Questions
3.1k
views4
comments
Generic way to cast to typeof<'a>? So inline (int v) vs. (float v)?
f#

I'm writing a generic numeric algorithm. I want to instantiate one copy of the algorithm for each type used, a la C++ templates. I don't want to "inline", because that would presumably expand the entire code each time the algorithm is used.

-------------------------------------------------

Q 1: Is there a template mechanism in F#?

NOTE: For now, I instead manually make a copy for each type:
Given:

1
2
3
4

// Pretend this is a function with lots of statements.
// Don't want to pay the code size of "inline" every time it is used.
let inline MyAlg0 (n1:'a) (n2:'a) (n3:'a) =
    ((n1 + n2) + (n1 + n3) + (n2 + n3))

I declare:

1
2
3

// SOLUTION: Create versions for each type used.
let MyAlgInt n1 n2 n3 :int = MyAlg0 n1 n2 n3
let MyAlgFloat n1 n2 n3 :float = MyAlg0 n1 n2 n3

-Or- I wrap inside a class declaration, so that I can declare these both as MyAlg, overloading the type. Better to use, but a bit annoying to set up:

1
2
3
4
5
6
7
8
9
10

type My(t: System.Type) =
    [<OverloadID("MyAlgInt")>]
    static member Alg (n1:int) n2 n3 :int =
        MyAlg0 n1 n2 n3
    [<OverloadID("MyAlgFloat")>]
    static member Alg (n1:float) n2 n3 :float =
        MyAlg0 n1 n2 n3

My.Alg 1 2 3
My.Alg 1. 2. 3.

---------------------------------------------------

Q 2: Is there a way to cast to "typeof<'a>"?
I want a constant to be compile-time cast to the type determined at inline expansion.
I can explicitly do this when I have a known type:

1
2
3

let MyConst = 123
MyAlgInt 1 2 (int MyConst)
MyAlgFloat 1. 2. (float MyConst)

But I haven't found a way to use such a constant inside my inline generic algorithm:

1
2
3
4

let inline MyAlg2 (n1:'a) (n2:'a) =
    ((n1 + n2) + (n1 + MyConst) + (n2 + MyConst))
MyAlg2 1 2
MyAlg2 1. 2. // ***TYPE ERROR***

E.g., I want to replace those "MyConst"s with "(int MyConst)" or "(float MyConst)", depending on the final expanded type.

~TMSteve

.

Citizens of hubFS rejoice! The Toolmaker is back.

By on 6/16/2009 6:01 PM ()

found in f# source

1
2
3
4
5
6
7

ToolmakerSteve:

let inline retype (x:'a) : 'b = (# "" x : 'b #)

let inline MyAlg2 (n1:'a) (n2:'a) =

    ((n1 + n2) + (n1 + (retype MyConst : 'a)) + (n2 + (retype MyConst : 'a)))

(# #) allows to write inline IL

By on 10/29/2008 5:13 PM ()

just to gain some experience I typed this in

1
2
3
4
5
6

let MyConst = 123
let inline retype (x:'a) : 'b=(# "" x: 'b #)
let inline MyAlg2 (n1:'a) (n2:'a) =
      ((n1+n2)+(n1 + (retype MyConst:'a)) + (n2 + (retype MyConst:'a)))
printfn "??? %02d" (MyAlg2 1 2)      // ==> 252
printfn "!!! %5.2f" (MyAlg2 1. 2.)   // ==> 6.00


either the deprication of the construct if faulting the float sample to 6.00 instead of
252.00 or what? it now look retype evaluates floats to 0.0!?

Just trying to gain insight in the language

By on 6/18/2009 12:29 AM ()

(rereading some old posts)

1
2
3
4
5
6
7
8

FYI, in the latest release - 1.9.6.16:

> let inline retype (x:'a) : 'b = (# "" x : 'b #);;

  let inline retype (x:'a) : 'b = (# "" x : 'b #);;
  ---------------------------------^^^^^^^^^^^^^

C:\Users\James\AppData\Local\Temp\stdin(3,34): warning FS0042: This construct is deprecated: it is only for use in the F# library
By on 6/16/2009 4:04 PM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper