Back to Questions
1.6k
views5
comments
0
score		+
-
Generic numeric function question
f#

I want to write something like this:

1
2
3

// 'a is numeric
let foo (value : int) (coeff : 'a) =
    coeff * convert_to_'a (value >>> 8)

I want to be able to pass either an int or a float as coeff, and have the correct type as result. Right now I pass an extra argument which is the convert_to_'a function (int() or float()). I was wondering if there's any way to do it without the extra argument and without ifs inside foo's body.

I'm a conciseness maniac :)

.Reply
Hide this comment

Capable of doing that would be great(hope to see it in some future release) and answer the question in the comment of Matthew's blog(what good is static member extension):

Is Enumerable.CharRange('a', 'z') really better than EnumerableExtensions.CharRange('a', 'z')?

The answer is of course, I can write these 'type class' style generic function.

By on 2/6/2010 6:32 PM ()Reply
Hide this comment

Seems to be another case where type class would help. That is if 'a implement fromintegral it would work.

the equivalent in F# I believe would be something like the following :

1
2
3
4

 

let inline foo (value:int) coeff = coeff * (^a : (static member fromIntegral : int -> ^a) value)

Unfortunately, 'a in this case must be some defined type rather than standard type like float or int as I failed to find a way to extend the builtin type like int/float. Even c# cannot do it for value type I would assume. I also failed to find the C# extension method equivalent in F# to extend builtin type like list or array.

By on 2/4/2010 6:00 PM ()Reply
Hide this comment

Hi,
I posted a more detailed answer about static constraints at StackOverflow here:
[link:stackoverflow.com]

Unfortunately, I don't think this will help you - as Gary wrote, his example won't work with standard F# data types and there is no pre-defined function in LanguagePrimitives that would allow you to do this.

In some cases, you could also use NumericAssociations (in Microsoft.FSharp.Math in PowerPack.dll), which is a lookup table that you can use to get basic operations for working with numeric types with runtime type info as the key. For more info, look for example here: [link:cs.hubfs.net] But again, it doesn't look like it has operations powerful enough to express what you want.

By on 2/5/2010 4:12 AM ()Reply
Hide this comment

Found a blog post by Mattew Podwysocki about extension methods.

[link:codebetter.com]

Though I don't understand why the following failed:

1
2
3
4
5
6
7
8
9
10
11
12

 

module MyExtensons

type System.Double with
  static member fromIntegral x = float(x)
  
let inline fromIntegral< ^a when ^a: (static member fromIntegral : int -> ^a) > x = 
  (^a : (static member fromIntegral : int -> ^a) x)
  
let x = fromIntegral<System.Double> 1

yet just saying 'System.Double.fromIntegral 1' does work.

By on 2/6/2010 12:41 PM ()Reply
Hide this comment

For now, inline static member constraints can't "see" extension methods. This could be useful, and is a suggestion we'll consider in a future release.

By on 2/6/2010 12:56 PM ()Reply

    Topic tags

    See all 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

    Logging in...