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Sorting confusion
f#

in my attemps to penetrate F# I found "List.sort" googling for samples I found the following variations

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List.sort (fun x y -> if (x = y) then 0 elif (x > y) then -1 else 1) [0..5] |> printfn "%A"
List.sortBy (fun x y -> if (x = y) then 0 elif (x > y) then -1 else 1) [0..5] |> printfn "%A"
List.sortWith (fun x y -> if (x = y) then 0 elif (x > y) then -1 else 1) [0..5] |> printfn "%A"

the first one doesn't compile since:

Error 4 This value is not a function and cannot be applied

the 2nd one compiles but throws a runtime error:

System.ArgumentException: Failure during generic comparison: the type Microsoft.FSharp.Core.OptimizedClosures+Invoke@2693[System.Int32,System.Int32,System.Int32] does not implement the System.IComparable interface. This error may be arise from the use of a function such as 'compare', 'max' or 'min' or a data structure such as 'Set' or 'Map' whose keys contain instances of this type

the last one compiles and neatly reverses the given sequence.

tried to use compare too but Int32 don't seem IComparable?

I do hope this is a VS2008 ctp issue, and that released VS2010 offers code samples in the helpsystem, and explains the 3 sort's above in detail.

Just a confused New kid on the block (despite my 50 years (hi hi))

A google later brought me to [link:cs.hubfs.net] (Sorry I didn't check first)

.

Hi,

The confusion comes from a recent change in F# library. Your first example used to work for a long time, but it has changed (to me, it's now cleaner and easier to use).

Yesterday, I wrote a set of examples for list functions, you can have a look here: [link:man.f-sharp.fr]. For details, you can find documentation here:

[link:research.microsoft.com]

Laurent.

By on 6/25/2009 5:34 AM ()

these examples may help you

List.sort uses generic comparison. See compare in Microsoft.FSharp.Core.Operators
note signature is 

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'T list -> 'T list
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let l = [5;4;3;2;1]

List.sort l |> printfn "%A"

sortBy uses generic comparison too but in keys

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type person = {name:string;age:int}
let persons = [{name="paul";age=28};{name="eric";age=56};{name="franck";age=21}]


// key is name
List.sortBy (fun p -> p.name) persons |> printfn "%A"
// key is age
List.sortBy (fun p -> p.age) persons |> printfn "%A"

// sort in reverse order using sortWith
List.sortWith (fun p1 p2 -> - compare p1.name p2.name) persons |> printfn "%A"
List.sortWith (fun p1 p2 -> - compare p1.age p2.age) persons |> printfn "%A"
By on 6/25/2009 3:03 AM ()

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