How to use anonymous functions?

Question

How can I properly use Anonymous Functions? I am trying use a generic compare function but I get the following error in the example bellow. Can someone explain why does this happen?

program Project11;

{$APPTYPE CONSOLE}
{$R *.res}

uses
  System.SysUtils,
  System.Classes;

type
  TSort<T> = record
  private
    type
      TCompare = reference to function(const L, R: T): Integer;
  public
    class procedure Sort(var A: Array of T; const First, Last: Integer; const Compare: TCompare); static;
  end;

{ TSort<T> }

class procedure TSort<T>.Sort(var A: array of T; const First, Last: Integer; const Compare: TCompare);
var
  I: Integer;
begin
  I := Compare(1, 2); // [dcc32 Error] Project11.dpr(30): E2010 Incompatible types: 'T' and 'Integer'
end;

var
  A: Array of Integer;
begin
  TSort<Integer>.Sort(A, 1, 2,
  function(const L, R: Integer): Integer
  begin
    // Do something with L & R
  end);
end.

FWIW, TArray.Sort<T> in Generics.Collections implements this already. And also, FWIW, your compare function is no good. Imagine what happens when L is high(Integer) and R is low(Integer). — David Heffernan
– David Heffernan, Commented Jun 25, 2014 at 12:57
@DavidHeffernan compare function can be anything thats the beauty of generics without any kind of interfaces. — user3764855
– user3764855, Commented Jun 25, 2014 at 13:01
You are reinventing things from System.Generics.Collections and System.Generics.Defaults. — Stefan Glienke
– Stefan Glienke, Commented Jun 25, 2014 at 13:31
You don't need to deal with any interfaces. You can just supply a compare function. And I don't call your compare function beautiful. I call it an overflow generator. Your entire code can be replaced by TArray.Sort<Integer>(A). If you want to supply a compare function you use TArray.Sort<Integer>(A, TComparer<Integer>.Construct(Comparison)) where Comparison is your anonymous method. — David Heffernan
– David Heffernan, Commented Jun 25, 2014 at 13:38
Also, +1 for a really good SSCCE with complete code and a full error message. — David Heffernan
– David Heffernan, Commented Jun 25, 2014 at 13:42

Daniel · Accepted Answer · 2014-06-25 12:34:06Z

2

I think that you should actually want

I := Compare(A[1], A[2]);

or

I := Compare(A[First], A[Last]);

instead of

I := Compare(1, 2);

As TLama already mentioned: Compare expects two parameters of type T. A is an array of T, so you can supply its members. 1 and 2 however are integers.

The fact that you later on say that you want T to be an integer is not relevant at this point: If you can say at this point that your code ALWAYS will use integer as T, then you shouldn't use a generic

answered Jun 25, 2014 at 12:34

Daniel

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2 Comments

user3764855 Over a year ago

T will be anything not just an Integer.

Daniel Over a year ago

@user3764855 Thought so (otherwise using a generic wouldn't make sense). So in that case Compare(1, 2) is definitely wrong.

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