Fluent Interfaces example using Delphi part II

Here is the second part of this interesting topic. As I'm still trying to redeem myself from my non popular first example, I'm sure this one will reach the expectations. For this example I'm trying to mimic the way LINQ works, using generics and delegates and I have adapted my solution using fluent Interfaces as well. This solution presents a IQueryList which contains a TList<T> which can be queried like we were using SQL. So, we can select certain values and apply a where clause to filter the final list. This example will give you a hint on how to correctly implement fluent interfaces and how to extend this functionality for your applications.

Delegates:

uses
  Generics.Collections, Generics.Defaults;

type
  TProc<T> = procedure (n : T) of Object;
  TProcList<T> = procedure (n : TList<T>) of Object;
  TFunc<T> = reference to function() : T;
  TFuncParam<T, TResult> = reference to function(param : T) : TResult;
  TFuncList<T, TResult> = reference to function(n : TList<T>) : TResult;
  TFuncListSelect<T, TResult> = reference to function(n : TList<T>) : TList<T>;

IQueryList:

IQueryList<T, TResult> = interface
    function Where(const param : TFuncParam<T, TResult>) : IQueryList<T, TResult>;
    function OrderBy(const AComparer: IComparer<T>) : IQueryList<T, TResult>;
    function Select(const param : TFuncListSelect<T, TResult>) : IQueryList<T, TResult>;
    function FillList(const param : TFuncList<T, TResult>) : IQueryList<T, TResult>;
    function Distinct() : IQueryList<T, TResult>;
    function List() : TList<T>;
  end;

  TQueryList<T, TResult> = class(TInterfacedObject, IQueryList<T, TResult>)
  private
    FList : TList<T>;
  protected
    function Where(const param : TFuncParam<T, TResult>) : IQueryList<T, TResult>;
    function FillList(const param : TFuncList<T, TResult>) : IQueryList<T, TResult>;
    function Select(const param : TFuncListSelect<T, TResult>) : IQueryList<T, TResult>;
    function Distinct() : IQueryList<T, TResult>;
    function List() : TList<T>;
    function OrderBy(const AComparer: IComparer<T>) : IQueryList<T, TResult>;
  public
    constructor Create();
    destructor Destroy(); override;
    class function New: IQueryList<T, TResult>;
  end;

constructor TQueryList<T, TResult>.Create();
begin
  FList := TList<T>.Create;
end;

destructor TQueryList<T, TResult>.Destroy;
begin
  if Assigned(FList) then
    FList.Free;
  inherited;
end;

function TQueryList<T, TResult>.Distinct: IQueryList<T, TResult>;
var
  list : TList<T>;
  i : integer;
begin
  list := TList<T>.Create();
  for i := 0 to FList.Count-1 do
  begin
    if not list.Contains(FList[i]) then
      list.Add(FList[i]);
  end;
  FList.Free;
  FList := list;
  result := Self;
end;

function TQueryList<T, TResult>.FillList(const param : TFuncList<T, TResult>): IQueryList<T, TResult>;
begin
  param(FList);
  Result := Self;
end;

function TQueryList<T, TResult>.List: TList<T>;
begin
  result := FList;
end;

class function TQueryList<T, TResult>.New: IQueryList<T, TResult>;
begin
  result := Create;
end;

function TQueryList<T, TResult>.OrderBy(const AComparer: IComparer<T>): IQueryList<T, TResult>;
begin
  FList.Sort(AComparer);
  result := Self;
end;

function TQueryList<T, TResult>.Select(const param: TFuncListSelect<T, TResult>): IQueryList<T, TResult>;
begin
  FList := param(FList);
  result := Self;
end;

function TQueryList<T, TResult>.Where(const param: TFuncParam<T, TResult>): IQueryList<T, TResult>;
var
  list : TList<T>;
  i: Integer;
  Comparer: IEqualityComparer<TResult>;
begin
  list := TList<T>.Create();
  for i := 0 to FList.Count-1 do
  begin
    Comparer := TEqualityComparer<TResult>.Default;
    if not Comparer.Equals(Default(TResult), param(FList[i])) then
      list.Add(FList[i]);
  end;
  FList.Free;
  FList := list;
  result := Self;
end;

Example Implementation <Integer, Boolean>:

procedure DisplayList();
var
  IqueryList : IQueryList<Integer, Boolean>;
  item : integer;
begin
  //Create the list and fill it up with random values
  IqueryList := TQueryList<Integer, Boolean>
    .New()
    .FillList(function ( list : TList<Integer> ) : Boolean
              var k : integer;
              begin
                for k := 0 to 100 do
                  list.Add(Random(100));
                result := true;
              end);

  //Display filtered values
  for item in IqueryList
    .Select(function ( list : TList<Integer> ) : TList<Integer>
              var
                k : integer;
                selectList : TList<Integer>;
              begin
                selectList := TList<Integer>.Create;
                for k := 0 to list.Count-1 do
                begin
                  if Abs(list.items[k]) > 0 then
                    selectList.Add(list.items[k]);
                end;
                list.Free;
                result := selectList;
              end)
    .Where(function ( i : integer) : Boolean
          begin
            result := (i > 50);
          end)
    .Where(function ( i : integer) : Boolean
          begin
            result := (i < 75);
          end)
    .OrderBy(TComparer<integer>.Construct(
         function (const L, R: integer): integer
         begin
           result := L - R; //Ascending
         end
     )).Distinct.List do
          WriteLn(IntToStr(item));
end;

This example fills up an initial list with 100 random numbers and then I query the list to give me all the values from the list which absolute value is greater than 0 and the values are between 50 and 75. From this list I want all the values ordered by value and I do not want repeated numbers (distinct method).

Example Implementation <String, String>:

procedure DisplayStrings();
const
   Chars = '1234567890ABCDEFGHJKLMNPQRSTUVWXYZ!';
var
  S: string;
  IqueryList : IQueryList<String, String>;
  item : String;
begin
  //Fill up the list with random strings
  IqueryList := TQueryList<String, String>
    .New
    .FillList(function ( list : TList<String> ) : String
              var
                k : integer;
                l : Integer;
              begin
                Randomize;
                for k := 0 to 100 do
                begin
                  S := '';
                  for l := 1 to 8 do
                    S := S + Chars[(Random(Length(Chars)) + 1)];
                  list.Add(S);
                end;
                result := '';
              end);
  //Query the list and retrieve all items which contains 'A'
  for item in IqueryList.Where(function ( i : string) : string
          begin
            if AnsiPos('A', i) > 0 then
              result := i;
          end).List do
    WriteLn(item);
end;

The string example is quite similar, it fills up a list with 100 random string values and then it filters the list displaying only items which contains the character "A".

Everything is based on interfaces so the garbage collector can step in and avoid memory leaks and you can find a sound widespread of generics, delegates and chaining methods all in once. This solution gives you control on the way data is treated and it can work with any type as it is using generics.

I have included all those examples in my personal repository on Google project. Please feel free to use it and  comment everything you like/dislike so we all can improve those examples. The Unit testing side includes interesting examples:

Enjoy it!.

I look forward to your comments.

Jordi

Related Links:

• LINQ C# Examples.
• DeHL project.