patterns.md

Common patterns and their functional equivalent

Creating new items for each element in a list

For code that looks like this:

for Item in List do
begin
  NewThing := TNewThing.Create;
  NewThing.Value := Item.Value;
  NewList.Add(NewThing);
end;

Instead, use the Map function like this:

NewList := _.Map<TThing, TNewThing>(List, 
  function(const Item: TThing): TNewThing
  begin
    Result := TNewThing.Create;
    Result.Value := Item.Value;
  end);

Filtering a list

For code that looks like this:

for Item in List do
begin
  if Item.ShouldFilter then
  begin
    NewList.Add(Item);
  end;
end;

Instead, use Filter:

NewList := _.Filter<TThing>(List,
  function(const Item: TThing): Boolean
  begin
    Result := Item.ShouldFilter;
  end);

Find an item in a list

For code that looks like this:

for Item in List do
begin
  if (Item.Value = 5) and Item.OtherValue then
  begin
    Result := Item;
    Exit;
  end;
end;

Instead, use Find (First in Spring4d):

Result := _.FindOrDefault<TThing>(List,
  function(const Item: TThing): Boolean
  begin
    Result := (Item.Value = 5) and Item.OtherValue;
  end, nil);

Creating a summary of items in a list

For code that looks like this:

for Item in List do
begin
  TotalVAT := TotalVAT + Item.Value * 0.21;
  TotalInclVAT := TotalInclVAT + Item.Value * 1.21;
end;

Instead, use Reduce:

Totals := _.Reduce<TThing, TVATTotals>(List,
  function(const Current: TVATTotals; const Item: TThing): TVATTotals
  begin
    Result.TotalVAT := Current.TotalVAT + Item.Value * 0.21;
    Result.TotalInclVAT := Current.TotalInclVAT + Item.Value * 1.21;
  end, EmptyTotals);

Check if all items match a certain condition

For code that looks like this:

Result := True;
for Item in List do
begin
  if Item.Value mod 2 <> 0 then
  begin
    Result := False;
    Exit;
  end;
end;

Instead, use All:

Result := _.All<TThing>(List,
  function(const Item: TThing): Boolean
  begin
    Result := Item.Value mod 2 <> 0;
  end);

Check if at least 1 item matches a certain condition

For code that looks like this:

Result := False;
for Item in List do
begin
  if Item.Value mod 2 = 0 then
  begin
    Result := True;
    Exit;
  end;
end;

Instead, use Any:

Result := _.Any<TThing>(List,
  function(const Item: TThing): Boolean
  begin
    Result := Item.Value mod 2 = 0;
  end);

Find the Object with a mininum or maximum value

For code that looks like this:

for Item in List do
begin
  if Item.SomeValue < MininumValue then
  begin
    MininumValue := Item.SomeValue;
    MininumItem := Item;
  end;
end;

Instead, use Min (or Max):

MininumItem := _.Min<TThing>(List,
  function(const Item: TThing): Integer
  begin
    Result := Item.SomeValue;
  end);