What is FluentQuery?

FluentQuery allows you to operate on containers of items in Delphi and AppMethod in a fluent, declarative fashion.

Huh?

OK, I agree, that was not very helpful.

Let's say you have a TListbox of names, and you want to display the first two that start with 'Miss' (case-insensitive) and contain a hyphen.

The standard solution in Delphi and AppMethod would be something like this:

var
  LName : String;
  LFoundCount : Integer;
begin
  LFoundCount := 0;
  for LName in Listbox1.Items do
    if (LName.StartsWith('Miss', True)) and
       (LName.Contains('-')) then
    begin
      Inc(LFoundCount);
      ShowMessage(LName);

      if LFoundCOunt = 2 then
        break;
    end;
end;

There's nothing incorrect about that code, but it's unclear both in terms of what items in the listbox we're acting on, and unclear what we're doing to those items. This is because the code that selects the strings we want to display, and the code that actually displays them, are all mixed in together.

FluentQuery lets you represent the same thing like this:

var
  LName : String;
begin
  for LName in StringQuery
                 .From(ListBox1.Items)
                 .StartsWith('Miss')
                 .Contains('-')
                 .Take(2) do
    ShowMessage(LName);
end;

Yes, it's less code, but more importantly, it is much clearer. The statement of what items we're going to act on (in the for..in statement) reads fairly naturally, and the code that acts on the items is seperate and self contained.

You can of course include the same operation multiple times in your query, eg:

StartsWith('Miss').Contains('-').Contains('VII').Take(2)

Not just for strings

FluentQuery is not just for strings in Listboxes though. Want all the char's in a string that are either letters or digits? Use:

for LChar in StringQuery.From(FMyString).IsLetterOrDigit do

If FluentQuery does not contain a query operation you need, no problem. You can use the Where operation which takes a TPredicate<T> to do your custom querying. This example finds all TPerson objects in your TObjectList<TPerson> that are over 18:

var
  LPerson : TPerson;
  LOver18 : TPredicate<TPerson>;
begin
  LOver18 := function (Person : TPerson) : boolean
             begin
               Result := Person.Age > 18;
             end;

  for LPerson in ObjectQuery<TPerson>
                   .Select
                   .From(FPersonCollection)
                   .Where(LOver18) do
  begin
      // do something here with your adult LPerson
  end;
end;

Not just for..in loops

FluentQuery is not just for for..in loops though. In the prior example, if you only want the First TPerson object over 18, your query could look like this:

  LPerson := ObjectQuery<TPerson>
                   .Select
                   .From(FPersonCollection)
                   .Where(LOver18)
                   .First;

You can export the results of a query as another collection, such as:

var
  LStrings : TStrings;
begin
  LStrings := StringQuery
                .From(FStrings)
                .Skip(4)
                .Take(3)
                .ToTStrings;

You can also use FluentQuery to define a TPredicate<T> for use in other classes, such as this example filtering a listbox:

Listbox.FilterPredicate := StringQuery.EndsWith('e').Predicate;

The goal is to define a declarative query language for any Delphi container.

Containers

FluentQuery currently supports querying over the following types of containers:

Anything with a TEnumerator<T>, such as TList<T>, TObjectList<T>, etc
Strings in a TStrings, such as TStringList, etc
Chars in a String
Integers in a generic container, such as a TList<Integer>, etc
Pointers in a TList

The latest list is on the wiki.

Query Operations

FluentQuery determines which query operations are available to you by looking at the type for which you are querying. For example, if you are querying for strings, you have operations such as StartsWith, EndsWith, Contains, etc.

For the latest details of the operations available for different types, check the wiki

Back Story

This issue of mixing Selection code and Action code has bugged me for awhile, but I've never really found a solution I'm happy with. I tried some experiments but was never satisfied that the cure was better than the disease. A series of functional languages sessions at the recent Yow Conference in Australia was the light bulb moment that led me to this.