Fluent interface

In software engineering, a fluent interface is an object-oriented API whose design relies extensively on method chaining. Its goal is to increase code legibility by creating a domain-specific language (DSL). The term was coined in 2005 by Eric Evans and Martin Fowler.

Implementation
A fluent interface is normally implemented by using method chaining to implement method cascading (in languages that do not natively support cascading), concretely by having each method return the object to which it is attached, often referred to as   or. Stated more abstractly, a fluent interface relays the instruction context of a subsequent call in method chaining, where generally the context is
 * Defined through the return value of a called method
 * Self-referential, where the new context is equivalent to the last context
 * Terminated through the return of a void context

Note that a "fluent interface" means more than just method cascading via chaining; it entails designing an interface that reads like a DSL, using other techniques like "nested functions and object scoping".

History
The term "fluent interface" was coined in late 2005, though this overall style of interface dates to the invention of method cascading in Smalltalk in the 1970s, and numerous examples in the 1980s. A common example is the iostream library in C++, which uses the  or   operators for the message passing, sending multiple data to the same object and allowing "manipulators" for other method calls. Other early examples include the Garnet system (from 1988 in Lisp) and the Amulet system (from 1994 in C++) which used this style for object creation and property assignment.

C#
C# uses fluent programming extensively in LINQ to build queries using "standard query operators". The implementation is based on extension methods.

Fluent interface can also be used to chain a set of method, which operates/shares the same object. Instead of creating a customer class, we can create a data context which can be decorated with fluent interface as follows.

C++
A common use of the fluent interface in C++ is the standard iostream, which chains overloaded operators.

The following is an example of providing a fluent interface wrapper on top of a more traditional interface in C++:

Java
An example of a fluent test expectation in the jMock testing framework is:

The jOOQ library models SQL as a fluent API in Java:

The fluflu annotation processor enables the creation of a fluent API using Java annotations.

The JaQue library enables Java 8 Lambdas to be represented as objects in the form of expression trees at runtime, making it possible to create type-safe fluent interfaces, i.e., instead of: One can write:

Also, the mock object testing library EasyMock makes extensive use of this style of interface to provide an expressive programming interface.

In the Java Swing API, the LayoutManager interface defines how Container objects can have controlled Component placement. One of the more powerful  implementations is the GridBagLayout class which requires the use of the   class to specify how layout control occurs. A typical example of the use of this class is something like the following.

This creates a lot of code and makes it difficult to see what exactly is happening here. The  class provides a fluent mechanism, so you would instead write:

There are many places where fluent APIs can simplify how software is written and help create an API language that helps users be much more productive and comfortable with the API because the return value of a method always provides a context for further actions in that context.

JavaScript
There are many examples of JavaScript libraries that use some variant of this: jQuery probably being the most well known. Typically, fluent builders are used to implement "database queries", for example in the Dynamite client library:

A simple way to do this in JavaScript is using prototype inheritance and.

Scala
Scala supports a fluent syntax for both method calls and class mixins, using traits and the  keyword. For example:

Raku
In Raku, there are many approaches, but one of the simplest is to declare attributes as read/write and use the  keyword. The type annotations are optional, but the native gradual typing makes it much safer to write directly to public attributes.

PHP
In PHP, one can return the current object by using the  special variable which represent the instance. Hence  will make the method return the instance. The example below defines a class  and three methods to set its name, surname and salary. Each return the instance of the  class allowing to chain methods.

Python
In Python, returning  in the instance method is one way to implement the fluent pattern.

It is however discouraged by the language’s creator, Guido van Rossum, and therefore considered unpythonic (not idiomatic) for operations that do not return new values. Van Rossum provides string processing operations as example where he sees the fluent pattern appropriate.

Swift
In Swift 3.0+ returning  in the functions is one way to implement the fluent pattern.

Immutability
It's possible to create immutable fluent interfaces that utilise copy-on-write semantics. In this variation of the pattern, instead of modifying internal properties and returning a reference to the same object, the object is instead cloned, with properties changed on the cloned object, and that object returned.

The benefit of this approach is that the interface can be used to create configurations of objects that can fork off from a particular point; Allowing two or more objects to share a certain amount of state, and be used further without interfering with each other.

JavaScript example
Using copy-on-write semantics, the JavaScript example from above becomes:

Errors cannot be captured at compile time
In typed languages, using a constructor requiring all parameters will fail at compilation time while the fluent approach will only be able to generate runtime errors, missing all the type-safety checks of modern compilers. It also contradicts the "fail-fast" approach for error protection.

Debugging and error reporting
Single-line chained statements may be more difficult to debug as debuggers may not be able to set breakpoints within the chain. Stepping through a single-line statement in a debugger may also be less convenient.

Another issue is that it may not be clear which of the method calls caused an exception, in particular if there are multiple calls to the same method. These issues can be overcome by breaking the statement into multiple lines which preserves readability while allowing the user to set breakpoints within the chain and to easily step through the code line by line:

However, some debuggers always show the first line in the exception backtrace, although the exception has been thrown on any line.

Logging
Adding logging into the middle of a chain of fluent calls can be an issue. E.g., given: To log the state of  after the   method call, it is necessary to break the fluent calls:

This can be worked around in languages that support extension methods by defining a new extension to wrap the desired logging functionality, for example in C# (using the same Java ByteBuffer example as above):

Subclasses
Subclasses in strongly typed languages (C++, Java, C#, etc.) often have to override all methods from their superclass that participate in a fluent interface in order to change their return type. For example:

Languages that are capable of expressing F-bound polymorphism can use it to avoid this difficulty. For example:

Note that in order to be able to create instances of the parent class, we had to split it into two classes —  and , the latter with no content (it would only contain constructors if those were needed). The approach can easily be extended if we want to have sub-subclasses (etc.) too:

In a dependently typed language, e.g. Scala, methods can also be explicitly defined as always returning  and thus can be defined only once for subclasses to take advantage of the fluent interface: