Method chaining

Method chaining, also known as named parameter idiom, is a common syntax for invoking multiple method calls in object-oriented programming languages. Each method returns an object, allowing the calls to be chained together in a single statement without requiring variables to store the intermediate results.[1]

Rationale

Local variable declarations are syntactic sugar because of the difficulty humans have with deeply nested method calls.[2][3]

Method chaining eliminate an extra variable for each intermediate steps. The developer is saved from the cognitive burden of naming the variable and keeping the variable in mind.

Method chaining has been referred to as producing a "train wreck" due to the increase in the number of methods that come one after another in the same line that occurs as more methods are chained together.[4]

A similar syntax is method cascading, where after the method call the expression evaluates to the current object, not the return value of the method. Cascading can be implemented using method chaining by having the method return the current object itself. Cascading is a key technique in fluent interfaces, and since chaining is widely implemented in object-oriented languages while cascading isn't, this form of "cascading-by-chaining by returning this" is often referred to simply as "chaining". Both chaining and cascading come from the Smalltalk language.

While chaining is syntax, it has semantic consequences, namely that requires methods to return an object, and if implementing cascading via chaining, this must be the current object. This prevents the return value from being used for some other purpose, such as returning an error value.

Examples

A common example is iostream in C++, where for example << returns the left object, allowing chaining.

Compare:

a << b << c;

equivalent to:

a << b;
a << c;

Another example in Javascript that utilize the build-in methods of Array:

somethings
  .filter(x => x.count > 10)
  .sort((a, b) => a.count - b.count)
  .map(x => x.name)

See also

References

  1. ↑ "Applying Method Chaining". http://firstclassthoughts.co.uk/: First Class Thoughts. Archived from the original on 2011-02-22. Retrieved 2011-04-13. In order to simplify repeated object interactions on the same object the old trick Method Chaining originating the world of Smalltalk should be enforced. The idea is to let methods return this rather than void, thus affecting especially set() and add() methods. Method chaining arose during the designers of Smalltalk pursuit to minimize the number of keywords in the language, which lead to the discovery that void is an unnecessary keyword!.
  2. ↑ "Session 18 Variable References". Today you learn that variable names are not necessary: they are really syntactic sugar.
  3. ↑ "CMSC 631 – Program Analysis and Understanding" (PDF). • Syntactic sugar for local declarations - let x = e1 in e2 is short for (λx.e2) e1
  4. ↑ Martin, Robert Cecil (2008). Clean Code: A Handbook of Agile Software Craftsmanship. Prentice Hall. ISBN 0-13-235088-2.
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