Lambda (programming)

Lambda is an operator used to denote anonymous functions or closures, following the usage of lambda calculus, in programming languages such as C#, Erlang, Lisp, Python, Ruby, Scala, and recently C++11, the latest iteration of C++.

Examples

C#

In C#, lambda expressions are often used with LINQ:

var allWikipediaPages = GetAllWikipediaPages();
var lambdaWikipediaPage = allWikipediaPages.First(wp => wp.Title == "Lambda (programming)");

C++

In C++, lambda expressions can take this form:

auto square = [] (int x) -> int { return x * x; };
square(5); // returns 25

Erlang

In Erlang, lambda expressions (usually called as "funs") can take this form:

F = fun(X) -> X * X end,
F(5). % returns 25

Haskell

In Haskell, lambda expressions can take this form:

Prelude> let f = \x -> x + 1
Prelude> :t f
f :: Integer -> Integer
Prelude> f 2
3

Python

In Python, an example of this use of lambda is this sample of computer code that sorts a list alphabetically by the last character of each entry:

>>> stuff = ['woman', 'man', 'horse', 'boat', 'plane', 'dog']
>>> sorted(stuff, key=lambda word: word[-1])
['horse', 'plane', 'dog', 'woman', 'man', 'boat']

Scala

In Scala, lambda expressions can take this form:

scala> (x:Int, y:Int) => x + y
res0: (Int, Int) => Int = <function2>
scala> res0(1, 2)
res1: Int = 3

Argument types can be inferred when applied to a list:

scala> List(1, 2, 3, 4)
res0: List[Int] = List(1, 2, 3, 4)
scala> res0.foldLeft(0)((x, y) => x + y)
res1: Int = 10