Lambda comes from the Lambda Calculus and refers to anonymous functions in programming.
Why is this cool? It allows you to write quick throw away functions without naming them. It also provides a nice way to write closures. With that power you can do things like this.
Python
def adder(x):
return lambda y: x + y
add5 = adder(5)
add5(1)
6
As you can see from the snippet of Python, the function adder takes in an argument x, and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.
Examples in other languages
Perl 5
sub adder {
my ($x) = @_;
return sub {
my ($y) = @_;
$x + $y
}
}
my $add5 = adder(5);
print &$add5(1) == 6 ? "ok\n" : "not ok\n";
JavaScript
var adder = function (x) {
return function (y) {
return x + y;
};
};
add5 = adder(5);
add5(1) == 6
JavaScript (ES6)
const adder = x => y => x + y;
add5 = adder(5);
add5(1) == 6
Scheme
(define adder
(lambda (x)
(lambda (y)
(+ x y))))
(define add5
(adder 5))
(add5 1)
6
C# 3.5 or higher
Func<int, Func<int, int>> adder =
(int x) => (int y) => x + y; // `int` declarations optional
Func<int, int> add5 = adder(5);
var add6 = adder(6); // Using implicit typing
Debug.Assert(add5(1) == 6);
Debug.Assert(add6(-1) == 5);
// Closure example
int yEnclosed = 1;
Func<int, int> addWithClosure =
(x) => x + yEnclosed;
Debug.Assert(addWithClosure(2) == 3);
Swift
func adder(x: Int) -> (Int) -> Int{
return { y in x + y }
}
let add5 = adder(5)
add5(1)
6
PHP
$a = 1;
$b = 2;
$lambda = fn () => $a + $b;
echo $lambda();
Haskell
(\x y -> x + y)
Java see this post
// The following is an example of Predicate :
// a functional interface that takes an argument
// and returns a boolean primitive type.
Predicate<Integer> pred = x -> x % 2 == 0; // Tests if the parameter is even.
boolean result = pred.test(4); // true
Lua
adder = function(x)
return function(y)
return x + y
end
end
add5 = adder(5)
add5(1) == 6 -- true
Kotlin
val pred = { x: Int -> x % 2 == 0 }
val result = pred(4) // true
Ruby
Ruby is slightly different in that you cannot call a lambda using the exact same syntax as calling a function, but it still has lambdas.
def adder(x)
lambda { |y| x + y }
end
add5 = adder(5)
add5[1] == 6
Ruby being Ruby, there is a shorthand for lambdas, so you can define adder
this way:
def adder(x)
-> y { x + y }
end
R
adder <- function(x) {
function(y) x + y
}
add5 <- adder(5)
add5(1)
#> [1] 6
A function is a piece of code that is called by name. It can be passed data to operate on (i.e. the parameters) and can optionally return data (the return value). All data that is passed to a function is explicitly passed.
A method is a piece of code that is called by a name that is associated with an object. In most respects it is identical to a function except for two key differences:
- A method is implicitly passed the object on which it was called.
- A method is able to operate on data that is contained within the class (remembering that an object is an instance of a class - the class is the definition, the object is an instance of that data).
(this is a simplified explanation, ignoring issues of scope etc.)
Best Answer
This was explained by Mads Torgersen in C# Design Meeting Notes where local functions were first discussed:
To expand on it some more, the advantages are:
Performance.
When creating a lambda, a delegate has to be created, which is an unnecessary allocation in this case. Local functions are really just functions, no delegates are necessary.
Also, local functions are more efficient with capturing local variables: lambdas usually capture variables into a class, while local functions can use a struct (passed using
ref
), which again avoids an allocation.This also means calling local functions is cheaper and they can be inlined, possibly increasing performance even further.
Local functions can be recursive.
Lambdas can be recursive too, but it requires awkward code, where you first assign
null
to a delegate variable and then the lambda. Local functions can naturally be recursive (including mutually recursive).Local functions can be generic.
Lambdas cannot be generic, since they have to be assigned to a variable with a concrete type (that type can use generic variables from the outer scope, but that's not the same thing).
Local functions can be implemented as an iterator.
Lambdas cannot use the
yield return
(andyield break
) keyword to implementIEnumerable<T>
-returning function. Local functions can.Local functions look better.
This is not mentioned in the above quote and might be just my personal bias, but I think that normal function syntax looks better than assigning a lambda to a delegate variable. Local functions are also more succinct.
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