Editing First-class function (section)

=== Higher-order functions: passing functions as arguments ===
{{further information|Higher-order function}}
In languages where functions are first-class citizens, functions can be passed as arguments to other functions in the same way as other values (a function taking another function as argument is called a higher-order function). In the language [[Haskell (programming language)|Haskell]]:
<syntaxhighlight lang="haskell">
map :: (a -> b) -> [a] -> [b]
map f []     = []
map f (x:xs) = f x : map f xs
</syntaxhighlight>

Languages where functions are not first-class often still allow one to write higher-order functions through the use of features such as [[function pointer]]s or [[Delegate (CLI)|delegate]]s. In the language [[C (programming language)|C]]:
<syntaxhighlight lang="c">
void map(int (*f)(int), int x[], size_t n) {
    for (int i = 0; i < n; i++)
        x[i] = f(x[i]);
}
</syntaxhighlight>

There are a number of differences between the two approaches that are ''not'' directly related to the support of first-class functions. The Haskell sample operates on [[List (computing)|list]]s, while the C sample operates on [[Array data structure|arrays]]. Both are the most natural compound data structures in the respective languages and making the C sample operate on linked lists would have made it unnecessarily complex. This also accounts for the fact that the C function needs an additional parameter (giving the size of the array.) The C function updates the array [[in-place]], returning no value, whereas in Haskell data structures are [[persistent data structure|persistent]] (a new list is returned while the old is left intact.) The Haskell sample uses [[recursion]] to traverse the list, while the C sample uses [[iteration]]. Again, this is the most natural way to express this function in both languages, but the Haskell sample could easily have been expressed in terms of a [[fold (higher-order function)|fold]] and the C sample in terms of recursion. Finally, the Haskell function has a [[Polymorphism (computer science)|polymorphic]] type, as this is not supported by C we have fixed all type variables to the type constant <code>int</code>.