In computer programming, a pure function is a function that has the following properties:[1][2]

  1. the function return values are identical for identical arguments (no variation with local static variables, non-local variables, mutable reference arguments or input streams), and
  2. the function application has no side effects (no mutation of local static variables, non-local variables, mutable reference arguments or input/output streams).

Thus a pure function is a computational analogue of a mathematical function. Some authors, particularly from the imperative language community, use the term "pure" for all functions that just have the above property 2[3][4] (discussed below).


Pure functions

The following examples of C++ functions are pure:

Impure functions

The following C++ functions are impure as they lack the above property 1:

The following C++ functions are impure as they lack the above property 2:

The following C++ functions are impure as they lack both the above properties 1 and 2:

I/O in pure functions

I/O is inherently impure: input operations undermine referential transparency, and output operations create side effects. Nevertheless, there is a sense in which function can perform input or output and still be pure, if the sequence of operations on the relevant I/O devices is modeled explicitly as both an argument and a result, and I/O operations are taken to fail when the input sequence does not describe the operations actually taken since the program began execution.

The second point ensures that the only sequence usable as an argument must change with each I/O action; the first allows different calls to an I/O-performing function to return different results on account of the sequence arguments having changed.[5][6]

The I/O monad is a programming idiom typically used to perform I/O in pure functional languages.

Compiler optimizations

Functions that have just the above property 2 allow for compiler optimization techniques such as common subexpression elimination and loop optimization similar to arithmetic operators.[3] A C++ example is the length method, returning the size of a string, which depends on the memory contents where the string points to, therefore lacking the above property 1. Nevertheless, in a single-threaded environment, the following C++ code

std::string s = "Hello, world!";
int a[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int l = 0;

for (int i = 0; i < 10; ++i) {
  l += s.length() + a[i];

can be optimized such that the value of s.length() is computed only once, before the loop.

Some programming languages allow for declaring a pure property to a function:

Unit testing

Since pure functions have identical return values for identical arguments, they are well suited to unit testing.

See also


  1. ^ Bartosz Milewski (2013). "Basics of Haskell". School of Haskell. FP Complete. Archived from the original on 2016-10-27. Retrieved 2018-07-13. Here are the fundamental properties of a pure function: 1. A function returns exactly the same result every time it's called with the same set of arguments. In other words a function has no state, nor can it access any external state. Every time you call it, it behaves like a newborn baby with blank memory and no knowledge of the external world. 2. A function has no side effects. Calling a function once is the same as calling it twice and discarding the result of the first call.
  2. ^ Brian Lonsdorf (2015). "Professor Frisby's Mostly Adequate Guide to Functional Programming". GitHub. Retrieved 2020-03-20. A pure function is a function that, given the same input, will always return the same output and does not have any observable side effect.
  3. ^ a b "GCC 8.1 Manual". GCC, the GNU Compiler Collection. Free Software Foundation, Inc. 2018. Retrieved 2018-06-28.
  4. ^ Fortran 95 language features#Pure Procedures
  5. ^ Peyton Jones, Simon L. (2003). Haskell 98 Language and Libraries: The Revised Report (PDF). Cambridge, United Kingdom: Cambridge University Press. p. 95. ISBN 0-521 826144. Retrieved 17 July 2014.
  6. ^ Hanus, Michael. "Curry: An Integrated Functional Logic Language" (PDF). Institut für Informatik, Christian-Albrechts-Universität zu Kiel. p. 33. Archived from the original (PDF) on 25 July 2014. Retrieved 17 July 2014.
  7. ^ Pure attribute in Fortran
  8. ^ Pure attribute in D language
  9. ^ "Common Function Attributes". Using the GNU Compiler Collection (GCC. Retrieved 22 July 2021.
  10. ^ constexpr attribute in C++