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(In contrast to applied, when distinguishing between applied and pure mathematics; the latter being divorced from the real world.)

In mathematics, functions are pure - they merely associate each possible input value with an output value, and nothing more. In Haskell, most functions uphold this principle. As a result, they usually are referentially transparent - each of them does not depend on anything other than its parameters, so when invoked in a different context or at a different time with the same arguments, it will produce the same result. In comparison, procedures or subroutines are more complicated - for example, they could also:

  • read from or write to global variables,
  • send data to a file,
  • or print to a screen.

A programming language may sometimes be known as purely functional if:

  • it only permits programs to be defined in terms of pure definitions,
  • and functions are its primary means of abstraction.

However there has been some debate in the past as to the precise meaning of these terms:

See also

  • The [[pure]] attribute, JTC1.22.32 Programming Language Evolution Working Group (2015).