Modules and Constants

Pure Functions

No Side Effects

A pure function promises no side effects and no I/O. The compiler enforces it.

Program

Play the program to square a number with a pure function.

pure_function.f90 
module math_mod
    implicit none
contains
    pure function square(x) result(y)
        integer, intent(in) :: x
        integer :: y
        y = x * x
    end function square
end module math_mod

program pure_function_demo
    use math_mod
    implicit none
    integer :: result_
    result_ = square(9)
    print '(I0)', result_
end program pure_function_demo
pure `pure function` cannot perform I/O or modify globals.
compile check The compiler rejects side effects inside `pure` procedures.
functional style Pure functions are safe to call from elemental and parallel contexts.