Description
It is a Fortran 95 feature. PURE attribute added to a function or subroutine means that it is side-effect free, output only depends on its arguments (in case of function it is also not allowed to write to the arguments). From the standard:
... a pure subprogram shall not contain any operation
that could conceivably result in an assignment or pointer assignment to a common variable, a
variable accessed by use or host association, or an INTENT (IN) dummy argument; nor shall
a pure subprogram contain any operation that could conceivably perform any external file
I/O or STOP operation. Note the use of the word conceivably; it is not sufficient for a pure
subprogram merely to be side-effect free in practice. For example, a function that contains an
assignment to a global variable but in a block that is not executed in any invocation of the
function is nevertheless not a pure function. The exclusion of functions of this nature is
required if strict compile-time checking is to be used.
Pure functions can be used in FORALL construct; also intrinsic functions are expected to be pure.
Example:
$ cat pure.f95
pure function f(x) result(y)
integer, intent(in) :: x
integer :: y
y = x * 5
end function
$ fort -fsyntax-only pure.f95
pure.f95:1:6: error: expected '='
pure function f(x) result(y)
^
pure.f95:2:10: error: expected '='
integer, intent(in) :: x
^
pure.f95:3:11: error: expected '='
integer :: y
^
pure.f95:5:1: error: expected 'end program'
end function
^
<unknown>:0: note: to match this 'program'
$ gfortran -fsyntax-only pure.f95
$
Checks will be needed to catch statements not allowed inside of a pure function. Also, pure function should produce IR function with appropriate metadata (something like readonly or argmemonly).