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The constraints on pure procedures may appear complicated, but it is not necessary for a programmer to be intimately familiar with them. From the programmer's point of view, these constraints can be summarized as follows: a pure procedure must not contain any operation that could conceivably result in an assignment or pointer assignment to a global variable or INTENT (IN) dummy argument, or perform any I/O or STOP operation. Note the use of the word conceivably; it is not sufficient for a pure procedure merely to be side-effect free in practice. For example, a function that contains an assignment to a global variable but in a branch that is not executed in any invocation of the function is nevertheless not a pure function. The exclusion of functions of this nature is unavoidable if strict compile-time checking is to be used. In the choice between compile-time checking and flexibility, the HPF committee decided in favor of enhanced checking.
It is expected that most library procedures will conform to the constraints required of pure procedures (by the very nature of library procedures), and so can be declared pure and referenced in FORALL statements and constructs and within user-defined pure procedures. It is also anticipated that most library procedures will not reference global data, whose use may sometimes inhibit concurrent execution.
The constraints on pure procedures are limited to those necessary to check statically for freedom from side effects, processor independence, and for lack of saved internal state. Subject to these restrictions, maximum functionality has been preserved in the definition of pure procedures. This has been done to make function calls in FORALL as widely available as possible, and so that quite general library procedures can be classified as pure.
A drawback of this flexibility is that pure procedures permit certain features whose use may hinder, and in the worst case prevent, concurrent execution in FORALL (that is, such references may have to be implemented by sequentialization). Foremost among these features are the access of global data, particularly distributed global data, and the fact that the arguments and, for a pure function, the result may be pointers or data structures with pointer components, including recursive data structures such as lists and trees. The programmer should be aware of the potential performance penalties of using such features.
(End of rationale.)