weyrich@csun1.UUCP (Orville Weyrich) (04/30/89)
My reading of the Ada LRM indicates that functions in Ada may not have OUT or IN OUT parameters, but may modify non-local variables. This means that while a function may not have explicit side effects (through the use of OUT or IN OUT parameters), it may have implicit side effects (through the modification of non-local variables or through calling a procedure which does). At best, this strikes me as a curious inconsistency in the design philosopy of Ada. DOES THE NET HAVE ANY COMMENTS ON THIS? One reason that comes to my mind for allowing functions to have implicit side effects is that as Ada is presently defined, it is not possible to reliably detect violations of a "no side effects" rule. IMHO it is better to not make a rule at all than to make a rule which cannot be enforced. IMHO the places in the LRM where phrases like "if XXX rule is broken then the program is erroneous", where XXX cannot be checked at compile time are, worse than useless. Another reason for allowing functions to have implicit side effects is to facilitate source-level instrumentation of Ada programs. This leads to the question "CAN the Ada language definition be modified in such a way that a prohibition on function side effects can be enforced without breaking some applications beyond repair?" I think that such a modification is both necessary and possible. I would propose the following: Introduce a new reserved word, NO_SIDE_EFFECTS, which can be placed following the formal_part of a procedure_specification to indicate that non-local variables may not be modified by the procedure. It is easy for the compiler to check at compile time that the procedure does not directly modify non-local variables, and the compiler can also easily complain if a procedure which is declared to have no side effects calls a procedure which is not so declared. Functions would be forbidden to modify non-local variables and would not be allowed to call procedures that were not declared with NO_SIDE_EFFECTS. For the RARE occaisions when functions need to have side effects, the additional reserved word UNCHECKED_SIDE_EFFECTS could be placed after the formal_part of the function_specification. When a function is declared with UNCHECKED_SIDE_EFFECTS, OUT and IN OUT parameters should be allowed. The above proposal would not badly break existing programs: only those which used identifiers NO_SIDE_EFFECTS or UNCHECKED_SIDE_EFFECTS, or which used functions which called procedures or modified non-local variables. In all of these cases, the compiler would complain (no mysterious changes in operation as happened with FORTRAN-66/FORTRAN-77 DO loops), and minor editing would suffice to restore proper program operation. I can see no execution penalty for the proposed changes, nor do I see any great problems for comnpiler writers. WHAT DOES THE NET THINK OF MY PROPOSAL? Is it in the spirit of Ada? Should it be propsed for Ada-9x? Are there problems which I have not anticipated? Is there a better way? -- Orville R. Weyrich, Jr. | UUCP : ...gatech!csun1!weyrich Department of Computer Science | INTERNET: weyrich@csun1.cs.uga.edu University of Georgia | Athens, GA 30602 USA | MA BELL : (404) 542-1082