proposal for setting default values of optional arguments

proposals/default_optional_arguments/proposal.txt

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+To: J3                                                     J3/XX-XXX
+From: Milan Curcic
+Subject: Default value for optional arguments
+Date: 2020-January-10
+
+Proposal for Fortran Standard: 202y (NOT 202x)
+
+
+Prior art: https://j3-fortran.org/doc/year/18/18-136r1.txt
+
+
+1. Problem
+
+Currently, standard Fortran does not allow setting a default value
+for optional arguments. Default value of an optional argument is 
+the value that the dummy argument would take if the corresponding 
+actual argument is not present. If declaring a dummy argument as 
+optional, the user must:
+
+  * Explicitly test for the presence of the actual argument using 
+    the intrinsic function present();
+  * Use a separate variable inside the procedure to assign the value
+    because the optional dummy argument that is not present must not
+    be referenced in expressions other than as actual argument to 
+    the intrinsic function present().
+
+This example function illustrates the problem:
+
+    real function quadratic(x, a, b, c)  
+      ! returns a + b * x + c * x**2 if c is present 
+      ! and a + b * x otherwise
+      real, intent(in) :: x, a, b
+      real, intent(in), optional :: c
+      real :: c_tmp ! use another var. to reference the missing arg
+      c_tmp = 0 ! default value if c is not present
+      if (present(c)) c_tmp = c
+      quadratic = a + b * x + c_tmp * x**2
+    end function quadratic
+
+For any dummy argument with the optional attribute, the programmer 
+must use the intrinsic function present() to check for the presence 
+of the argument. Furthermore, if the optional dummy argument is 
+meant to be used in multiple places in the procedure, the programmer 
+is likely to use the pattern from the example above, where a 
+"temporary" variable is declared and used in place of the dummy 
+argument, which disconnects the implementation from the user 
+interface. Furthermore, this requires at least 3 lines of code
+(declaration of c_tmp, initialization of c_tmp, and testing for the 
+presence of c) only to handle the scenario of a missing optional
+argument.
+
+This proposal seeks to address the issue that explicitly checking 
+for presence of the optional dummy argument and using a helper 
+variable is cumbersome and error-prone. The primary benefit of this 
+feature is the reduction in source code needed to handle optional 
+arguments. This benefit is even greater in scenarios where the 
+optional argument is used in many places in the procedure, and a 
+helper variable is used for its value instead. Reduction in needed 
+source code would result in more readable and more correct programs. 
+The secondary benefit of this is programmer happiness, as working 
+with optional arguments would require less typing.
+
+2. Proposed solution
+
+As suggested by Van Snyder in 18-136r1, the problem could be solved 
+by allowing an optional argument to be initialized using a constant 
+expression. The optional argument would then only be initialized if 
+the corresponding actual argument is not provided by the caller. 
+Example:
+
+    real function quadratic(x, a, b, c)
+      ! returns a + b * x + c * x**2 if c is present 
+      ! and a + b * x otherwise
+      real, intent(in) :: x, a, b
+      real, intent(in), optional :: c = 0
+      quadratic = a + b * x + c * x**2
+    end function quadratic
+
+In this snippet, we use the assignment operator (=) to specify the
+default value of the optional dummy argument. 
+
+Like initializer, the optional argument can be assigned any constant 
+expression, as defined in Section 10.1.12 of 18-007r1. 
+
+While there may be concerns that the same syntax is used to 
+implicitly set the save attribute for variables in procedures, there 
+is no conflict because the language prohibits dummy arguments 
+from having an initializer, or the save attribute. The change to the 
+standard to allow this feature would thus be to allow an optional 
+dummy argument to have an initializer, which would be triggered only 
+when corresponding actual argument is not passed by the caller. 
+
+This improvement has already been suggested by Van Snyder in 
+18-136r1, has received votes in the user survey for 202X (see
+https://isotc.iso.org/livelink/livelink?func=ll&objId=19530634&objAction=Open&viewType=1), 
+and appeared on Data subgroup's wishlist at meeting 215 (see 
+https://j3-fortran.org/doc/year/18/18-122r1.txt).
+
+3. Backward compatibility
+
+This addition to the language would not break any existing standard
+conforming Fortran program, and thus preserves Fortran's backward
+compatibility.
+
+4. Further discussion
+
+Online discussion that led to this proposal can be found at
+https://github.com/j3-fortran/fortran_proposals/issue/22.