optval example

[ivan-pi]

The Fortran-lang stdlib defines a function optval that can be used to define provide a fallback value for an optional argument.

The function is currently implemented with the help of the fypp preprocessor, by looping through all the desired types and kinds:

  #:for k1, t1 in KINDS_TYPES
    pure elemental function optval_${t1[0]}$${k1}$(x, default) result(y)
    ${t1}$, intent(in), optional :: x
    ${t1}$, intent(in) :: default
    ${t1}$ :: y

    if (present(x)) then
       y = x
    else
       y = default
    end if
  end function optval_${t1[0]}$${k1}$
  #:endfor

Without generics, a Fortran programmer could also resort to using a preprocessor directly, such as with the following fypp macro:

#:def optval(lhs,opt,default)
  if (present(${opt}$)) then
    ${lhs}$ = ${opt}$
  else
    ${lhs}$ = ${default}$
  end if
#:enddef

This macro however cannot be used in expressions. With Fortran 202X the new ternary if operator will make this possible using the following fypp macro:

#:def optval(opt,default)
present(${opt}$)) ? ${opt}$ : ${default}$
#:enddef

or its cpp/fpp equivalent:

#define optval(opt,value) present(opt) ? (opt) : (value)

Using the Fortran 2018 enhanced C interoperability features, it is possible to interoperate optional arguments with C, where a null pointer means the argument is not present. A Fortran procedure interface could be provided behind the scenes by a procedure in C++ with an extern "C" interface. In C++ a function template is an elegant and type-sage solution for this usage case:

template<typename T>
inline T optval(const T* opt, T value) {
    T r = value;
    if (opt) r = *opt;
    return r;
}

What would the Fortran solution with the generics look like? Would this be a good example to motivate the generics proposals?