Are Fortran named constants evaluated each time the containing module is used?

Question

Say I define the parameter pi = -acos(-1.0) and save it in a module. When I use this module in another program, is the value of pi computed from the function ACOS each time?

If that's the case, is it better to define pi = -3.1415.... to whatever precision I require?

Answer 1

If you have a named constant defined in a module then its value must be given by a constant expression. In a compile/execute module this constant expression will typically be evaluated when compiling the module itself, rather than when it is ultimately referenced.

Either way, there is no possibility for something using a module to affect the evaluation of the named constant's value in that module.

For the example of the question, ACOS(-1.0) is evaluated using whatever the default real kind is at the time of compiling. If this is changed to something like

module pidef
  use, intrinsic :: iso_fortran_env, only : piprec => real64
  implicit none
  real(piprec), parameter :: pi = ACOS(-1.0_piprec)
end module

then the constant expression uses the value of piprec in scope at that point. Being a constant expression every value must be well-defined by then. It will not be the case that something like

program piuse
  use, intrinsic :: iso_fortran_env, only : piprec => real32
  use pidef
  implicit none
end program

will somehow evaluate pi using real32. Equally there is no way to reference in the module's constant expression a variable defined globally later on, after the module is compiled.