Specify function result kind based on argument value

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1

Is there a way to specify a function(not type-bounded) result kind based on input argument value(not type)? For example, intrinsic function real(x, kind). This function returns different kind result based on input kind value.

2

No, it is not possible. But you can come close by passing a mold rather than a kind. A little tedious, but it’s the closest approximation I know of.

Module Myreal_m
Use, intrinsic :: iso_fortran_env, only: real32, real64

Private
Public :: myreal

Interface myreal
Myreal_r32
Myreal_r64
End interface

Contains

Function myreal_r32(x, mold) result(y)
Integer, intent(in) :: x
Real(real32), intent(in) :: mold
Real(real32) :: y
...
End function

Function myreal_r64(x, mold) result(y)
Integer, intent(in) :: x
Real(real64), intent(in) :: mold
Real(real64) :: y
...
End function

End module myreal_m
3

Not directly, but you can get this effect with an interface having module procedures for the various types. Here is an example:

module twice_mod
implicit none
integer, parameter :: sp = kind(1.0), dp = kind(1.0d0)
interface twice
   module procedure twice_r_sp, twice_r_dp
end interface twice
contains
function twice_r_sp(x) result(y)
real(kind=sp), intent(in) :: x
real(kind=sp)             :: y
y = 2*x
end function twice_r_sp
!
function twice_r_dp(x) result(y)
real(kind=dp), intent(in) :: x
real(kind=dp)             :: y
y = 2*x
end function twice_r_dp
end module twice_mod
!
program main
use twice_mod, only: twice, sp, dp
implicit none
print*,twice(3.1_sp),twice(3.1_dp)
print*,kind(twice(3.1_sp)),kind(twice(3.1_dp))
end program main
! gfortran output:
!    6.19999981       6.2000000000000002     
!            4           8
4

Related thread at Discourse: Fortran function return value polymorphism

Proposition in the J3-Fortran GitHub: Function kind specified by optional input · Issue #91 · j3-fortran/fortran_proposals · GitHub