Fortran syntax for assignments - syntax

Question

The Fortran syntax is driving me mad! Can anyone explain how I can call the assignment (I'm pretty sure that is not the right terminology either...). I'm trying to assign a type according to the value type. I have the following:
module test_module
implicit none
type :: mytype
integer :: i
real :: r
logical :: l
contains
generic :: assignment(=) => mytype_to_type
procedure, pass(me) :: mytype_to_type
end type mytype
contains
subroutine mytype_to_type(t, me)
implicit none
class(*), intent(inout) :: t
class(mytype), intent(in) :: me
!.. process based on input type
select type (t)
type is (integer)
t = me%i
type is (real)
t = me%r
type is (logical)
t = me%l
class default
stop "none"
return
end select
end subroutine mytype_to_type
end module test_module
program test
use test_module
implicit none
type(mytype) :: t_type
integer :: i = 1
real :: r = 1.
logical :: l = .true.
t_type = i !! how is this supposed to work?
select type(t_type)
type is (integer)
write(*,*) "is int"
type is (real)
write(*,*) "is real"
type is (logical)
write(*,*) "is logical"
class default
return
end select
end program test
Would this even work? Could anyone help me with this?
Thanks!

Answer 1

In a subroutine supporting defined assignment the two arguments are such that the first corresponds to the left-hand side of the assignment statement and the second the right-hand side.1
Here, then the subroutine you provide is assignment from a my_type expression to an unlimited polymorphic object. This isn't what you want, seeing t_type on the left.
Instead, you should provide defined assignment to a my_type object.
subroutine stuff_to_mytype(me,t)
class(mytype), intent(out) :: me
class(*), intent(in) :: t
!.. process based on input type
select type (t)
type is (integer)
me%i = t
type is (real)
me%r = t
type is (logical)
me%l = t
class default
stop "none"
return
end select
end subroutine stuff_to_mytype
That said, you could do this with a specific subroutine for each type you support, rather than an unlimited polymorphic right-hand side, with generic resolution. In this case you could also consider generic structure constructors (t_type=mytype(i)).
1 Precisely, the second argument is the right-hand side enclosed in parentheses.