Where to put `implicit none` in Fortran

Solution 1:

The implicit statement (including implicit none) applies to a scoping unit. Such a thing is defined as

BLOCK construct, derived-type definition, interface body, program unit, or subprogram, excluding all nested scoping units in it

This "excluding all nested scoping units in it" suggests that it may be necessary/desirable to have implicit none in each function and subroutine (collectively, procedures) defined in a module. However, inside procedures contained within a module there is a default mapping based on the host scoping unit. So, with implicit none in the module it isn't necessary to have that in contained procedures.

This host scoping unit rule applies equally to internal programs. This means that implicit none in the main program covers all procedures contained in it; and that the same applies for internal programs of module procedures. Block constructs see this also, and the implicit statement isn't even allowed within one of these.

However, external functions/subroutines will not inherit implicit behaviour from a program or module, and modules don't inherit it from programs/other modules which use them. This clearly makes sense as the implicit typing must be known at compile time and be well defined regardless of their ultimate use.

Further, one cannot apply implicit rules from one program unit to a module it uses, such as in:

implicit none
use somemodule

end program

An implicit statement must follow all use statements.

Equally, a submodule is a program unit in itself, distinct from its ancestors. A module or submodule is a parent, not a host, of a submodule which extends it: the host scoping unit rule doesn't apply and the submodule doesn't inherit the mapping rules from its parent. Without an implicit statement in the submodule's scope the default rules will apply there.

The host scoping unit rule notably doesn't apply to interface bodies. IanH's answer motivates that exception, but it's an important enough thing to re-stress. It has caused much confusion.

module mod
 implicit none

  interface
    subroutine external_sub()
      ! The default implicit typing rules apply here unless
      ! there is an implicit statement, such as implicit none.
      ! Those from the module aren't in force here.
    end subroutine
  end interface

end module

Regarding the test of removing implicit none from a subroutine: if the code is valid with implicit none then it must be valid and identical without that statement. All entities must be explicitly declared in the former case, so no implicit rules would be applied in the latter.

Solution 2:

No, Yes (sort of) and No.

Once per program unit (which is not the same thing as once per program) and in each interface body is enough.

A program unit is either a main program, a module, an external subprogram (a function or a subroutine subprogram that does not appear after the CONTAINS statement of another type of program unit), a block data program unit or a submodule. Unless specified otherwise with an IMPLICIT statement, the default in each program unit is the default mapping of things starting with I-N as default integer and everything else as default real.

The same principle applies to interface bodies - as they are supposed to be a snapshot of the specification part of a procedure defined in another program unit. That other program unit would have the default mapping unless otherwise specified differently, so the interface body has the default mapping unless otherwise specified differently.

Inside a program unit, internal subprograms or module subprograms inherit whatever implicit typing is specified in their host, in the absence of a "local" IMPLICIT statement within the subprogram proper.

Redundant specification of IMPLICIT NONE is harmless. You often see it where subprograms that were formerly external subprograms have been put into a module.

Solution 3:

Here is an informal answer based on what works for me.

My Fortran code is in two types of files -- those containing a main program and those containing a single module. In each kind of file, IMPLICIT NONE appears just after the "program foo" or "module foo" statements and after the USE statements at the top. It does not appear within subroutines or functions, because that would be redundant.

Solution 4:

If you are using gfortran, you may as well simply add an -fimplicit-none parameter.

Note that this is a compiler-specific solution. Other widespread compilers may not support this parameter. For example, Intel's ifort ignores this as an unknown option.