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Conditions for Omitting Explicit Interfaces

Under certain conditions, an explicit interface for a subprogram is not required. The conditions in Fortran under which this is allowable are tightened considerably for HPF programs that use mapping directives.

Advice to users.These conditions are complex. The important thing to realize is that you don't have to read any of this if you have an explicit interface. So if there is any doubt in your mind, just make sure you have an explicit interface. (End of advice to users.)

An explicit interface is required except when all of the following conditions hold:

1. Fortran does not require one, and
2. No dummy argument is distributed transcriptively or with the INHERIT attribute, and
3. For each pair of corresponding actual and dummy arguments, either:
   1. They are both implicitly mapped, or
   2. They are both explicitly mapped and the mapping of the actual argument is a specialization of the mapping of the dummy argument,

   and

4. For each pair of corresponding actual and dummy arguments, either:
   1. Both are sequential, or
   2. Both are nonsequential.

Rationale.This has the following consequences:
• A plain Fortran program (i.e., with no HPF directives) will be HPF-conforming without the need to add additional interfaces, at least in a compilation environment in which all variables are sequential by default. This is insured by items 1, 2, 3(a), and 4(a).
• If remapping is necessary, this fact will be visible to the caller. Thus the implementation may choose to have all remapping performed by the caller.
(End of rationale.)

Advice to users.This requirement pushes the user strongly in the direction of always providing explicit interfaces. This is a good thing--explicit interfaces allow many errors to be caught at compile-time and greatly speed up the process of robust software development.

Note that an explicit interface can be provided in three ways:

1. A module subprogram has an explicit interface.
2. An internal subprogram has an explicit interface.
3. An explicit interface may be provided by an interface block.

In addition, an intrinsic procedure always has an explicit interface by definition.

The idiomatic Fortran way of programming makes extensive use of modules; every subprogram, for instance, can be in a module. This provides explicit interfaces automatically, with no extra effort on the part of the programmer. It should very seldom be necessary to write an interface block. (End of advice to users.)