The above examples show clearly that it is important to provide the caller with essential information about the called procedure's arguments; this information must include the types, number and order of dummy arguments.
If, in the above examples, the main program CONTAINS the function sort, all the faulty examples above will result in compilation errors. We say that internal procedures have an explicit interface with the host. This means that the host (main program) has all the information needed about the procedure's argument list.
Explicit interfaces provide the compiler with all the needed information to
In general, procedures have an explicit interface with the calling program (the calling program has complete knowledge about the procedure's argument list) in the following situations: