PWR008: Declare the intent for each procedure parameter

Issue

Each procedure parameter should have its intent declared to facilitate reasoning about the flow of data in and out of the function.

Relevance

By declaring the intent of each procedure parameter the flow of data in and out of the function can be understood from merely looking at the procedure declaration. This not only improves code legibility but also allows reasoning about what the procedure does: by including all data inputted and outputted by a procedure in its signature, the interactions of the procedure with external data can be easily determined by inspecting the procedure call sites. Compilers can use this information to enforce correct usage of procedure parameters.

Actions

Declare the proper intent for each procedure parameter.

Code example

The following code compiles correctly:

PROGRAM p
    IMPLICIT NONE
    INTEGER :: s = 2
    CALL f(s, 2)
CONTAINS
    SUBROUTINE f(a, b)
        IMPLICIT NONE
        INTEGER :: a
        INTEGER :: b
        a = 5
        b = a * 2
    END SUBROUTINE f
END PROGRAM

However, there are 2 runtime errors that could be avoided by declaring the proper intents for the subroutine parameters:

PROGRAM p
    IMPLICIT NONE
    INTEGER :: s = 2
    CALL f(s, 2)
CONTAINS
    SUBROUTINE f(a, b)
        IMPLICIT NONE
        INTEGER, INTENT(IN) :: a
        INTEGER, INTENT(OUT) :: b
        a = 5
        b = a * 2
    END SUBROUTINE f
END PROGRAM

This code will not compile, since now the compiler has enough information to detect the errors:

1. ‘a’ has INTENT(IN) and thus can not be modified.
2. ‘b’ has INTENT(OUT) and thus the subroutine can not receive a value for that parameter.

You can find this and more examples at GitHub.

Related resources

• PWR008 examples at GitHub
• INTENT – Intel® Fortran Compiler Developer Guide and Reference, April 2022 [last checked September 2022]