# Component of derived type function value

**URL:** <https://fortran-lang.discourse.group/t/component-of-derived-type-function-value/7474>\
**Category:** Language enhancement\
**Created:** [February 26, 2024, 8:42pm UTC](https://fortran-lang.discourse.group/t/component-of-derived-type-function-value/7474 "2024-02-26T20:42:00Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Harper](https://avatars.discourse-cdn.com/v4/letter/h/b5ac83/32.png) [@Harper](https://fortran-lang.discourse.group/u/Harper)\
**Post date:** [February 26, 2024, 8:42pm UTC](https://fortran-lang.discourse.group/t/component-of-derived-type-function-value/7474/1 "2024-02-26T20:42:00Z")

</div>

Recently I was surprised by the commented-out statement in the program below being disallowed by gfortran, ifort and ifx. They were all correct: Intel’s error message “The leftmost part-ref in a data-ref can not be a function reference.” showed me that `log(minus1)%im` was being treated as if it were a component of a derived type, and F2023 prohibits the derived type being a function reference. This prohibition is reminiscent of the F66 prohibition of things like `X*Y` in lists to be written by `WRITE` statements, forcing one to invent and use a new variable e.g. `XY=X*Y` and then write `XY`. F77 removed that annoying prohibition. Could F202Y remove the similarly annoying prohibition of components of derived types which are function values?

```auto
  implicit none
  complex,parameter :: minus1 = -1, z = log(minus1)
! print *,log(minus1)%im
  print *,z%im
end program

```

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<div class="post-metadata">

**Author:** ![Beliavsky](https://avatars.discourse-cdn.com/v4/letter/b/ba8739/32.png) [@Beliavsky](https://fortran-lang.discourse.group/u/Beliavsky)\
**Post date:** [February 26, 2024, 9:07pm UTC](https://fortran-lang.discourse.group/t/component-of-derived-type-function-value/7474/2 "2024-02-26T21:07:47Z")

</div>

You can’t write

`log(minus1)%im`

but you can write

`aimag(log(minus1))`

A similar request has been to allow subscripting of expressions when they are array-valued. The answer has been a suggestion to ASSOCIATE the expression with a variable and subscript that variable. One could similarly ASSOCIATE an expression returning a complex variable or a derived type to a variable and then access fields of the associated variable with the usual %.

The code

```auto
  implicit none
  complex, parameter :: minus1 = -1, z = log(minus1)
! print *,log(minus1)%im
  print*,z%im
  print*,aimag(log(minus1))
  associate (x => log(minus1))
     print*,x
     print*,aimag(x)
! print*,x%im ! gfortran says Error: Symbol 'x' at (1) has no IMPLICIT type
  end associate
end program

```

gives

```auto
   3.14159274    
   3.14159274    
             (0.00000000,3.14159274)
   3.14159274

```

with gfortran 13.0.0 20221218. I think the last commented out line should also work and will test it with gfortran 14 on a different computer.

---

<div class="post-metadata">

**Author:** ![Harper](https://avatars.discourse-cdn.com/v4/letter/h/b5ac83/32.png) [@Harper](https://fortran-lang.discourse.group/u/Harper)\
**Post date:** [February 27, 2024, 4:34am UTC](https://fortran-lang.discourse.group/t/component-of-derived-type-function-value/7474/3 "2024-02-27T04:34:10Z")

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@Beliavsky is of course right about `aimag` being usable in circumstances where  
`%im` is not. Metcalf et al. in Modern Fortran Explained (2023) said on p.23 that the real and imaginary parts of a complex variable are accessible, less conveniently, by the intrinsic functions `real` and `aimag`. My suggestion was given in the hope that F202Y will allow a more convenient method in the general case of `f(x)%comp`, where `f` is a function whose value is of a user-defined derived type, and `comp` is a component of that derived type. The original example using `%im` merely allowed the point to be made by a much shorter program.

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<div class="post-metadata">

**Author:** ![Beliavsky](https://avatars.discourse-cdn.com/v4/letter/b/ba8739/32.png) [@Beliavsky](https://fortran-lang.discourse.group/u/Beliavsky)\
**Post date:** [February 27, 2024, 11:33am UTC](https://fortran-lang.discourse.group/t/component-of-derived-type-function-value/7474/4 "2024-02-27T11:33:35Z")

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Even gfortran 14.0.1 20240121 refuses to compile `print*,x%im` in the previous code. Intel Fortran allows it. I think this is a bug in gfortran.

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**Author:** ![Arjen](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@Arjen](https://fortran-lang.discourse.group/u/Arjen)\
**Post date:** [February 27, 2024, 3:25pm UTC](https://fortran-lang.discourse.group/t/component-of-derived-type-function-value/7474/5 "2024-02-27T15:25:25Z")

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If it was not a bug, then the program would not be able to do anything with x, especially not printing it as a complex number. (There was a connection outage here, so I did not finish my sentence)