Channel Directions in Co-array Fortran

program channel_directions
  use iso_fortran_env
  implicit none

  type :: string_channel
    character(len=100) :: message
    logical :: is_full = .false.
  end type string_channel

  type(string_channel) :: pings, pongs

  call ping(pings, "passed message")
  call pong(pings, pongs)
  print *, pongs%message

contains

  ! This `ping` subroutine only accepts a channel for sending
  ! values. In Co-array Fortran, we simulate this by using
  ! the `intent(out)` attribute.
  subroutine ping(pings, msg)
    type(string_channel), intent(out) :: pings
    character(len=*), intent(in) :: msg

    pings%message = msg
    pings%is_full = .true.
  end subroutine ping

  ! The `pong` subroutine accepts one channel for receives
  ! (pings) and a second for sends (pongs).
  subroutine pong(pings, pongs)
    type(string_channel), intent(inout) :: pings
    type(string_channel), intent(out) :: pongs

    if (pings%is_full) then
      pongs%message = pings%message
      pongs%is_full = .true.
      pings%is_full = .false.
    end if
  end subroutine pong

end program channel_directions

When using channels as function parameters in Co-array Fortran, we can specify if a channel is meant to only send or receive values. This specificity increases the type-safety of the program.

In this example, we’ve simulated channels using a custom type string_channel. The ping subroutine is designed to only send values, while the pong subroutine both receives and sends.

To run the program:

$ gfortran -coarray=single channel_directions.f90 -o channel_directions
$ ./channel_directions
passed message

Note that Co-array Fortran doesn’t have built-in channel semantics like Go. Instead, we’ve used a simple struct-like type to simulate channel behavior. The intent attributes in Fortran help enforce the directionality of our simulated channels.

In a real Co-array Fortran program, you might use co-arrays for inter-image communication, which would be more idiomatic for the language. However, this example focuses on simulating Go’s channel directions concept.

Channel Synchronization in Co-array Fortran

Select in Co-array Fortran

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Channel Directions in Co-array Fortran