happy-path.rb

# given file path, print number of lines or report I/O errors from opening/reading

with file_contents(filename) do 
  case error_free (contents): 
      print number_of_lines(contents)
  
  case io_error(errno):
      print "I/O error opening file: #{perror(errno)}"
  end
end

def file_contents(filename) : { error_free: contents, io_error: errno }

  with syscall_open(filename) do
    case error_free (fd):
        with syscall_read(fd) do
          case error_free(contents):
              yield error_free<contents>
              
           case io_error(errno):
               yield io_error<errno>
           end
        end            

   case io_error (errno):
       yield io_error<errno>
   end
end

def syscall_open(filename) : { error_free: fd, io_error: errno }
   # ...FFI...
end