OpenEuphoria

Re: goto considered essential

• Posted by Vincent <darkvincentdude at yahoo.com> Jan 21, 2006
• 525 views

Tom Dailey wrote:
> 
> 
> Consider the following program fragment, which copies one text file into
> another text file:
> 
>     sequence cmd_args
>     sequence cmd_line_words
>     sequence input_file_name
>     integer  input_file_nr
>     object   line
>     sequence output_file_name
>     integer  output_file_nr
> 
>     cmd_line_words = command_line ( )
>     if length ( cmd_line_words ) != 4 then
>         Printf ( "Exactly two arguments expected -- %d supplied.\n",
>             { length ( cmd_line_words ) - 2 } )
>     else
>         cmd_args = cmd_line_words [ 3..4 ]
>         input_file_name = cmd_args [ 1 ]
>         input_file_nr = open ( input_file_name, "r" )
>         if input_file_nr = -1 then
>             Printf ( "Can't open input file %s.\n",
>                 { input_file_name } )
>         else
>             output_file_name = cmd_args [ 2 ]
>             output_file_nr = open ( output_file_name, "w" )
>             if output_file_nr = -1 then
>                 Printf ( "Can't open output file %s.\n",
>                     { output_file_name } )
>             else
>                 while 1 do
>                     line = gets ( input_file_nr )
>                     if atom ( line ) then
>                         exit
>                     end if
>                     puts ( output_file_nr, line )
>                 end while
>                 Printf ( "%s copied to %s.\n",
>                     { input_file_name, output_file_name } )
>             end if
>         end if
>     end if
> 
> Without a goto statement, or something like it, we have the creeping margin
> problem.  In production code, with many error checks in the main logic, this
> can make code very hard to understand.  With a goto, we have
> 
>     cmd_line_words = command_line ( )
>     if length ( cmd_line_words ) != 4 then
>         goto arg_count_is_wrong
>     end if
>     cmd_args = cmd_line_words [ 3..4 ]
>     input_file_name = cmd_args [ 1 ]
>     input_file_nr = open ( input_file_name, "r" )
>     if input_file_nr = -1 then
>         goto cant_open_input_file
>     end if
>     output_file_name = cmd_args [ 2 ]
>     output_file_nr = open ( output_file_name, "w" )
>     if output_file_nr = -1 then
>         goto cant_open_output_file
>     end if
>     while 1 do
>         line = gets ( input_file_nr )
>         if atom ( line ) then
>             exit
>         end if
>         puts ( output_file_nr, line )
>     end while
>     Printf ( "%s copied to %s.\n",
>         { input_file_name, output_file_name } )
>     goto halt
> 
>     arg_count_is_wrong:
>         Printf ( "Exactly two arguments expected -- %d supplied.\n",
>             { length ( cmd_line_words ) - 2 } )
>         goto halt
>     cant_open_input_file:
>         Printf ( "Can't open input file %s.\n", { input_file_name } )
>         goto halt
>     cant_open_output_file:
>         Printf ( "Can't open output file %s.\n", { output_file_name } )
>         goto halt
>     halt:
> 
> Here, the main (non-error) logic is much more apparent.  But what, you ask,
> is then to prevent evil programmers from generating spaghetti code? Well,
> nothing.  So perhaps a more constrained construct would be better.  In the
> spirit of Ada exceptions, we might try something like this:
> 
>     --
>     -- Function/procedure body or main program starts here.
>     --
>     cmd_line_words = command_line ( )
>     exception arg_count_is_wrong if length ( cmd_line_words ) != 4
>     cmd_args = cmd_line_words [ 3..4 ]
>     input_file_name = cmd_args [ 1 ]
>     input_file_nr = open ( input_file_name, "r" )
>     exception cant_open_input_file if input_file_nr = -1
>     output_file_name = cmd_args [ 2 ]
>     output_file_nr = open ( output_file_name, "w" )
>     exception cant_open_output_file if output_file_nr = -1
>     while 1 do
>         line = gets ( input_file_nr )
>         exit if atom ( line )
>         puts ( output_file_nr, line )
>     end while
>     Printf ( "%s has been copied to %s.\n",
>         { input_file_name, output_file_name } )
>     --
>     -- Normal function/procedure return or program termination occurs here.
>     -- Exception handlers follow function/procedure body or main program.
>     --
>     when arg_count_is_wrong do
>         Printf ( "Exactly two arguments expected -- %d supplied.\n",
>             { length ( cmd_line_words ) - 2 } )
>         -- implicit return or halt here
>     when cant_open_input_file do
>         Printf ( "Can't open input file %s.\n", { input_file_name } )
<snip>

Robert Craig has expressed his opionion about the goto statement many times in
the past.

According to him, there is almost no chance of "goto" ever being added into
official Euphoria. He and others feel it causes "spaghetti code" execution. I am
against the goto construct for the bad programming habits it taught me in Liberty
Basic.

If you really want the goto and labels, implement them yourself in the public
domain or C back-end source code. Or use Matt's Object Oriented Euphoria and/or
Bach.


Regards,
Vincent