[Grace-core] Assignment return values and null

[James Noble]

Hi Jan

that's a good question! 

In the case of looping over a file, I guess I'd hope for something like

> for (myfile.lines) do { line -> 
>    print (line) 
> } 

but that doesn't help the larger problem. 

I'd have thought the code below could work

>  var line
>  while { line := myfile.getline; !myfile.eof } do {
>      print(line)
>  }


James

On 26/06/2012, at 19:19 PM, Jan Larres wrote:

> Hi,
> 
> I just came across a situation that doesn't really have clean solution
> in Grace. If you want to read a line from a file and check whether the
> end has been reached you can do this in C:
> 
>  while ((read = getline(&line, &len, fp)) != -1) {
>      printf("%s", line);
>  }
> 
> Or this in Java:
> 
>  while ((line = br.readLine()) != null) {
>      System.out.println(line);
>  }
> 
> In Grace you can't do it in such a clean way since assignments don't
> return their assigned values and there are no nulls. I can think of two
> ways to do this currently:
> 
>  while { !myfile.eof } do {
>      var line := myfile.getline
>      if (!myfile.eof) then {
>          print(line)
>      }
>  }
> 
>  var line := myfile.getline
>  while { !myfile.eof } do {
>      print(line)
>      line := myfile.getline
>  }
> 
> but they're both neither really intuitive nor pretty since they require
> unnecessary duplication, and the second one is kind of backwards.
> 
> I've also tried this:
> 
>  var line
>  while { line := myfile.getline; !myfile.eof } do {
>      print(line)
>  }
> 
> but that fails with a "use of undefined identifier line" in the print
> statement (this may be an implementation issue, I haven't checked yet).
> 
> Is there a specific reason why assignments don't return their value? I
> can't see any harm in doing that. The null is of course a more
> controversial issue, but at least at the moment there doesn't seem to be
> a way to implement a getline() method in a way that indicates the end of
> the file instead of just an empty line.
> 
> Jan