[PROPOSAL] with_cur_ios() function in the stdlib

[apache2]

Hi list,

As per discussion with Mohammad-Reza and Jose, here is a function
proposed for inclusion in the stdlib:

//
// with_cur_ios(fn, cb) calls cb with fn opened as the current ios.
// if an exception is raised, fn will be closed before the exception
// is re-raised, allowing the enclosed code to fail without global
// side effects.
//
type void_lambda = ()void;
fun with_cur_ios = (string filename, void_lambda callback)void:
  {
    var old_ios = get_ios ?! E_no_ios ? get_ios : -1;
    var new_ios = open(fn);
    set_ios(new_ios);
    try {
      callback();
      close(new_ios);
      if (-1 != old_ios) set_ios(old_ios);
    } catch (Exception exc) {
      close(new_ios);
      if (-1 != old_ios) set_ios(old_ios);
      raise exc;
    }
  }

As an example of where this would be useful, consider:

fun cast_to_bytes = (uint32[] tmp)byte[]
  {
    var ret = byte[]();
    with_cur_ios("*data*", lambda void:{
      // if anything goes wrong here, we do not leak the *data* fd:
      var tmp2 = uint32[tmp'length] @ 0#b;
      for (var i = 0; i < tmp'length; i++) {
         tmp2[i] = tmp[i];
      }
      for(var i = 0; i < 4*4; i++){
        ret += byte[1] @ i#B;
      }
    });
  }