Question about implementation in source (disclaimer: not really an issue)

[matteyas]

I noticed a very weird implementation in here: https://github.com/rsdn/nemerle/blob/master/macros/core.n

Look at the WhileOtherwise macro. First of all, it defines firstHit as a mutable. Secondly, it sets firstHit = true all the time (this will probably be optimized by the compiler, but still seems unnecessary).

Wouldn't it be possible to just use def firstHit = $cond instead? No need for a mutable variable, no need to set it in the loop. Maybe the language doesn't work that way?

[NN---]

It is possible but then you have to write $cond in while once again.

[matteyas]

macro WhileOtherwise(cond, body, otherwiseBody)
syntax ("while", "(", cond, ")", body, "otherwise", otherwiseBody)
{
<[
  def firstHit = $cond;

  while ($cond)
  {
    $body
  }

  when (!firstHit)
    $otherwiseBody;
]>
}

Is that valid? The only changes is taking away firstHit = true; inside the loop and setting firstHit to an immutable instead of a mutable.

Do you mean that using $cond twice is less efficient? That can't be the case; $cond will be evaluated on each loop anyway.

I am certainly open for other perspectives, but please explain. :smiley:

[Ziaw]

@matteyas $cond will be evaluated at least twice

macro without mutable might look like this:

macro WhileOtherwise(cond, body, otherwiseBody)
syntax ("while", "(", cond, ")", body, "otherwise", otherwiseBody)
{
<[
  def loop(firstHit)
  {
    match ($cond, firstHit) 
    {
      | false, true => $otherwiseBody;
      | false, false => (); // end loop
      | true, _ => $body; loop(false);
    }
  }

  loop(true)
]>
}

[ionoy]

@matteyas $cond might have a mutating behavior, which would lead to very obscure bugs.