Re^7: Use of wantarray Considered Harmful

[dragonchild]

What I meant by that was that the subroutine should always return the same information - changing context should be lossless. Hence, an array vs. an iterator (or an arrayref). But, not an array vs. the first element.

As for your examples: localtime does array vs. joined array. That's on the border of acceptable. The diamond operator and readline are lossless - they are their own iterators (which is another rant for another day). m//g always returns a list - what you do with that list is your decision.

[ikegami]

m//g always returns a list - what you do with that list is your decision.

Aside from the fact that it's impossible for an operator to return a list in scalar context, m//g behaves quite differently in scalar and in list context.

It can act as an iterator (which I think you consider as being the same as a list for the discussion):

while (/.../g) {
   ...
}

But it's not an iterator in this tokenizer:

/\G( ... )/g
   or die;
my $token1 = $1;

/\G( ... )/g
   or die;
my $token2 = $1;

...

[BrowserUk]

What I meant by that was that the subroutine should always return the same information - changing context should be lossless. Hence, an array vs. an iterator (or an arrayref). But, not an array vs. the first element.

I still think you are missing the point. The assumption is that in the latter case, that the routine must be calculating the entire array and then discarding most of the data in a scalar context. But that just isn't the case.

In the case of the each sub the routine can detect the scalar context and avoid a potentially costly determination of the value if the user only wants the key. That's a 50% or potentially much higher optimisation. And that for a sub that returns just a 2 element list:

sub each {
    ...
    return wantarray ? ( getKey(...), getValue(...) ) : getKey(...);
}

For a sub like caller, if the user only wants the calling package name--which probably accounts for 90% of uses--then detecting scalar context to avoid walking a complex AST to gather the rest of the information that the user would only then discard, is again, a very useful optimisation. Think about something like Moose which needs to detect the caller package a lot, and how much more slowly it would run if caller went through the process of gathering all the 11 possible values, everytime the calling code just need the first.