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Date: Mon, 26 Aug 2024 20:12:19 +0100
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Subject: Re: [PHP-DEV] [RFC] Default expression
To: Stephen Reay <php-lists@koalephant.com>
Cc: "internals@lists.php.net" <internals@lists.php.net>
References: <0c8ed5d6-5507-4c41-8d7f-05d14ba8aa4c@scriptfusion.com>
 <B37D6F96-092D-4DC0-B100-CBFCCC9AD47E@koalephant.com>
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From: bilge@scriptfusion.com (Bilge)

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On 26/08/2024 15:20, Stephen Reay wrote:
> Hi,
Hi :)
>
> I haven't followed the entire thread in depth so I apologise if this 
> was already answered, but I haven't noticed it being 
> mentioned/clarified yet.
Don't worry, you're right, this is an important topic that I was still 
finalising in the past 48 hours and is still an omission from the RFC, 
and as such we haven't discussed it on the list yet.
> Can you clarify in the following, is the arm comparing against match's 
> default or the parameter's default? Or to put it another way, in the 
> second call, If `$arg` is 2, will the match error out due to an 
> unmatched subject, or will it pass 1 to `F`?
>
> function F(int $foo =1) {}
>
> F(match(default) {default =>default });
> F(match($arg) {'a' =>0,default =>default });

Thank you for your (excellent) question. The answer is it will pass `1`, 
and the reason is as follows.

`F(match(default) { default => default });` is interpreted as `match 
(default expression) { default arm => default expression }`, therefore 
the first and last `default`s will be substituted with the argument's 
default, but not the middle one. However, that is only the case when the 
default arm is written exactly as `default`.

You can turn the condition into an expression, in which case all three 
will act as expressions and be substituted accordingly, e.g. 
`F(match(default) { (int) default => default });`.

Since the default condition is now an expression, you can still have a 
default arm in addition to this, e.g. the following would be valid:

F(match(default) {
     (int) default => default,
     default => default,
});

Whilst this is a curiosity, consider that passing match expressions 
directly to arguments is something I personally have never witnessed and 
that goes doubly for combining it with `default`. So, whilst it is 
interesting to know, and important for the RFC to state the specific 
semantics of this scenario, the practical applications are presumed slim 
to none.

Cheers,
Bilge

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    <div class="moz-cite-prefix">On 26/08/2024 15:20, Stephen Reay
      wrote:<br>
    </div>
    <blockquote type="cite"
      cite="mid:B37D6F96-092D-4DC0-B100-CBFCCC9AD47E@koalephant.com">
      <div>Hi,</div>
    </blockquote>
    Hi :)<br>
    <blockquote type="cite"
      cite="mid:B37D6F96-092D-4DC0-B100-CBFCCC9AD47E@koalephant.com">
      <div><br>
      </div>
      <div> <font color="#000000"><span
            style="caret-color: rgb(0, 0, 0);">I haven't followed the
            entire thread in depth so I apologise if this was already
            answered, but I haven't noticed it being mentioned/clarified
            yet.</span></font></div>
    </blockquote>
    Don't worry, you're right, this is an important topic that I was
    still finalising in the past 48 hours and is still an omission from
    the RFC, and as such we haven't discussed it on the list yet.<br>
    <blockquote type="cite"
      cite="mid:B37D6F96-092D-4DC0-B100-CBFCCC9AD47E@koalephant.com">
      <div>Can you clarify in the following, is the arm comparing
        against match's default or the parameter's default? Or to put it
        another way, in the second call, If `$arg` is 2, will the match
        error out due to an unmatched subject, or will it pass 1 to `F`?</div>
      <div><br>
      </div>
      <div>
        <div
style="background-color: rgb(255, 255, 255); color: rgb(75, 77, 85);">
          <pre
style="font-family: &quot;JetBrains Mono&quot;, monospace; font-size: 9.8pt;"><span
          style="color: rgb(174, 93, 52);">function </span><span
          style="color: rgb(28, 130, 227);">F</span>(<span
          style="color: rgb(174, 93, 52);">int </span><span
          style="color: rgb(155, 112, 177);">$foo </span>= <span
          style="color: rgb(97, 255, 255);">1</span>) {}

<span style="color: rgb(25, 129, 225);">F</span>(<span
          style="color: rgb(174, 93, 52);">match</span>(<span
          style="color: rgb(174, 93, 52);">default</span>) { <span
          style="color: rgb(174, 93, 52);">default </span>=&gt; <span
          style="color: rgb(174, 93, 52);">default </span>})<span
          style="color: rgb(167, 172, 181);">;
</span></pre>
          <pre
style="font-family: &quot;JetBrains Mono&quot;, monospace; font-size: 9.8pt;"><span
          style="color: rgb(167, 172, 181);">
</span></pre>
          <pre
style="font-family: &quot;JetBrains Mono&quot;, monospace; font-size: 9.8pt;"><div
          style="white-space: normal;"><pre
style="font-family: &quot;JetBrains Mono&quot;, monospace; font-size: 9.8pt;"><span
          style="color: rgb(25, 129, 225);">F</span>(<span
          style="color: rgb(174, 93, 52);">match</span>(<span
          style="color: rgb(155, 112, 177);">$arg</span>) { <span
          style="color: rgb(100, 181, 111);">'a' </span>=&gt; <span
          style="color: rgb(97, 255, 255);">0</span>, <span
          style="color: rgb(174, 93, 52);">default </span>=&gt; <span
          style="color: rgb(174, 93, 52);">default </span>})<span
          style="color: rgb(167, 172, 181);">;</span></pre></div></pre>
        </div>
      </div>
    </blockquote>
    <p> Thank you for your (excellent) question. The answer is it will
      pass `1`, and the reason is as follows.</p>
    <p>`F(match(default) { default =&gt; default });` is interpreted as
      `match (default expression) { default arm =&gt; default expression
      }`, therefore the first and last `default`s will be substituted
      with the argument's default, but not the middle one. However, that
      is only the case when the default arm is written exactly as
      `default`.</p>
    <p>You can turn the condition into an expression, in which case all
      three will act as expressions and be substituted accordingly, e.g.
      `F(match(default) { (int) default =&gt; default });`.</p>
    <p>Since the default condition is now an expression, you can still
      have a default arm in addition to this, e.g. the following would
      be valid:<br>
      <br>
      F(match(default) {<br>
          (int) default =&gt; default,<br>
          default =&gt; default,<br>
      });</p>
    <p>Whilst this is a curiosity, consider that passing match
      expressions directly to arguments is something I personally have
      never witnessed and that goes doubly for combining it with
      `default`. So, whilst it is interesting to know, and important for
      the RFC to state the specific semantics of this scenario, the
      practical applications are presumed slim to none.</p>
    <p>Cheers,<br>
      Bilge<br>
    </p>
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