code review 104680043: spec: permit "for range x" (no index variables)

spec: permit "for range x" (no index variables)

This is a fully backward-compatible language change.

There are not a lot of cases in the std library, but
there are some. Arguably this makes the syntax a bit
more regular - any trailing index variable that is _
can be left away, and there's some analogy to type
switches where the temporary can be left away.

Implementation-wise the change should be trivial as
it can be done completely syntactically. For instance,
the respective change in go/parser is a dozen lines
(see https://codereview.appspot.com/112970044 ).

Fixes issue 6102.

[gri, 2014-07-10 21:30:33]

Hello r@golang.org, rsc@golang.org, iant@golang.org, ken@golang.org (cc:
golang-codereviews@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go/

[gri, 2014-07-10 21:32:09]

PS: I am slightly in favor of this for the added regularity and because
making the change is trivial. If we don't go forward with it, at least I
can close the issue.



On Thu, Jul 10, 2014 at 2:30 PM, <gri@golang.org> wrote:

> Reviewers: r, rsc, iant, ken2,
>
> Message:
> Hello r@golang.org, rsc@golang.org, iant@golang.org, ken@golang.org (cc:
> golang-codereviews@googlegroups.com),
>
> I'd like you to review this change to
> https://code.google.com/p/go/
>
>
> Description:
> spec: permit "for range x" (no index variables)
>
> This is a fully backward-compatible language change.
>
> There are not a lot of cases in the std library, but
> there are some. Arguably this makes the syntax a bit
> more regular - any trailing index variable that is _
> can be left away, and there's some analogy to type
> switches where the temporary can be left away.
>
> Implementation-wise the change should be trivial as
> it can be done completely syntactically. For instance,
> the respective change in go/parser is a dozen lines
> (see https://codereview.appspot.com/112970044 ).
>
> Fixes issue 6102.
>
> Please review this at https://codereview.appspot.com/104680043/
>
> Affected files (+20, -16 lines):
>   M doc/go_spec.html
>
>
> Index: doc/go_spec.html
> ===================================================================
> --- a/doc/go_spec.html
> +++ b/doc/go_spec.html
> @@ -1,6 +1,6 @@
>  <!--{
>         "Title": "The Go Programming Language Specification",
> -       "Subtitle": "Version of June 24, 2014",
> +       "Subtitle": "Version of July 10, 2014",
>         "Path": "/ref/spec"
>  }-->
>
> @@ -4714,41 +4714,42 @@
>  A "for" statement with a "range" clause
>  iterates through all entries of an array, slice, string or map,
>  or values received on a channel. For each entry it assigns <i>iteration
> values</i>
> -to corresponding <i>iteration variables</i> and then executes the block.
> +to corresponding <i>iteration variables</i> if present and then executes
> the block.
>  </p>
>
>  <pre class="ebnf">
> -RangeClause = ( ExpressionList "=" | IdentifierList ":=" ) "range"
> Expression .
> +RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range"
> Expression .
>  </pre>
>
>  <p>
>  The expression on the right in the "range" clause is called the <i>range
> expression</i>,
>  which may be an array, pointer to an array, slice, string, map, or
> channel permitting
>  <a href="#Receive_operator">receive operations</a>.
> -As with an assignment, the operands on the left must be
> +As with an assignment, if present the operands on the left must be
>  <a href="#Address_operators">addressable</a> or map index expressions;
> they
> -denote the iteration variables. If the range expression is a channel, only
> -one iteration variable is permitted, otherwise there may be one or two.
> In the latter case,
> -if the second iteration variable is the <a href="#Blank_identifier">blank
> identifier</a>,
> -the range clause is equivalent to the same clause with only the first
> variable present.
> +denote the iteration variables. If the range expression is a channel, at
> most
> +one iteration variable is permitted, otherwise there may be up to two.
> +If the last iteration variable is the <a href="#Blank_identifier">blank
> identifier</a>,
> +the range clause is equivalent to the same clause without that identifier.
>  </p>
>
>  <p>
>  The range expression is evaluated once before beginning the loop,
> -with one exception. If the range expression is an array or a pointer to
> an array
> -and only the first iteration value is present, only the range expression's
> -length is evaluated; if that length is constant
> -<a href="#Length_and_capacity">by definition</a>,
> +with one exception: If the range expression is an array or a pointer to
> an array
> +and at most one iteration variable is present, only the range expression's
> +length is evaluated; if that length is constant,
> +<a href="#Length_and_capacity">by definition</a>
>  the range expression itself will not be evaluated.
>  </p>
>
>  <p>
>  Function calls on the left are evaluated once per iteration.
> -For each iteration, iteration values are produced as follows:
> +For each iteration, iteration values are produced as follows
> +if the respective iteration variables are present:
>  </p>
>
>  <pre class="grammar">
> -Range expression                          1st value          2nd value
> (if 2nd variable is present)
> +Range expression                          1st value          2nd value
>
>  array or slice  a  [n]E, *[n]E, or []E    index    i  int    a[i]       E
>  string          s  string type            index    i  int    see below
>  rune
> @@ -4760,7 +4761,7 @@
>  <li>
>  For an array, pointer to array, or slice value <code>a</code>, the index
> iteration
>  values are produced in increasing order, starting at element index 0.
> -If only the first iteration variable is present, the range loop produces
> +If at most one iteration variable is present, the range loop produces
>  iteration values from 0 up to <code>len(a)-1</code> and does not index
> into the array
>  or slice itself. For a <code>nil</code> slice, the number of iterations
> is 0.
>  </li>
> @@ -4814,7 +4815,7 @@
>  var testdata *struct {
>         a *[7]int
>  }
> -for i, _ := range testdata.a {
> +for i := range testdata.a {
>         // testdata.a is never evaluated; len(testdata.a) is constant
>         // i ranges from 0 to 6
>         f(i)
> @@ -4841,6 +4842,9 @@
>  for w := range ch {
>         doWork(w)
>  }
> +
> +// empty a channel
> +for range ch {}
>  </pre>
>
>
>
>
>

[iant, 2014-07-10 21:52:33]

LGTM

https://codereview.appspot.com/104680043/diff/40001/doc/go_spec.html
File doc/go_spec.html (right):

https://codereview.appspot.com/104680043/diff/40001/doc/go_spec.html#newcode4738
doc/go_spec.html:4738: with one exception: If the range expression is an array
or a pointer to an array
s/: If/: if/

[gri, 2014-07-10 22:19:59]

PTAL

https://codereview.appspot.com/104680043/diff/40001/doc/go_spec.html
File doc/go_spec.html (right):

https://codereview.appspot.com/104680043/diff/40001/doc/go_spec.html#newcode4738
doc/go_spec.html:4738: with one exception: If the range expression is an array
or a pointer to an array
On 2014/07/10 21:52:33, iant wrote:
> s/: If/: if/

Done.

[r, 2014-07-14 21:57:01]

LGTM

[rsc, 2014-07-14 22:02:05]

LGTM

I think of this as similar to not requiring the _  in switch _ := x.(type) {.

https://codereview.appspot.com/104680043/diff/60001/doc/go_spec.html
File doc/go_spec.html (left):

https://codereview.appspot.com/104680043/diff/60001/doc/go_spec.html#oldcode4817
doc/go_spec.html:4817: for i, _ := range testdata.a {
This _ might be worth keeping. I don't think it's spelled out that i, _ counts
as only one range variable, and even if it is, this is good emphasis.

[gri, 2014-07-14 22:06:03]

https://codereview.appspot.com/104680043/diff/60001/doc/go_spec.html
File doc/go_spec.html (left):

https://codereview.appspot.com/104680043/diff/60001/doc/go_spec.html#oldcode4817
doc/go_spec.html:4817: for i, _ := range testdata.a {
On 2014/07/14 22:02:04, rsc wrote:
> This _ might be worth keeping. I don't think it's spelled out that i, _ counts
> as only one range variable, and even if it is, this is good emphasis.

Reverted.

[gri, 2014-07-14 22:08:14]

*** Submitted as https://code.google.com/p/go/source/detail?r=fcb7e7aa1740 ***

spec: permit "for range x" (no index variables)

This is a fully backward-compatible language change.

There are not a lot of cases in the std library, but
there are some. Arguably this makes the syntax a bit
more regular - any trailing index variable that is _
can be left away, and there's some analogy to type
switches where the temporary can be left away.

Implementation-wise the change should be trivial as
it can be done completely syntactically. For instance,
the respective change in go/parser is a dozen lines
(see https://codereview.appspot.com/112970044 ).

Fixes issue 6102.

LGTM=iant, r, rsc
R=r, rsc, iant, ken
CC=golang-codereviews
https://codereview.appspot.com/104680043