PPC for allowing alternate iteration variable in map and grep

Author: haarg

ppcs/ppc0033-map-grep-with-topic.md

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+# Map with different topic variable
+
+## Preamble
+
+    Author:     Graham Knop <[email protected]>
+    ID:         0033
+    Status:     Draft
+
+## Abstract
+
+Allow `map` and `grep` to be given a different variable to be used as the topic
+variable, rather than using `$_`. Also allow multiple variables to be given to
+do n-at-a-time iteration.
+
+## Motivation
+
+Traditionally, `map` and `grep` loop over a list, aliasing `$_` to each list
+entry. While this can be convenient in many cases, it gets awkward when nested
+maps are needed. You need to manually save the `$_` value in another variable.
+
+```perl
+my %hash = (
+  foo => [ 1, 2, 3, 4 ],
+  bar => [ 5, 6, 7, 8 ],
+);
+
+my @out = map {
+  my $key = $_;
+  map { "$key: $_" } $hash{$key}->@*;
+} keys %hash;
+```
+
+Using `$_` can also be dangerous if you need to call code not under your
+direct control, due to it being the implicit target of operations like
+`readline`.
+
+It would be more convenient if you could use a different variable for the
+topic, similar to what `for` allows.
+
+```perl
+my @out = map my $key {
+  map my $i { "$key: $i" } $hash{$key}->@*;
+} keys %hash;
+```
+
+A natural extension of this syntax would be to allow n-at-a-time iteration, as
+`for` can do on perl 5.36+.
+
+```perl
+my @out = map my ($key, $val) {
+  map my $i { "$key: $i" } $val->@*;
+} %hash;
+```
+
+All of this would apply to `grep` as well.
+
+As the syntax proposed is currently invalid, a feature should not be needed to
+enable it.
+
+## Rationale
+
+The syntax chosen is meant to follow from the syntax of `for`, treating `map`
+as "`for` as an expression". The chosen syntax does not create any
+ambiguities, and naturally extends to follow the syntax used by `for` for
+n-at-a-time iteration.
+
+## Specification
+
+### `map my VAR BLOCK LIST`
+
+This will evaluate `BLOCK` for each element of `LIST`, aliasing `VAR` to each
+element. Its behavior will otherwise match `map BLOCK LIST`. It is only
+possible to use `my` variables for this.
+
+### `map my (VAR, VAR) BLOCK LIST`
+
+This will evaluate `BLOCK` for each set of two elements in `LIST`, aliasing
+the `VAR` to the first of each set, and the second `VAR` to the second. If
+there is not a multiple of two items in `LIST`, the last run of `BLOCK` will
+assign `undef` to the extra `VAR` slot not corresponding to an element in
+`LIST`. More than two variables can be used to iterate over sets of three or
+more items.
+
+## Backwards Compatibility
+
+As the syntax chosen is currently invalid, it should not present any backwards
+compatibility concerns.
+
+While the parsing of `map` and `grep` is complex and has ambiguities, the
+additions proposed are bounded and simple to parse, so they do not introduce
+any new ambiguities.
+
+## Security Implications
+
+None foreseen.
+
+## Examples
+
+```perl
+my @my_array = (11 .. 14);
+
+my @trad_map = map { $_ + 1 } @my_array;
+# ( 12, 13, 14, 15 )
+
+my @new_map = map my $f { $f + 1 } @my_array;
+# ( 12, 13, 14, 15 )
+
+my @pair_map = map my ($f, $g) { $f + $g } @my_array;
+# ( 23, 27 )
+
+my @new_grep = grep my $f { $f > 12 } @my_array;
+# ( 13, 14 )
+
+my %my_hash = (
+   first_key => 11,
+   second_key => 14,
+);
+
+my %pair_grep = grep my ($f, $g) { $f =~ /sec/ } %my_hash;
+# ( second_key => 14 )
+```
+
+## Prototype Implementation
+
+  - List::Util includes the functions `pairmap` and `pairgrep` which can do
+    n-at-a-time iteration. These use the "magic" `$a` and `$b` variables.
+
+## Future Scope
+
+  - Doing n-at-a-time iteration is useful, but authors may not have a use for
+    all of the variables, especially with `grep`. Could the extra variables be
+    eliminated? Maybe `grep my ($key, undef) { ... } ...` could be supported?
+
+  - `map`, `grep`, and even `for` could possibly be extended to allow
+    refaliasing syntax for their variables.
+
+    ```perl
+    my %hash = (
+      key1 => [ 1, 2, 3 ],
+      key2 => [ 4, 5, 6 ],
+    );
+    my %out = map my ($key, \@values) { ... } %hash;
+    ```
+
+  - Subs with a `(&@)` prototype are, in part, meant to allow mimicking the
+    syntax of `map` and `grep`. An example of this being `List::Util::any`:
+
+    ```perl
+    use List::Util qw(any);
+
+    my $found = any { /ab/ } qw( 1234 abcd );
+    ```
+
+    It would be desirable to be able to to write a function that accepted the
+    same syntax this this PPC proposes.
+
+## Rejected Ideas
+
+  - `my` is required because the behavior of `for` when used with non-`my` or
+    predeclared variables is confusing and hard to explain. The newer syntax
+    `for my ($var1, $var2) { ... }` also requires `my`, both for that reason
+    as well as implementation details. Making the `my` implicit also
+    introduces a difference from `for` making it harder to document and
+    explain.
+
+## Open Issues
+
+Nothing at this time.
+
+## References
+
+  - Pre-PPC discussion: https://www.nntp.perl.org/group/perl.perl5.porters/2022/11/msg265104.html
+
+## Copyright
+
+Copyright (C) 2022, Graham Knop
+
+This document and code and documentation within it may be used, redistributed
+and/or modified under the same terms as Perl itself.