Merge pull request #68 from haarg/ampersand-method-calls

PPC0033: method-like calls to lexical subs

Author: haarg

ppcs/ppc0033-ampersand-method-calls.md

@@ -0,0 +1,138 @@
+# Allow calling subs using method-like syntax
+
+## Preamble
+
+    Author:  Graham Knop <[email protected]>
+    ID:      0033
+    Status:  Implemented
+
+## Abstract
+
+Add a syntax for calling lexical methods that is similar to the existing
+method syntax, rather than needing to call them like subs. The new syntax
+`$object->&method(@args)` would be equivalent to sub call to `method`, but
+using a syntax closer to method calls.
+
+## Motivation
+
+When using a lexical sub inside an object class, the subs must be called as
+`method($object, @args)`. This works, but for many people, this feels wrong
+as a way to call something thought of as a private method. This has resulted
+in some people using syntax like `$object->${\&method}(@args)`, or continuing
+to use code refs stored in a lexical variable (`$object->$method(@args)`).
+
+## Rationale
+
+`$object->method(@args)` always does a method lookup, ignoring the local
+context. Changing this would result in a lot of broken code. Changing this
+only for new code (via feature or other lexical effect) would introduce
+confusion and would also require some new (or pessimised) syntax to allow
+the previous behavior.
+
+`$object->&method(@args)` would be an unambiguous way to call the `method` in
+the current scope, whether a lexical or package sub.
+
+A truly private method does not need to do any lookup via `@ISA`, so being
+equivalent to a sub call makes sense.
+
+This also would pair well with
+[PPC0021](ppc0021-optional-chaining-operator.md) to allow optional sub calls
+as part of a chain.
+
+## Specification
+
+`$object->&method(@args)` would behave exactly the same as `&method($object,
+@args)`. `method` would be searched for in the current lexical scope. If not
+found, it would be looked up in the current package. If still not found, the
+current package's `AUTOLOAD` would be called. If `AUTOLOAD` does not exist, an
+error would be issued. The call would ignore prototypes, just as traditional
+method calls and `&subroutine()` calls do.
+
+Methods defined using the `class` syntax would also be callable using this
+syntax.
+
+## Backwards Compatibility
+
+The syntax `->&word` is currently a syntax error, so no backwards
+compatibility issues are forseen.
+
+## Security Implications
+
+As this is purely a syntax feature that is currently a syntax error, no
+security implications are forseen.
+
+## Examples
+
+```perl
+use v5.36;
+
+package Foo {
+    sub new ($class) {
+        return bless {
+            field => 5,
+        }, $class;
+    }
+
+    my sub private ($self) {
+        return $self->{field};
+    }
+
+    sub _old_school_private ($self) {
+        return $self->{field};
+    }
+
+    sub public ($self) {
+
+        # call via lexical method is allowed
+        say "my field: " . $self->&private;
+        # exactly equivalent to:
+        say "my field: " . private($self);
+
+
+        # can also be used to call package methods. But won't look up via @ISA
+        say "my field: " . $self->&_old_school_private;
+        # exactly equivalent to:
+        say "my field: " . _old_school_private($self);
+    }
+}
+
+my $foo = Foo->new;
+$foo->public;
+
+# this is an error, as there is no sub named "private" in scope
+$foo->&private;
+
+# this is also an error, as "public" does not exist in the current scope
+$foo->&public;
+```
+
+## Prototype Implementation
+
+* [Object::Pad::LexicalMethods](https://metacpan.org/pod/Object::Pad::LexicalMethods)
+  implements lexical methods and `->&` method calls for
+  [Object::Pad](https://metacpan.org/pod/Object::Pad).
+
+## Future Scope
+
+Lexical methods (`my method foo`) are not currently supported under the
+`class` syntax. If they were added, `->&method` would be the preferred way to
+call them.
+
+## Rejected Ideas
+
+Allowing `$object->method` to call lexical subs would break existing code and
+would interfere with the ability to call the actual method. Perl doesn't know
+what type `$object` is, so it can't know when it is appropriate to do method
+lookup and when to call a local sub. Using a different syntax at the call site
+avoids these issues.
+
+## Open Issues
+
+None currently.
+
+## Copyright
+
+Copyright (C) 2025, Graham Knop
+
+This document and code and documentation within it may be used, redistributed
+and/or modified under the same terms as Perl itself.