PPC 0032: Optional Strictures and Warnings

ppcs/ppc0032-optional-strictures.md

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+# Adding Optional Strictures and Warnings Classifications to Perl
+
+## Preamble
+
+    Author:  Curtis "Ovid" Poe <[email protected]>
+    Sponsor:
+    ID:      0032
+    Status:  Draft
+
+## Abstract
+
+This PPC proposes adding mechanisms for optional strictures and warnings to
+Perl that would not be enabled by the standard `use strict` and `use warnings`
+pragmas. This allows for introducing new strict checks and warnings without
+affecting existing code that uses these pragmas. As a concrete example, this
+PPC proposes adding `use strict 'strings'` as the first such optional
+stricture, which would prevent implicit stringification of references, similar
+to the functionality provided by the CPAN module `stringification`.
+
+## Motivation
+
+The current `strict` and `warnings` pragmas in Perl effectively enable all
+available strictures and warnings when used in their recommended forms (`use
+strict` and `use warnings`). This creates a challenge when wanting to
+introduce new strict checks or warnings that might be too strict or noisy for
+general use but valuable for specific cases. Currently, such functionality
+must be implemented as separate pragmatic modules (like `indirect`,
+`multidimensional`, or `bareword::filehandles`), leading to inconsistent
+interfaces and documentation scattered across different locations.
+
+The specific case of implicit reference stringification illustrates this
+problem well. When references are implicitly stringified (e.g., `"$ref"`),
+they produce output like `ARRAY(0x1234567)` which is rarely useful and often
+indicates a programming error. However, this behavior cannot be prohibited
+under the current `strict` pragma as it would break existing code.
+
+## Rationale
+
+Adding support for optional strictures and warnings provides several benefits:
+
+1. It allows for introducing new strict checks and warnings without breaking
+   backward compatibility
+2. It provides a consistent interface for enabling such checks (`use strict
+   'feature'` rather than `no feature`)
+3. It groups related functionality under the established `strict` and
+   `warnings` pragmas
+4. It provides a clear path for experimental features to graduate into core
+   Perl
+
+The stringification case demonstrates these benefits. Rather than using a
+separate `no stringification` pragma, the functionality can be enabled with
+`use strict 'strings'`, which is more consistent with existing Perl syntax and
+more discoverable through the standard documentation.
+
+## Specification
+
+### Optional Strictures
+
+Add support for optional strict modes that are not enabled by a bare `use
+strict` statement. These would be enabled explicitly:
+
+```perl
+use strict 'strings';  # Enable reference stringification checks
+```
+
+The first such optional stricture would be 'strings', which prevents implicit
+stringification of references. When enabled, the following operations would
+die with an error if attempted on a non-object reference:
+
+- String interpolation (`"$ref"`)
+- String concatenation (`$ref . "foo"`)
+- Case conversion (`lc`, `lcfirst`, `uc`, `ucfirst`)
+- Pattern matching (`$ref =~ m//`)
+- String operations (`split`, `join`)
+- Output operations (`print`, `say`)
+
+Error messages would be specific to the operation being performed, e.g.:
+
+```
+Attempted to stringify a reference at line X
+Attempted to concat a reference at line Y
+```
+
+### Optional Warnings
+
+Add a new warnings classification for optional warnings that are not
+enabled by `use warnings` or `use warnings 'all'`. These would require
+explicit enabling:
+
+```perl
+use warnings 'optional';  # Enable all optional warnings
+use warnings 'optional::specific';  # Enable specific optional warning
+```
+
+## Backwards Compatibility
+
+This proposal is explicitly designed to maintain backward compatibility:
+
+- Existing code using `use strict` or `use warnings` will continue to work
+  unchanged
+- New strictures and warnings must be explicitly enabled
+- The mechanism allows for gradual adoption of new features
+- Existing modules like `stringification` can be updated to use the new
+  mechanism while maintaining their current interface
+
+## Security Implications
+
+The proposed changes improve security by allowing developers to catch more
+potential programming errors. The stringification case in particular can help
+prevent information leaks where reference addresses might be accidentally
+exposed in output, though that seems unlikely to be a significant security
+issue.
+
+## Examples
+
+Here's a trivial example:
+
+```perl
+use strict 'all', 'strings';
+use warnings;
+
+my $data = [ sensitive_info() ];
+log("Processing $data");  # Dies
+```
+
+## Prototype Implementation
+
+The [`stringification` module on
+CPAN](https://metacpan.org/release/PEVANS/stringification-0.01_004/view/lib/stringification.pm)
+provides a prototype for the reference stringification checks. The
+implementation can be adapted to work as a core feature, using similar
+op-hooking techniques.
+
+## Future Scope
+
+1. Additional optional strictures could include:
+   - `strict 'direct'` (current `indirect` module)
+   - `strict 'filehandles'` (current `bareword::filehandles` feature)
+
+2. Warning modes for optional strictures:
+   - `use strict 'strings=warn'` to warn rather than die
+
+## Rejected Ideas
+
+1. Adding new strictures to the default `strict` mode:
+   - Would break backward compatibility
+   - Would make `strict` too restrictive for general use
+
+2. Using negative pragmas (`no feature`):
+   - Less consistent with Perl's existing pragma system
+   - Makes features harder to discover
+
+## Open Issues
+
+1. The PPC specifically mentions "non-object" stringification. Should this be
+   more general and apply to all references?
+
+2. How should string overloading be handled? Presumably, stringification
+   should be allowed for objects that overload stringification. Hence, the
+   earlier restriction on "non-object" stringification, but it's unclear if
+   that's the best approach.
+
+3. Should there be a way to enable all optional strictures at once?
+
+   ```perl
+   use strict 'optional';  # ?
+   ```
+
+See also [Grinnz's feature suggestion on this
+matter](https://github.com/Perl/perl5/issues/18543).
+
+## Copyright
+
+Copyright (C) 2025, Curtis "Ovid" Poe.
+
+This document and code and documentation within it may be used, redistributed and/or modified under the same terms as Perl itself.