Patternia v0.8.3 Release Note

Release Date: March 9, 2026
Version: 0.8.3

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Overview

Patternia v0.8.3 was an API ergonomics release in the 0.8 line. The direct match(subject, case...) entry introduced there has since been removed as the library converged on the pipeline form.

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Highlights

Historical Note: Direct Match Entry

v0.8.3 temporarily added a direct match(subject, case...) overload. That form is no longer part of the public API. The modern spelling is:

int r = match(x) | on(
    lit(1) >> 100,
    lit(2) >> 200,
    _ >> 0
);

Wildcard Alias: _

_ is the public wildcard spelling:

match(x) | on(
    lit(1) >> 10,
    _ >> 0
);

Bind Shorthand: $

$ is a concise alias for bind(), reducing noise in value-capture patterns:

match(x) | on(
    $[_0 > 0] >> [](int v) { return v * 2; },
    _ >> 0
);

Guard Placeholders: _0, _1, _2, _3

New shorthand aliases for arg<0> through arg<3>. The bare _ guard placeholder is deprecated in favor of _0:

// Before
bind()[_ > 0 && _ < 10]

// After
$[_0 > 0 && _0 < 10]

Bug Fixes

• Fixed pred_and / pred_or failing to unwrap tuple when invoking raw callables in mixed guard expressions (e.g., _0 > 1u && is_even). Added detail::invoke_guard to handle tuple unpacking for non-guard predicates.

Deprecations

• The older single-value guard placeholder backing _ is deprecated. Use _0 instead. The _ name is now reserved for the wildcard pattern.

Internal

• Migrated all tests, samples, and benchmarks from _ to _0 for guard expressions.
• Updated documentation to reflect new placeholder naming.