[Issue]: [Feature] pred(callable) — predicate pattern

[sentomk]

Issue Type

Feature Proposal

Description

Lift an arbitrary unary predicate into a first-class pattern, evaluated during the match phase rather than as a post-bind guard.

match(x) | on(
  pred([](int v) { return v % 2 == 0; }) >> "even",
  _ >> "odd"
);

Rationale / Motivation

Currently, evaluating a predicate requires a post-bind guard (e.g., using PTN_WHERE or operator []). A first-class predicate pattern reads more naturally and allows the matching engine to evaluate the condition earlier in the pipeline. It also enables native composition within combinators like any(...) or all(...) without requiring intermediate bindings. This is tracked under the WIP section of the roadmap.md.

Alternatives Considered

Using standard guards (pattern[guard]), but pred(...) offers better ergonomics for simple unary callables and integrates seamlessly with other pattern combinators.

Checklist

• I have searched existing issues and discussions
• This issue aligns with Patternia’s design goals