Errors serve callers and operators
A typed enum lets callers handle expected categories without parsing human text. `Display` remains an operator-facing description, while `std::error::Error` enables generic boundaries and optional source chaining.
rust / LEVEL 3
The error string nobody can branch on
EST.35 MIN
01
THEORY / RETRIEVAL
What to restore
- Separate invalid syntax from domain-range failures
- Implement Display and Error without losing machine-readable variants
Do not erase context too early
Converting every failure to `String` or `Box<dyn Error>` at a low layer removes exhaustiveness and stable branching. Erasure is useful at application boundaries, after domain code has preserved the distinctions it owns.
CHECKPOINT
Why can `PortError` be passed as `&dyn std::error::Error` after implementing the trait?
02 / IMPLEMENTATION
Implement the contract
Implement the typed port parser and its stable Display messages. Empty input, invalid decimal syntax, and values outside 1..=65535 must stay distinct.
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OUTPUT
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