Capitalization is part of the language
Some names must start with an uppercase letter, and others with a lowercase one. The checker enforces this; it is not just a convention.
| Must start uppercase | Must start lowercase |
|---|---|
| types, enum variants, traits | variables, functions, fields, parameters |
Because of this rule, a bare uppercase name in a pattern always refers to a fieldless variant. A name that doesn't resolve is an error, with a suggested fix such as "did you mean None?". It never silently binds a new variable.
Why
The parser can't tell a pattern like None from a binding without name resolution (#30). In languages where capitalization is only a convention, a misspelled variant such as Nnoe => 0 silently becomes a catch-all that matches everything. That is exactly the kind of bug agents produce and reviewers miss.
Making capitalization part of the language removes the ambiguity, without needing heuristics. A second rule closes the remaining gap: a binding or _ arm followed by further arms is an "unreachable arm" error.
Considered options
- Heuristics in the checker. For example, warn when a binding's name matches a variant name. Rejected: still ambiguous, and easy to miss.
- Requiring qualified variants. Always write
Option.None. Rejected: too verbose for the most common patterns.
Consequences
Programs that break the capitalization rule do not compile. Agents already follow these conventions, so in practice the cost is close to zero.