Lint rules

The linter ships a small pack of built-in rules and reads project rules from a lint.metta file. Each rule has an id you can raise, lower, or turn off per project. The built-in pack:

Rule	Default	What it flags
constant-if-true	warn	The condition is always True, so the if reduces to the then-branch.
constant-if-false	warn	The condition is always False, so the if reduces to the else-branch.
if-same-branches	warn	Both branches of the if are identical, so the condition does not matter.
if-true-false	warn	(if c True False) is just c.
superposed-empty	warn	(superpose ()) is equivalent to (empty).
superposed-single	warn	superpose with a single value is redundant; it fans out one branch.
duplicate-binder	warn	A let binds the same variable twice, which is an equality constraint, not a fresh binding.
missing-type-declaration	off	A function has no (: ...) type declaration. Off by default; a declared type speeds interpretation.

Configuring rules

A lint.metta file at or above a source file sets per-rule severity and adds project rules. Severities are deny (error), warn, or off:

(lint-severity constant-if-true off)
(lint-severity missing-type-declaration warn)

The semantic rules (missing-recursive-type, inconsistent-arity) parse inert definitions and ignore bang forms. They run only when metta.diagnostics.semanticLint is on, since they are an extra background pass.

Project rules

Project rules are structural patterns over source code. They are written in the same MeTTa syntax as the code they match, but the rule file is parsed as data and is not evaluated.

(lint-rule no-debug
  (pattern (debug! $X))
  (message "leftover debug! around {$X}")
  (severity warn)
  (fix $X))

Captures are source spans. $X captures one subterm, $$$Args captures a run of arguments, and $_ matches one subterm without capturing it. Reusing the same capture name requires the same source text.

You can refine a rule with constraints:

(lint-rule capitalized-function-name
  (pattern (= ($Name $$$Args) $$$Body))
  (metavariable-regex $Name "^[A-Z]")
  (message "function {$Name} starts with a capital letter"))

metavariable-regex applies a JavaScript regular expression to the captured source text. Matching is anchored at the start, like Semgrep's metavariable-regex: use .* when you want to allow a prefix. String captures include their quotes because the matcher works on source text.