Credence.Pattern.NoMapAsSet (credence v0.4.2)

Style rule: Detects using a Map with boolean literal values (true/false) purely for membership tracking, when MapSet is more appropriate.

Map.put(seen, item, true) paired with Map.has_key?(seen, item) is a manual reimplementation of MapSet.put/2 and MapSet.member?/2. Using MapSet makes the intent clearer and avoids storing meaningless values.

This rule is not auto-fixable because a correct transformation requires companion changes beyond the flagged call site — initialising the variable with MapSet.new() instead of %{}, and replacing the corresponding Map.has_key?/2 check with MapSet.member?/2. Those changes involve data-flow analysis that cannot be done safely with a local AST rewrite.

Bad

# Boolean value used purely for membership tracking
Map.put(seen, item, true)
Map.put(seen, item, false)

# Typical pattern this catches
if Map.has_key?(seen, item) do
  {seen, acc}
else
  {Map.put(seen, item, true), [item | acc]}
end

Good

if MapSet.member?(seen, item) do
  {seen, acc}
else
  {MapSet.put(seen, item), [item | acc]}
end