Credence.Pattern.NoDeadMapUpdate (credence v0.7.0)

Copy Markdown

Detects a no-op Map.update(key, literal, & &1) |> Map.drop([key]) (and the Map.delete/direct-call variants) where the updated value is immediately thrown away by dropping the same key.

Safe core only

In general Map.update(map, key, default, fun) runs fun on the existing value when key is present, and eagerly evaluates default. Removing the update would therefore drop any exception or side effect that fun/default produces — e.g. %{prev: "x"} |> Map.update(:prev, 0, &(&1 - 1)) raises ArithmeticError, whereas Map.drop(map, [:prev]) returns %{}. That is a behaviour change, so those forms are intentionally not flagged.

This rule fires only when removing the update is provably output-identical:

  • fun is the identity capture & &1 — fun.(value) == value for every value, so it can never raise or side-effect; and
  • default is a literal — a pure value whose eager evaluation has no observable effect.

Under those two conditions the update is a genuine no-op before the key is dropped, so it can be removed for any input.

Bad

map |> Map.update(prev, 0, & &1) |> Map.drop([prev])
Map.delete(Map.update(map, key, 0, & &1), key)

Good

Map.drop(map, [prev])
Map.delete(map, key)

Auto-fix

Removes the dead identity Map.update call, keeping only the drop/delete.