View Source Credo.Check.Refactor.DoubleBooleanNegation (Credo v1.7.5)

Basics

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This check is tagged :controversial

This means that this check is more opinionated than others and not for everyone's taste.

This check has a base priority of low and works with any version of Elixir.

Explanation

Having double negations in your code can obscure the parameter's original value.

# NOT preferred

!!var

This will return false for false and nil, and true for anything else.

At first this seems like an extra clever shorthand to cast anything truthy to true and anything non-truthy to false. But in most scenarios you want to be explicit about your input parameters (because it is easier to reason about edge-cases, code-paths and tests). Also: nil and false do mean two different things.

A scenario where you want this kind of flexibility, however, is parsing external data, e.g. a third party JSON-API where a value is sometimes null and sometimes false and you want to normalize that before handing it down in your program.

In these case, you would be better off making the cast explicit by introducing a helper function:

# preferred

defp present?(nil), do: false
defp present?(false), do: false
defp present?(_), do: true

This makes your code more explicit than relying on the implications of !!.

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