Basics
This check is disabled by default.
Learn how to enable it via .credo.exs.
This check has a base priority of high and works with any version of Elixir.
Explanation
Replace Enum.map/2 |> Enum.reject/2 with a comprehension.
Why
The pipe walks the list twice and allocates an intermediate list of
transformed elements before rejection. A comprehension fuses both
steps in one pass, preserves order naturally, and avoids the
reduce + reverse anti-pattern.
How to fix (in order of preference)
Preferred: comprehension. A = binding inside a comprehension
lets you compute the transformed value once and filter on it:
# BEFORE
things
|> Enum.map(&transform/1)
|> Enum.reject(&drop?/1)
# AFTER
for x <- things, v = transform(x), not drop?(v), do: vOne pass, in-order, no intermediate list, no reverse step.
Last resort: Enum.reduce/3. Only when a comprehension is awkward
and the consumer does not care about order:
Enum.reduce(things, [], fn x, acc ->
v = transform(x)
if drop?(v), do: acc, else: [v | acc]
end)What NOT to do
Do not switch to Enum.reduce/3 and append |> Enum.reverse/1 to
restore order. That second pass is exactly the cost the comprehension
exists to avoid.
For the common map |> reject(&is_nil/1) case, see also
ForgeCredoChecks.MapRejectNil which targets that specific shape.
Check-Specific Parameters
There are no specific parameters for this check.
General Parameters
Like with all checks, general params can be applied.
Parameters can be configured via the .credo.exs config file.