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A static code analysis tool with a focus on code consistency and teaching.
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lib/credo/check/refactor/append_single_item.ex
defmodule Credo.Check.Refactor.AppendSingleItem do
@moduledoc """
When building up large lists, it is faster to prepend than
append. Therefore: It is sometimes best to prepend to the list
during iteration and call Enum.reverse/1 at the end, as it is quite
fast.
Example:
list = list_so_far ++ [new_item]
# refactoring it like this can make the code faster:
list = [new_item] ++ list_so_far
# ...
Enum.reverse(list)
"""
@explanation [check: @moduledoc]
use Credo.Check, base_priority: :low
@doc false
def run(%SourceFile{} = source_file, params \\ []) do
issue_meta = IssueMeta.for(source_file, params)
Credo.Code.prewalk(source_file, &traverse(&1, &2, issue_meta))
end
# [a] ++ b is OK
defp traverse(ast = {:++, _, [[_], _]}, issues, _issue_meta) do
{ast, issues}
end
# a ++ [b] is not
defp traverse(ast = {:++, meta, [_, [_]]}, issues, issue_meta) do
{ast, [issue_for(issue_meta, meta[:line], :++) | issues]}
end
defp traverse(ast, issues, _issue_meta) do
{ast, issues}
end
defp issue_for(issue_meta, line_no, trigger) do
format_issue issue_meta,
message: "Appending a single item to a list is inefficient, use [head | tail]
notation (and Enum.reverse/1 when order matters)",
trigger: trigger,
line_no: line_no
end
end