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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
use Credo.Check,
id: "EX4002",
base_priority: :low,
tags: [:controversial],
explanations: [
check: """
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)
"""
]
@doc false
@impl true
def run(%SourceFile{} = source_file, params) do
ctx = Context.build(source_file, params, __MODULE__)
result = Credo.Code.prewalk(source_file, &walk/2, ctx)
result.issues
end
# [a] ++ b is OK
defp walk({:++, _, [[_], _]} = ast, ctx) do
{ast, ctx}
end
# a ++ [b] is not
defp walk({:++, meta, [_, [_]]} = ast, ctx) do
{ast, put_issue(ctx, issue_for(ctx, meta))}
end
defp walk(ast, ctx) do
{ast, ctx}
end
defp issue_for(ctx, meta) do
format_issue(
ctx,
message:
"Appending a single item to a list is inefficient, use `[head | tail]` notation (and `Enum.reverse/1` when order matters).",
trigger: "++",
line_no: meta[:line]
)
end
end