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A static code analysis tool with a focus on code consistency and teaching.
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Files
lib/credo/check/warning/expensive_empty_enum_check.ex
defmodule Credo.Check.Warning.ExpensiveEmptyEnumCheck do
use Credo.Check,
id: "EX5003",
base_priority: :high,
explanations: [
# TODO: improve checkdoc
check: """
Checking if the size of the enum is `0` can be very expensive, since you are
determining the exact count of elements.
Checking if an enum is empty should be done by using
Enum.empty?(enum)
or
list == []
For Enum.count/2: Checking if an enum doesn't contain specific elements should
be done by using
not Enum.any?(enum, condition)
"""
]
@doc false
@impl true
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
@enum_count_pattern quote do: {
{:., _, [{:__aliases__, _, [:Enum]}, :count]},
_,
_
}
@length_pattern quote do: {:length, _, [_]}
@comparisons [
{@enum_count_pattern, 0, "Enum.count"},
{0, @enum_count_pattern, "Enum.count"},
{@length_pattern, 0, "length"},
{0, @length_pattern, "length"}
]
@operators [:==, :===]
for {lhs, rhs, trigger} <- @comparisons,
operator <- @operators do
defp traverse(
{unquote(operator), _meta, [unquote(lhs), unquote(rhs)]} = ast,
issues,
issue_meta
) do
{ast, issues_for_call(unquote(trigger), issues, issue_meta, ast)}
end
end
defp traverse(ast, issues, _issue_meta) do
{ast, issues}
end
defp issues_for_call(trigger, issues, issue_meta, ast) do
meta = get_meta(ast)
[issue_for(issue_meta, meta, trigger, suggest(ast)) | issues]
end
defp suggest({_op, _, [0, {_pattern, _, args}]}), do: suggest_for_arity(Enum.count(args))
defp suggest({_op, _, [{_pattern, _, args}, 0]}), do: suggest_for_arity(Enum.count(args))
defp get_meta({_op, _, [0, {{:., _, [{:__aliases__, meta, _}, _]}, _, _}]}), do: meta
defp get_meta({_op, _, [0, {_, meta, _}]}), do: meta
defp get_meta({_op, _, [{{:., _, [{:__aliases__, meta, _}, _]}, _, _}, 0]}), do: meta
defp get_meta({_op, _, [{_, meta, _}, 0]}), do: meta
defp suggest_for_arity(2), do: "`not Enum.any?/2`"
defp suggest_for_arity(1), do: "`Enum.empty?/1` or `list == []`"
defp issue_for(issue_meta, meta, trigger, suggestion) do
format_issue(
issue_meta,
message: "#{trigger} is expensive, prefer #{suggestion}.",
trigger: trigger,
line_no: meta[:line],
column: meta[:column]
)
end
end