defmodule Credo.Check.Refactor.NegatedConditionsWithElse do
use Credo.Check,
id: "EX4019",
base_priority: :high,
explanations: [
check: """
An `if` block with a negated condition should not contain an else block.
So while this is fine:
if not allowed? do
raise "Not allowed!"
end
The code in this example ...
if not allowed? do
raise "Not allowed!"
else
proceed_as_planned()
end
... should be refactored to look like this:
if allowed? do
proceed_as_planned()
else
raise "Not allowed!"
end
The same goes for negation through `!` instead of `not`.
The reason for this is not a technical but a human one. It is easier to wrap
your head around a positive condition and then thinking "and else we do ...".
In the above example raising the error in case something is not allowed
might seem so important to put it first. But when you revisit this code a
while later or have to introduce a colleague to it, you might be surprised
how much clearer things get when the "happy path" comes first.
"""
]
@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
defp traverse({:if, meta, arguments} = ast, issues, issue_meta) do
negator = negated_condition(arguments)
if negator && Credo.Code.Block.else_block?(ast) do
new_issue = issue_for(issue_meta, meta[:line], negator)
{ast, issues ++ [new_issue]}
else
{ast, issues}
end
end
defp traverse(ast, issues, _issue_meta) do
{ast, issues}
end
defp negated_condition({:!, _, _}), do: "!"
defp negated_condition({:not, _, _}), do: "not"
# parentheses around the condition wrap it in a __block__
defp negated_condition({:__block__, _, arguments}) do
negated_condition(arguments)
end
defp negated_condition(arguments) when is_list(arguments) do
arguments |> List.first() |> negated_condition()
end
defp negated_condition(_), do: nil
defp issue_for(issue_meta, line_no, trigger) do
format_issue(
issue_meta,
message: "Avoid negated conditions in if-else blocks.",
trigger: trigger,
line_no: line_no
)
end
end