defmodule Credo.Check.Warning.MapGetUnsafePass do
use Credo.Check,
id: "EX5009",
base_priority: :normal,
tags: [:controversial],
explanations: [
check: """
`Map.get/2` can lead into runtime errors if the result is passed into a pipe
without a proper default value. This happens when the next function in the
pipe cannot handle `nil` values correctly.
Example:
%{foo: [1, 2 ,3], bar: [4, 5, 6]}
|> Map.get(:missing_key)
|> Enum.each(&IO.puts/1)
This will cause a `Protocol.UndefinedError`, since `nil` isn't `Enumerable`.
Often times while iterating over enumerables zero iterations is preferable
to being forced to deal with an exception. Had there been a `[]` default
parameter this could have been averted.
If you are sure the value exists and can't be nil, please use `Map.fetch!/2`.
If you are not sure, `Map.get/3` can help you provide a safe default value.
"""
]
@call_string "Map.get"
@unsafe_modules [:Enum]
@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({:|>, _meta, _args} = ast, issues, issue_meta) do
pipe_issues =
ast
|> Macro.unpipe()
|> Enum.with_index()
|> find_pipe_issues(issue_meta)
{ast, issues ++ pipe_issues}
end
defp traverse(ast, issues, _issue_meta) do
{ast, issues}
end
defp find_pipe_issues(pipe, issue_meta) do
pipe
|> Enum.reduce([], fn {expr, idx}, acc ->
required_length = required_argument_length(idx)
{next_expr, _} = Enum.at(pipe, idx + 1, {nil, nil})
case {expr, nil_safe?(next_expr)} do
{{{{:., meta, [{_, _, [:Map]}, :get]}, _, args}, _}, false}
when length(args) != required_length ->
acc ++ [issue_for(issue_meta, meta[:line], @call_string)]
_ ->
acc
end
end)
end
defp required_argument_length(idx) when idx == 0, do: 3
defp required_argument_length(_), do: 2
defp nil_safe?(expr) do
case expr do
{{{:., _, [{_, _, [module]}, _]}, _, _}, _} ->
!(module in @unsafe_modules)
_ ->
true
end
end
defp issue_for(issue_meta, line_no, trigger) do
format_issue(
issue_meta,
message:
"`Map.get` with no default return value is potentially unsafe in pipes, use `Map.get/3` instead.",
trigger: trigger,
line_no: line_no
)
end
end