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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/with_clauses.ex
defmodule Credo.Check.Refactor.WithClauses do
use Credo.Check,
id: "EX4029",
base_priority: :high,
explanations: [
check: ~S"""
`with` statements are useful when you need to chain a sequence
of pattern matches, stopping at the first one that fails.
But sometimes, we go a little overboard with them (pun intended).
If the first or last clause in a `with` statement is not a `<-` clause,
it still compiles and works, but is not really utilizing what the `with`
macro provides and can be misleading.
with ref = make_ref(),
{:ok, user} <- User.create(ref),
:ok <- send_email(user),
Logger.debug("Created user: #{inspect(user)}") do
user
end
Here, both the first and last clause are actually not matching anything.
If we move them outside of the `with` (the first ones) or inside the body
of the `with` (the last ones), the code becomes more focused and .
This `with` should be refactored like this:
ref = make_ref()
with {:ok, user} <- User.create(ref),
:ok <- send_email(user) do
Logger.debug("Created user: #{inspect(user)}")
user
end
"""
]
@message_first_clause_not_pattern "`with` doesn't start with a <- clause, move the non-pattern <- clauses outside of the `with`"
@message_last_clause_not_pattern "`with` doesn't end with a <- clause, move the non-pattern <- clauses inside the body of the `with`"
@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({:with, meta, [_, _ | _] = clauses_and_body} = ast, issues, issue_meta)
when is_list(clauses_and_body) do
# If clauses_and_body is a list with at least two elements in it, we think
# this might be a call to the special form "with". To be sure of that,
# we get the last element of clauses_and_body and check that it's a keyword
# list with a :do key in it (the body).
# We can hard-match on [maybe_body] here since we know that clauses_and_body
# has at least two elements.
{maybe_clauses, [maybe_body]} = Enum.split(clauses_and_body, -1)
if Keyword.keyword?(maybe_body) and Keyword.has_key?(maybe_body, :do) do
{ast, issues_for_with(maybe_clauses, meta[:line], issue_meta) ++ issues}
else
{ast, issues}
end
end
defp traverse(ast, issues, _issue_meta) do
{ast, issues}
end
defp issues_for_with(clauses, line, issue_meta) do
issue_if_not_starting_with_pattern_clause(clauses, line, issue_meta) ++
issue_if_not_ending_with_pattern_clause(clauses, line, issue_meta)
end
defp issue_if_not_starting_with_pattern_clause(
[{:<-, _meta, _args} | _rest],
_line,
_issue_meta
) do
[]
end
defp issue_if_not_starting_with_pattern_clause(_clauses, line, issue_meta) do
[
format_issue(issue_meta,
message: @message_first_clause_not_pattern,
line_no: line,
trigger: "with"
)
]
end
defp issue_if_not_ending_with_pattern_clause(clauses, line, issue_meta) do
if length(clauses) > 1 and not match?({:<-, _, _}, Enum.at(clauses, -1)) do
[
format_issue(issue_meta,
message: @message_last_clause_not_pattern,
line_no: line,
trigger: "with"
)
]
else
[]
end
end
end