Sourceror (Sourceror v0.2.2) View Source
Installation
Add :sourceror
as a dependency to your project's mix.exs
:
defp deps do
[
{:sourceror, "~> 0.2.2"}
]
end
A note on compatibility
Sourceror is compatible with Elixir versions down to 1.10 and OTP 21. For Elixir versions prior to 1.13 it uses a vendored version of the Elixir parser and formatter modules.
Documentation
Background
There have been several attempts at source code manipulation in the Elixir
community. Thanks to its metaprogramming features, Elixir provides builtin tools
that let us get the AST of any Elixir code, but when it comes to turning
the AST back to code as text, we had limited options. Macro.to_string/2
is a
thing, but the produced code is generally ugly, mostly because of the extra
parenthesis or because it turns string interpolations into calls to erlang
modules, to name some examples. This meant that, even if we could use
Macro.to_string/2
to get a string and then give that to the Elixir formatter
Code.format_string!/2
, the output would still be suboptimal, as the formatter
is not designed to change the semantics of the code, only to pretty print it.
For example, call to erlang modules would be kept as is instead of being turned
back to interpolations.
We also had the additional problem of comments being discarded by the tokenizer, and literals not having information like line numbers or delimiter characters. This makes the regular AST too lossy to be useful if what we want is to manipulate the source code, because we need as much information as possible to be able to stay as close to the source as possible. There have been several proposal in the past to bring all this information to the Elixir AST, but they all meant a change that would either break macros due to the addition of new types of AST nodes, or making a compromise in core Elixir itself by storing comments in the nods metadata.
Despite of all these issues, the Elixir formatter is still capable of manipulating the source code to pretty print it. Under the hood it does some neat tricks to have all this information available: on one hand, it tells the tokenizer to extract the comments from the source code and keep it at hand(not in the AST itself, but as a separate data structure), and on the other hand it tells the parser to wrap literals in block nodes so metadata can be preserved. Once it has all it needs, it can start converting the AST and comments into an algebra document, and ultimately convert that to a string. This functionality was private, and if we wanted to do it ourselves we would have to replicate or vendor the Elixir formatter with all its more than 2000 lines of code. This approach was explored by Wojtek Mach in wojtekmach/fix, but it involved vendoring the elixir Formatter code, was tightly coupled to the formatting process, and any change in Elixir would break the code.
Since Elixir 1.13 this functionality from the formatter was finally exposed via
the Code.string_to_quoted_with_comments/2
and Code.quoted_to_algebra/2
functions. The former gives us access to the list of comments in a shape the
Elixir formatter is able to use, and the latter lets us turn any arbitrary
Elixir AST into an algebra document. If we also give it the list of comments,
it will merge them together, allowing us to format AST and preserve the
comments. Now all we need to care about is of manipulating the AST, and let the
formatter do the rest.
Sourceror's AST
Having the AST and comments as separate entities allows Elixir to expose the code formatting utilities without making any changes to it's AST, but also delegates the task of figuring out what's the most appropiate way to work with them to us.
Sourceror's take is to use the node metadata to store the comments. This allows you to move nodes around without worrying about leaving a comment behind and ending up with misplaced comments. Two fields are required for this:
:leading_comments
- holds the comments directly above the node or are in the same line as it. For example:test "parses leading comments" do quoted = """ # Comment for :a :a # Also a comment for :a """ |> Sourceror.parse_string() assert {:__block__, meta, [:a]} = quoted assert meta[:leading_comments] == [ %{line: 1, previous_eol_count: 1, next_eol_count: 1, text: "# Comment for :a"}, %{line: 2, previous_eol_count: 0, next_eol_count: 1, text: "# Also a comment for :a"}, ] end
:trailing_comments
- holds the comments that are inside of the node, but aren't leading any children, for example:test "parses trailing comments" do quoted = """ def foo() do :ok # A trailing comment end # Also a trailing comment for :foo """ |> Sourceror.parse_string() assert {:def, meta, _} = quoted assert meta[:trailing_comments] == [ %{line: 3, previous_eol_count: 1, next_eol_count: 1, text: "# A trailing comment"}, %{line: 4, previous_eol_count: 0, next_eol_count: 1, text: "# Also a trailing comment for :foo"}, ] end
Note Sourceror considers leading comments to the ones that are found in the same
line as a node, and trailing coments to the ones that are found in the same line
or before the ending line of a node, based on the end
, closing
or
end_of_expression
line.
Working with line numbers
The way the Elixir formatter combines AST and comments depends on their line numbers and the order in which the AST is traversed. This means that whenever you move a node around, you need to also change the line numbers to reflect their position in the node. This is best seen with an example. Lets imagine you have a list of atoms and you want to sort them in alphabetical order:
:a
# Comment for :j
:j
:c
# Comment for :b
:b
Sorting it is trivial, as you just need to use Enum.sort_by
with
Atom.to_string(atom)
. But if we consider the line numbers:
1 :a
2 # Comment for :j
3 :j
4 :c
5 # Comment for :b
6 :b
If we sort them, we end up with this:
1 :a
6 :b
4 :c
3 :j
And the comments will be associated to the line number of the node they're leading:
6 # Comment for :b
3 # Comment for :j
When the formatter traverses the AST, it will find node :b
with line 6
and
will see comment with line 6
, and it will print that comment. But it will also
see the comment with line 3
and will go like "hey, this comment has a line
number smaller than this node, so this is a trailing comment too!" and will
print that comment as well. That will make it output this code:
:a
# Comment for :b
# Comment for :j
:b
:c
:j
And that's not what we want at all. To avoid this issue, we need to calculate
how line numbers changed while the sorting and correct them appropiately.
Sourceror provides a correct_lines(node, line_correction)
that takes care of
correcting all the line numbers associated to a node, so all you have to do is
figure out the line correction numbers. One way to do it in this example is by
getting the line numbers before the change, reorder the nodes, zip the old line
numbers with the nodes, and correct their line numbers by the difference between
the new and the old one. Translated to code, it would look something like this:
test "sorts atoms with correct comments placement" do
{:__block__, meta, atoms} = """
:a
# Comment for :j
:j
:c
# Comment for :b
:b
""" |> Sourceror.parse_string()
lines = Enum.map(atoms, fn {:__block__, meta, _} -> meta[:line] end)
atoms =
Enum.sort_by(atoms, fn {:__block__, _, [atom]} ->
Atom.to_string(atom)
end)
|> Enum.zip(lines)
|> Enum.map(fn {{_, meta, _} = atom, old_line} ->
line_correction = old_line - meta[:line]
Macro.update_meta(atom, &Sourceror.correct_lines(&1, line_correction))
end)
assert Sourceror.to_string({:__block__, meta, atoms}) == """
:a
# Comment for :b
:b
:c
# Comment for :j
:j
""" |> String.trim()
end
Which will produce the code we expect:
:a
# Comment for :b
:b
:c
# Comment for :j
:j
In other cases, you may want to add lines to the code, which would cause the new
nodes to have higher line numbers than the nodes that come after it, and that
would also mess up the comments placement. For those use cases Sourceror
provides the Sourceror.postwalk/3
function. It's a wrapper over
Macro.postwalk/3
that lets you set the line correction that should be applied
to subsequent nodes, and it will automatically correct them for you before
calling your function on each node. You can see this in action in the
examples/expand_multi_alias.exs
example.
Examples
You can find usage examples in the examples
folder. You can run them with
elixir examples/<example_file>.exs
.
License
Copyright (c) 2021 dorgandash@gmail.com
Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
Link to this section Summary
Functions
Shifts the line numbers of the node by the given line_correction
.
Parses the source code into an extended AST suitable for source manipulation
as described in Code.quoted_to_algebra/2
.
Performs a depth-first post-order traversal of a quoted expression, correcting line numbers as it goes.
Performs a depth-first post-order traversal of a quoted expression with an accumulator, correcting line numbers as it goes.
Converts a quoted expression to a string.
Link to this section Types
Specs
postwalk_function() :: (Macro.t(), traversal_state() -> {Macro.t(), traversal_state()})
Specs
traversal_state() :: %{line_correction: integer()}
Link to this section Functions
Specs
Shifts the line numbers of the node by the given line_correction
.
This function will update the :line
, :closing
, :do
, :end
and
:end_of_expression
line numbers of the node metadata if such fields are
present.
Specs
Parses the source code into an extended AST suitable for source manipulation
as described in Code.quoted_to_algebra/2
.
Two additional fields are added to nodes metadata:
:leading_comments
- a list holding the comments found before the node.:trailing_comments
- a list holding the comments found before the end of the node. For example, comments right before theend
keyword.
Comments are the same maps returned by Code.string_to_quoted_with_comments/2
.
Specs
postwalk(Macro.t(), postwalk_function()) :: Macro.t()
Performs a depth-first post-order traversal of a quoted expression, correcting line numbers as it goes.
See postwalk/3
for more information.
Specs
postwalk(Macro.t(), term(), postwalk_function()) :: {Macro.t(), term()}
Performs a depth-first post-order traversal of a quoted expression with an accumulator, correcting line numbers as it goes.
fun
is a function that will receive the current node as a first argument and
the traversal state as the second one. It must return a {quoted, state}
,
in the same way it would return {quoted, acc}
when using Macro.postwalk/3
.
Before calling fun
in a node, its line numbers will be corrected by the
state.line_correction
. If you need to manually correct the line number of
a node, use correct_lines/2
.
The state is a map with the following keys:
:line_correction
- an integer representing how many lines subsequent nodes should be shifted. If the function adds more nodes to the tree that should go in a new line, the line numbers of the subsequent nodes need to be updated in order for comments to be correctly placed during the formatting process. If the function does this kind of change, it must update the:line_correction
field by adding the amount of lines that should be shifted. Note that this field is cumulative, setting it to 0 will reset it for the whole traversal. Starts at0
.:acc
- The accumulator. Defaults tonil
if none is given.
Specs
Converts a quoted expression to a string.
The comments line number will be ignored and the line number of the associated node will be used when formatting the code.
Options
:indent
- how many indentations to insert at the start of each line. Note that this only prepends the indents without checking the indentation of nested blocks. Defaults to0
.:indent_type
- the type of indentation to use. It can be one of:spaces
,:single_space
or:tabs
. Defaults to:spaces
;