gopatch is a tool to match and transform Go code. It is meant to aid in refactoring and restyling.
gopatch operates like the Unix patch
tool: given a patch file and another
file as input, it applies the changes specified in the patch to the provided
file.
.-------. .-------.
/_| |. /_| |.
| ||. +---------+ | ||.
| .go |||>-->| gopatch |>-->| .go |||
| ||| +---------+ | |||
'--------'|| ^ '--------'||
'--------'| | '--------'|
'--------' | '--------'
.-------. |
/_| | |
| +----'
| .patch |
| |
'--------'
What specifically differentiates it from patch
is that unlike plain text
transformations, it can be smarter because it understands Go syntax.
Download a pre-built binary of gopatch from the Releases page or by
running the following command in your terminal and place it on your $PATH
.
VERSION=0.4.0
URL="https://github.com/uber-go/gopatch/releases/download/v$VERSION/gopatch_${VERSION}_$(uname -s)_$(uname -m).tar.gz"
curl -L "$URL" | tar xzv gopatch
Alternatively, if you have Go installed, build it from source and install it with the following command.
go install github.com/uber-go/gopatch@latest
Note: If you're using Go < 1.16, use go get github.com/uber-go/gopatch@latest
instead.
Write your first patch.
$ cat > ~/s1028.patch
# Replace redundant fmt.Sprintf with fmt.Errorf
@@
@@
-import "errors"
-errors.New(fmt.Sprintf(...))
+fmt.Errorf(...)
This patch is a fix for staticcheck S1028. It searches for uses of
fmt.Sprintf
with errors.New
, and simplifies them by replacing them
with fmt.Errorf
.
For example,
return errors.New(fmt.Sprintf("invalid port: %v", err))
// becomes
return fmt.Errorf("invalid port: %v", err)
cd
to your Go project's directory.
$ cd ~/go/src/example.com/myproject
Run gopatch
on the project, supplying the previously written patch with the
-p
flag.
$ gopatch -p ~/s1028.patch ./...
This will apply the patch on all Go code in your project.
Check if there were any instances of this issue in your code by running
git diff
.
-
Instead,
cd
to your Go project's directory.$ cd ~/go/src/example.com/myproject
Run
gopatch
on the project, supplying the previously written patch with the-p
flag along with '-d' flag.$ gopatch -d -p ~/s1028.patch ./...
This will turn on diff mode and will write the diff to stdout instead of modifying all the Go code in your project. To provide more context on what the patch does, if there were description comments in the patch, they will also get displayed at the top. To learn more about description comments jump to section here
For example if we applied patch ~/s1028 to our testfile error.go
$ gopatch -d -p ~/s1028.patch ./testdata/test_files/diff_example/
Output would be :
gopatch/testdata/test_files/diff_example/error.go:Replace redundant fmt.Sprintf with fmt.Errorf --- gopatch/testdata/test_files/diff_example/error.go +++ gopatch/testdata/test_files/diff_example/error.go @@ -7,7 +7,7 @@ func foo() error { err := errors.New("test") - return errors.New(fmt.Sprintf("error: %v", err)) + return fmt.Errorf("error: %v", err) } func main() {
Note: Only the description comments of patches that actually apply are displayed.
To learn how to write your own patches, move on to the Patches section. To dive deeper into patches, check out Patches in depth.
To experiment with other sample patches, check out the Examples section.
To use the gopatch command line tool, provide the following arguments.
gopatch [options] pattern ...
Where pattern specifies one or more Go files, or directories containing Go files. For directories, all Go code inside them and their descendants will be considered by gopatch.
gopatch supports the following command line options.
-
-p file
,--patch=file
Path to a patch file specifying a transformation. Read more about the patch file format in Patches.
Provide this flag multiple times to apply multiple patches in-order.
$ gopatch -p foo.patch -p bar.patch path/to/my/project
If this flag is omitted, a patch is expected on stdin.
$ gopatch path/to/my/project << EOF @@ @@ -foo +bar EOF
-
-d
,--diff
Flag to turn on diff mode. Provide this flag to write the diff to stdout instead of modifying the file and display applied patches' description comments if they exist. Use in conjunction with -p to provide patch file.
Only need to apply the flag once to turn on diff mode
$ gopatch -d -p foo.patch -p bar.patch path/to/my/project
If this flag is omitted, normal patching occurs which modifies the file instead.
-
--print-only
Flag to turn on print-only mode. Provide this flag to write the changed code to stdout instead of modifying the file and display applied patches' description comments to stderr if they exist.
$ gopatch --print-only -p foo.patch -p bar.patch path/to/my/project
-
--skip-import-processing
Flag to turn on skip-import-processing mode. Provide this flag to disable import formatting for imports that were not part of the patch changes.
$ gopatch --skip-import-processing -p foo.patch -p bar.patch path/to/my/project
-
--skip-generated
Flag to turn on skip-generated code mode. Provide this flag to skip running the tool on generated code. A file is considered containing generated code if it has
@generated
or^// Code generated .* DO NOT EDIT\.$
in the comment header.$ gopatch --skip-generated -p foo.patch -p bar.patch path/to/my/project
Patch files are the input to gopatch that specify how to transform code. Each patch file contains one or more patches. This section provides an introduction to writing patches; look at Patches in depth for a more detailed explanation.
Each patch specifies a code transformation. These are formatted like unified
diffs: lines prefixed with -
specify matching code should be deleted, and
lines prefixed with +
specify that new code should be added.
Consider the following patch.
@@
@@
-foo
+bar
It specifies that we want to search for references to the identifier foo
and
replace them with references to bar
. (Ignore the lines with @@
for now.
We will cover those below.)
A more selective version of this patch will search for uses of foo
where it
is called as a function with specific arguments.
@@
@@
-foo(42)
+bar(42)
This will search for invocations of foo
as a function with the specified
argument, and replace only those with bar
.
gopatch understands Go syntax, so the above is equivalent to the following.
@@
@@
-foo(
+bar(
42,
)
Searching for hard-coded exact parameters is limited. We should be able to generalize our patches.
The previously ignored @@
section of patches is referred to as the
metavariable section. That is where we specify metavariables for the
patch.
Metavariables will match any code, to be reproduced later. Think of them like holes to be filled by the code we match. For example,
@@
var x expression
@@
# rest of the patch
This specifies that x
should match any Go expression and record its match
for later reuse.
What is a Go expression?
Expressions usually refer to code that has value. You can pass these as arguments to functions. These include
x
,foo()
,user.Name
, etc.Check the Identifiers vs expressions vs statements section of the appendix for more.
So the following patch will search for invocations of foo
with a single
argument---any argument---and replace them with invocations of bar
with the
same argument.
@@
var x expression
@@
-foo(x)
+bar(x)
Input | Output |
---|---|
foo(42) |
bar(42) |
foo(answer) |
bar(answer) |
foo(getAnswer()) |
bar(getAnswer()) |
Metavariables hold the entire matched value, so we can add code around them without risk of breaking anything.
@@
var x expression
@@
-foo(x)
+bar(x + 3, true)
Input | Output |
---|---|
foo(42) |
bar(42 + 3, true) |
foo(answer) |
bar(answer + 3, true) |
foo(getAnswer()) |
bar(getAnswer() + 3, true) |
For more on metavariables see Patches in depth/Metavariables.
gopatch patches are not limited to transforming basic expressions. You can also transform statements.
What is a Go statements?
Statements are instructions to do things, and do not have value. They cannot be passed as parameters to other functions. These include assignments (
foo := bar()
), if statements (if foo { bar() }
), variable declarations (var foo Bar
), and so on.Check the Identifiers vs expressions vs statements section of the appendix for more.
For example, consider the following patch.
@@
var f expression
var err identifier
@@
-err = f
-if err != nil {
+if err := f; err != nil {
return err
}
The patch declares two metavariables:
f
: This represents an operation that possibly returns anerror
err
: This represents the name of theerror
variable
The patch will search for code that assigns to an error variable immediately
before returning it, and inlines the assignment into the if
statement. This
effectively reduces the scope of the variable to just the if
statement.
Input | Output |
---|---|
err = foo(bar, baz)
if err != nil {
return err
} |
if err := foo(bar, baz); err != nil {
return err
} |
err = comment.Submit(ctx)
if err != nil {
return err
} |
if err := comment.Submit(ctx); err != nil {
return err
} |
For more on transforming statements, see Patches In Depth/Statements.
Matching a single argument is still too selective and we may want to match a wider criteria.
For this, gopatch supports elision of code by adding ...
in many places.
For example,
@@
@@
-foo(...)
+bar(...)
The patch above looks for all calls to the function foo
and replaces them
with calls to the function bar
, regardless of the number of arguments they
have.
Input | Output |
---|---|
foo(42) |
bar(42) |
foo(42, true, 1) |
bar(42, true, 1) |
foo(getAnswer(), x(y())) |
bar(getAnswer(), x(y())) |
Going back to the patch from Statements, we can instead write the following patch.
@@
var f expression
var err identifier
@@
-err = f
-if err != nil {
+if err := f; err != nil {
return ..., err
}
This patch is almost exactly the same as before except the return
statement
was changed to return ..., err
. This will allow the patch to operate even on
functions that return multiple values.
Input | Output |
---|---|
err = foo()
if err != nil {
return false, err
} |
if err := foo(); err != nil {
return false, err
} |
For more on elision, see Patches in depth/Elision.
Patches come with comments to give more context about what they do.
Comments are prefixed by '#'
For example:
# Replace time.Now().Sub(x) with time.Since(x)
@@
# var x is in the metavariable section
var x identifier
@@
-time.Now().Sub(x)
+time.Since(x)
# We replace time.Now().Sub(x)
# with time.Since(x)
Description comments are comments that appear directly above a patch's first
@@
line.
gopatch will record these descriptions and display them to users with use of
the --diff
or --print-only
flags.
For example,
# Replace time.Now().Sub(x) with time.Since(x)
@@
# Not a description comment
var x identifier
@@
-time.Now().Sub(x)
+time.Since(x)
# Not a description comment
# Not a description comment
Patch files with multiple patches can have a separate description for each patch.
# Replace redundant fmt.Sprintf with fmt.Errorf
@@
@@
-import "errors"
-errors.New(fmt.Sprintf(...))
+fmt.Errorf(...)
# Replace time.Now().Sub(x) with time.Since(x)
@@
var x identifier
@@
-time.Now().Sub(x)
+time.Since(x)
# Not a description comment
As these are messages that will be printed to users of the patch, we recommend the following best practices for description comments.
- Keep them short and on a single-line
- Use imperative mood ("replace X with Y", not "replaces X with Y")
When diff mode is turned on by the -d
/--diff
flag, gopatch will print
description comments for patches that matched different files to stderr.
$ gopatch -d -p ~/s1028.patch testdata/test_files/diff_example/error.go
error.go:Replace redundant fmt.Sprintf with fmt.Errorf
--- error.go
+++ error.go
@@ -7,7 +7,7 @@
func foo() error {
err := errors.New("test")
- return errors.New(fmt.Sprintf("error: %v", err))
+ return fmt.Errorf("error: %v", err)
}
func main() {
Note that gopatch will print only the description comments in diff mode. Other comments will be ignored.
This section lists various example patches you can try in your code. Note that some of these patches are not perfect and may have false positives.
- s1012.patch: Fix for staticcheck S1012.
- s1028.patch: Fix for staticcheck S1028.
- s1038.patch: Fix for staticcheck S1038.
- gomock-v1.5.0.patch: Drops unnecessary call to
Finish
method for users of gomock. - destutter.patch: Demonstrates renaming a type and updating its consumers.
The project is currently is in a beta state. It works but significant features are planned that may result in breaking changes to the patch format.
gopatch aims to be a generic power tool that you can use in lieu of simple search-and-replace.
gopatch will attempt to do 80% of the work for you in a transformation, but it cannot guarantee 100% correctness or completeness. Part of this is owing to the decision that gopatch must be able to operate on code that doesn't yet compile, which can often be the case in the middle of a refactor. We may add features in the future that require compilable code, but we plan to always support transformation of partially-valid Go code.
Beyond the known issues highlighted above, there are a handful of other issues with using gopatch today.
- It's very quiet, so there's no indication of progress. #7
- Error messages for invalid patch files are hard to decipher. #8
- Matching elisions between the
-
and+
sections does not always work in a desirable way. We may consider replacing anonymous...
elision with a different named elision syntax to address this issue. #9 - When elision is used, gopatch stops replacing after the first instance in the given scope which is often not what you want. #10
- Formatting of output generated by gopatch isn't always perfect.
Besides addressing the various limitations and issues we've already mentioned, we have a number of features planned for gopatch.
- Contextual matching: match context (like a function declaration), and then run a transformation inside the function body repeatedly, at any depth. #11
- Collateral changes: Match and capture values in one patch, and use those in a following patch in the same file.
- Metavariable constraints: Specify constraints on metavariables, e.g. matching a string, or part of another metavariable.
- Condition elision: An elision should match only if a specified condition is also true.
If you'd like to contribute to gopatch, you may find the following documents useful:
- HACKING documents the architecture, code organization, and other information necessary to contribute to the project.
- RELEASE documents the process for releasing a new version of gopatch.
- rf is a refactoring tool with a custom DSL
- gofmt rewrite rules support simple transformations on expressions
- eg supports basic example-based refactoring
- Coccinelle is a tool for C from which gopatch takes inspiration heavily
- Semgrep is a cross-language semantic search tool
- Comby is a language-agnostic search and transformation tool
gopatch is heavily inspired by Coccinelle.