gopatch

gopatch is a tool to match and transform Go code. It is meant to aid in refactoring and restyling.

Table of contents

• Introduction
• Getting started
  • Installation
  • Your first patch
  • Apply the patch
  • Next steps
• Usage
  • Options
• Patches
  • Metavariables
  • Statements
  • Elision
• Examples
• Project status
  • Goals
  • Known issues
  • Upcoming
• Similar Projects
• Credits

Introduction

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.

Getting started

Installation

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=0.1.0
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.

Your first patch

Write your first patch.

$ cat > ~/s1028.patch
@@
@@
-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)

Apply the patch

To apply the patch, 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.

Next steps

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.

Usage

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.

Options

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

Patches

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,
 )

Metavariables

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)

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)

For more on metavariables see Patches in depth/Metavariables.

Statements

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 an error
• err: This represents the name of the error 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.

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.

Elision

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.

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.

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.

Examples

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.

Project status

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.

Goals

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.

Known issues

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.

Upcoming

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.

Contributing

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.

Similar Projects

• 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

Credits

gopatch is heavily inspired by Coccinelle.