Go Faster Gooners Golang Refactor Tools

Go Refactoring Tools

Decision table

Workflow: dry-run first (gopatch -d / rf -diff), review the diff, then apply. After any tool run: goimports -w . && go build ./... && go test ./.... Commit before bulk rewrites.

gofmt -r — quickest, ships with Go

Syntax-aware but not type-aware. Pattern variables must be single lowercase letters; everything else is matched literally. -w . recurses naturally — ./... is a Go package pattern and will error.

gofmt -r 'a.OldMethod(b) -> a.NewMethod(b)' -w .
gofmt -r 'ioutil.ReadAll(a) -> io.ReadAll(a)' -w .   # ioutil/io are literal; a is wildcard

After running: fix imports with goimports -w . (gofmt -r never touches imports).

gopatch — structural patch files

Operates on syntax, understands statements and imports. Can work on partially-invalid code.

go install github.com/uber-go/gopatch@latest
gopatch -p my.patch ./...      # rewrite in place
gopatch -d -p my.patch ./...   # dry-run: print diff
gopatch ./... <<'EOF'          # read patch from stdin (omit -p)
...
EOF

Patch anatomy

# Description (shown in -d output)
@@
var x expression    # matches any Go expression
var n identifier    # matches any single identifier
@@
-old code
+new code

Metavariable types: expression (calls, field accesses, literals…), identifier (single name). Undeclared identifiers are matched literally — a, b, c in the body match only args named exactly a, b, c. Use ... (elision) or declare var a expression explicitly. ...: matches zero or more statements/arguments. Multiple @@…@@ blocks = multiple patches run in order.

Examples

Replace call + fix import in one patch:

@@
@@
-import "io/ioutil"
+import "io"

-ioutil.ReadAll(...)
+io.ReadAll(...)

Remove boilerplate with elision:

# Delete redundant gomock.Controller.Finish()
@@
var ctrl, gomock identifier
var t expression
@@
 import gomock "github.com/golang/mock/gomock"

 ctrl := gomock.NewController(t)
 ...
-defer ctrl.Finish()

Replace expression with sub-expression wildcard:

@@
var x expression
@@
-time.Now().Sub(x)
+time.Since(x)

rsc.io/rf — rename, move, expression rewrite

⚠ Experimental: rf's README says "incredibly rough and likely to be buggy and change incompatibly." Prefer gopatch for structural work; use rf when type-awareness is required (renames, moves, typed expression rewrites).

Takes a single script argument and operates on the module in the current directory — no ./....

go install rsc.io/rf@latest
rf -diff 'mv Foo Bar'    # show diff without writing

mv — rename or move

rf 'mv OldType NewType'
rf 'mv pkg.OldFunc pkg.NewFunc'
rf 'mv mypkg.Foo otherpkg.Foo'
rf 'mv MyStruct.OldField MyStruct.NewField'

ex — type-aware expression rewrite

Variables inside { } are typed metavariables — only values of the declared type match, so there are no false positives on same-named identifiers from other packages. If a type doesn't precisely match the AST node, the rule is silently skipped. Multiple rules go in one block.

rf 'ex {
  var s string
  var r io.Reader
  fmt.Sprintf("%s", s) -> s
  ioutil.ReadAll(r)    -> io.ReadAll(r)
}'

avoid prevents rewriting inside named method bodies:

rf 'ex {
  var m myMap
  var k, v int
  avoid myMap.Get
  m[k] -> m.Get(k)
}'

rm / add

rf 'rm OldFunc'
rf 'rm MyType.DeprecatedField'
rf 'add MyStruct.NewField int `json:"new_field"`'

Common recipes

Rename a public type

rf 'mv pkg.OldName pkg.NewName'

Source: go-faster/gooners — distributed by TomeVault.