modern-go

modern-go

Modernize Go source code by applying version-appropriate idioms, APIs, and language features. Works like go fix plus additional transformations curated from the Go team's modernize analysis passes and community best practices.

Usage

Invoke this skill when the user asks to modernize Go code. By default, modernize the entire project; the user may specify a file or directory instead.

When invoked:

1. Detect the project's Go version from go.mod (the go directive).
2. Find all .go files in the target scope (excluding vendor/, .git/, testdata/).
3. For each file, apply all transformations for versions ≤ the project's Go version, starting from the oldest to the newest.
4. After all transformations, print a summary of what was changed and what was skipped.

If the user specifies a file or directory, limit the scope to that path.

Transformation Catalog

Each transformation includes a Go version gate—only apply when the project's go.mod version ≥ that version. Never apply a transformation that requires a version higher than the project declares.

Go 1.0+ — time.Since

Before	After
time.Now().Sub(start)	time.Since(start)

// before
elapsed := time.Now().Sub(start)
// after
elapsed := time.Since(start)

Go 1.8+ — time.Until

Before	After
deadline.Sub(time.Now())	time.Until(deadline)

// before
remaining := deadline.Sub(time.Now())
// after
remaining := time.Until(deadline)

Go 1.10+ — strings.Builder (loop concatenation)

Before	After
s += item in a loop	var b strings.Builder; b.WriteString(item)

// before
s := ""
for _, item := range items {
    s += item
}
// after
var b strings.Builder
for _, item := range items {
    b.WriteString(item)
}
s := b.String()

Only when += concatenation happens inside a loop.

Go 1.13+ — errors.Is

Before	After
err == io.EOF	errors.Is(err, io.EOF)