goimports — Hands-on Tasks¶

Work through these in order. Each has explicit acceptance criteria. Use Go 1.21+.

Task 1: Install and run¶

Install goimports and confirm the binary is on your PATH.

Acceptance criteria - [ ] go install golang.org/x/tools/cmd/goimports@latest completes without error. - [ ] goimports -h prints usage from the terminal. - [ ] which goimports resolves to a path under $(go env GOBIN) or $(go env GOPATH)/bin.

Task 2: Rewrite a file with stray and missing imports¶

Create hello.go that uses fmt.Println but has no fmt import, and imports "os" it does not use:

package main

import "os"

func main() {
    fmt.Println("hi")
}

Acceptance criteria - [ ] goimports hello.go prints the corrected file to stdout (without changing the file). - [ ] goimports -w hello.go rewrites the file: fmt added, os removed. - [ ] The file now compiles (go build ./... succeeds).

Task 3: Group local imports with -local¶

In a module named example.com/myorg/app, write a file that imports stdlib, a third-party package (e.g., github.com/google/uuid), and a local subpackage (e.g., example.com/myorg/app/internal/util).

Acceptance criteria - [ ] Without -local, goimports -w main.go produces two groups (stdlib + everything else). - [ ] With goimports -w -local example.com/myorg main.go, the file now has three groups separated by blank lines: stdlib, third-party, example.com/myorg/.... - [ ] You can explain why -local only changes grouping, not which packages are imported.

Task 4: Wire goimports -l as a CI gate¶

Add a shell script scripts/fmt-check.sh that fails if any .go file under the repo is not formatted with your project's -local prefix.

Acceptance criteria - [ ] The script runs goimports -l -local <your-prefix> . and exits non-zero when the output is non-empty. - [ ] Deliberately add a stray import to one file; the script fails. - [ ] Run goimports -w -local <your-prefix> .; the script now passes. - [ ] The script also prints the diff (goimports -d) on failure so a reviewer sees what is wrong.

Task 5: Configure your editor to run goimports on save¶

Pick one editor and configure format-on-save with goimports (or gopls organizeImports).

Acceptance criteria - [ ] Editing a .go file and saving it triggers an automatic format pass. - [ ] Adding a reference to fmt.Println to a file without the import and saving causes the fmt import to be added automatically. - [ ] Your editor uses the same -local value as your CI script (check the settings file).

Task 6: Compare output to gofmt¶

Take the file from Task 2 (before fixing).

Acceptance criteria - [ ] gofmt -d hello.go shows only whitespace/layout changes, does not add fmt or remove os. - [ ] goimports -d hello.go shows the import-block change in addition. - [ ] You can state in one sentence why gofmt-formatted broken code still does not compile while goimports-formatted code does.

Task 7: Replace with gopls organize_imports¶

In your editor, disable goimports as the format tool and use gopls's source.organizeImports code action instead (alongside gopls's formatter).

Acceptance criteria - [ ] Saving a file still adds missing imports and removes unused ones. - [ ] The local-prefix grouping still works (configure gopls settings: "local": "example.com/myorg"). - [ ] On a large file in a repo with many dependencies, saving feels noticeably snappier than the goimports CLI invocation did (informal comparison is fine).

Task 8: Disambiguate two packages with the same name¶

Write code that calls rand.Intn(100) without an explicit import. Have both math/rand and crypto/rand available (they are both in the stdlib, so both are always available).

Acceptance criteria - [ ] Run goimports -w; observe which rand was chosen. - [ ] Manually replace the import with the other rand package; run goimports -w again — the explicit import is preserved, not changed back. - [ ] You can describe what you would do in a code review if you saw rand used without an explicit import in a security-sensitive context.

Task 9: Pre-commit hook on changed files¶

Add a Git pre-commit hook (.git/hooks/pre-commit) that runs goimports -w -local <prefix> on staged .go files only.

Acceptance criteria - [ ] Committing a file with bad formatting auto-fixes it and re-stages it. - [ ] Committing changes to a non-Go file does not run goimports. - [ ] The hook is fast on a large repo because it only touches changed files (test by editing one file in a tree with thousands of .go files — it should complete in well under a second).