gofmt / go fmt — Hands-on Tasks¶

Go 1.21+. Each task has explicit acceptance criteria.

Task 1: Format a messy file¶

Write a .go file with bad indentation and spacing, then format it.

Acceptance criteria - [ ] gofmt -d messy.go shows a diff before formatting. - [ ] gofmt -w messy.go rewrites it to canonical style. - [ ] A second gofmt -l messy.go prints nothing (it is now clean).

Task 2: List vs write vs print¶

Understand the flag behaviors.

Acceptance criteria - [ ] gofmt file.go prints formatted output but leaves the file unchanged. - [ ] gofmt -l file.go lists the file only if it needs formatting. - [ ] gofmt -w file.go modifies the file in place.

Task 3: Simplification with -s¶

Write code with redundant forms.

Acceptance criteria - [ ] A file containing s[0:len(s)] and []T{T{1}} exists. - [ ] gofmt -s -d file.go shows the simplifications. - [ ] After gofmt -s -w file.go, the redundant forms are gone.

Task 4: go fmt over a module¶

Format every package.

Acceptance criteria - [ ] go fmt ./... formats all packages and lists changed files. - [ ] You confirm go fmt equals gofmt -l -w by comparing behavior.

Task 5: goimports for import management¶

Install and use goimports.

Acceptance criteria - [ ] go install golang.org/x/tools/cmd/goimports@latest succeeds. - [ ] Adding an unused import, then goimports -w file.go, removes it. - [ ] Using a package without importing it, then goimports -w, adds the import.

Task 6: CI-style check¶

Write a check that fails on unformatted code.

Acceptance criteria - [ ] test -z "$(gofmt -l .)" exits 0 when everything is formatted. - [ ] Introducing an unformatted file makes it exit non-zero and list the file. - [ ] You explain why CI uses gofmt -l, not go fmt.

Task 7: Format generated code¶

Generate Go programmatically and ensure it is canonical.

Acceptance criteria - [ ] A small program builds a Go source string and writes it to a file. - [ ] Passing it through go/format.Source produces gofmt-clean output. - [ ] gofmt -l on the generated file prints nothing.

Task 8: Editor format-on-save (reflection)¶

Configure your editor to format on save.

Acceptance criteria - [ ] Saving a messy file auto-formats it (via gopls/the Go extension). - [ ] Organize-imports-on-save adds/removes imports automatically. - [ ] You write 2 sentences on why this makes the CI check rarely fail.