golang

Simplicity is the highest Go virtue. Three biases decide most calls:

• Resist abstraction until the cost of not abstracting is proven. An interface, a type parameter, or a layer added in anticipation of a second caller is a defect.
• Reach for the standard library first. A dependency earns its place by doing what the standard library does not.
• The go directive decides what may be written, not the installed toolchain.

Language Version

The go directive in go.mod sets the language version a module may use. Read it before writing code and never reach for a feature above it. The stdversion analyzer reports a standard-library symbol newer than the directive: Go 1.27 and later fail go test on it, earlier toolchains report it under go vet.

Floor version per feature. Anything older than 1.21 is available in every supported module.

• 1.21 — min, max, clear builtins; slices, maps, cmp, log/slog; context.WithoutCancel; errors.ErrUnsupported; sync.OnceValue
• 1.22 — per-iteration loop variables; for range int; math/rand/v2; method and wildcard patterns in net/http.ServeMux
• 1.23 — range-over-func iterators; iter; iterator functions in slices and maps; unique; unbuffered timer channels
• 1.24 — generic type aliases; os.Root; testing.B.Loop; t.Context; runtime.AddCleanup; omitzero struct tag; tool directives; iterator functions in strings
• 1.25 — testing/synctest; sync.WaitGroup.Go; container-aware GOMAXPROCS
• 1.26 — new(expr); self-referential type constraints; errors.AsType; go fix modernizers
• 1.27 — generic methods; field-selector keys in struct literals; encoding/json/v2; uuid; strings.CutLast

Read [${CLAUDE_SKILL_DIR}/references/versions/go1.NN.md] — one file per version, go1.21.md through go1.27.md — when writing against a feature near the module's floor, and whenever raising the directive. Each carries what its version added, which behavior changes the go directive gates, and the traps it introduced.