The Go standard library, learnable in an afternoon.

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If you're new to Go, learn these in this order. They cover almost everything a real program needs.

1. 1 fmt Print things, format strings, build error messages. Try: Print a struct three ways with %v, %+v, %#v.
2. 2 errors Idiomatic error handling — sentinels, wrapping, Is/As. Try: Wrap an error with fmt.Errorf %w and check it with errors.Is.
3. 3 strings Read, search, and reshape UTF-8 text. Try: Use strings.Builder to assemble a CSV row.
4. 4 strconv Parse "42" into an int and back. Try: Read a number from the command line with Atoi.
5. 5 os Files, env vars, args, exit codes. Try: Read a file with os.ReadFile and print its size.
6. 6 io Streams: Reader/Writer interfaces glue everything. Try: Pipe a file into stdout with io.Copy.
7. 7 bufio Read a file or stdin one line at a time. Try: Count lines in a file with bufio.Scanner.
8. 8 encoding/json Encode/decode Go values as JSON. Try: Round-trip a struct: Marshal, then Unmarshal back.
9. 9 time Wall time, durations, sleep, format/parse. Try: Format time.Now() as RFC3339; parse it back.
10. 10 context Cancellation and deadlines through your call graph. Try: Run a slow function with a 1-second WithTimeout.
11. 11 net/http HTTP client + server in one package. Try: Build a server that responds "hello, {name}" using path values.
12. 12 testing Write tests, benchmarks, and table-driven cases. Try: Write a TestX with t.Run subtests over a table.
13. 13 slices Generic slice helpers — sort, search, transform. Try: Sort a []User by Age with slices.SortFunc and cmp.Compare.
14. 14 log/slog Structured logging in the stdlib. Try: Log a request with slog.Info("req", "method", m, "path", p).