playground.shiny.space/go-cheatsheet.md

Go Patterns Cheat Sheet

Defer - Cleanup Pattern

initDB()
defer db.Close()  // Runs when function exits, no matter how

• Schedules function to run when surrounding function returns
• Executes in LIFO order if multiple defers
• Great for resource cleanup (files, connections, etc.)

Formatted Output to HTTP

fmt.Fprintf(w, "%d", rng.Intn(1000))

• fmt.Fprintf = formatted print to a writer (not stdout)
• w = HTTP response writer
• %d = format placeholder for integers
• Alternative: fmt.Fprint(w, value) for no formatting

Ticker + Select for Scheduled Tasks

func startMiningTicker() {
    ticker := time.NewTicker(1 * time.Minute)
    defer ticker.Stop()
    
    for {
        select {
        case <-ticker.C:
            updateMiningValue()
        }
    }
}

• time.NewTicker() creates repeating timer
• select waits for channel events
• <-ticker.C receives from ticker's channel
• Common pattern for background jobs

Database Scanning into Structs

type MiningRecord struct {
    ID        int       `json:"id"`
    Timestamp time.Time `json:"timestamp"`
    Value     float64   `json:"value"`
    Rate      float64   `json:"rate"`
}

var latest MiningRecord
err := db.QueryRow("SELECT id, timestamp, value, rate FROM mining ORDER BY timestamp DESC LIMIT 1").
    Scan(&latest.ID, &latest.Timestamp, &latest.Value, &latest.Rate)

• Scan() copies SQL results into Go variables
• Order matters: first column → first Scan argument
• Use & to pass addresses so Scan can write to them
• Struct tags (json:"id") control JSON serialization

Blank Identifier for Ignoring Values

_, err = db.Exec("INSERT INTO mining (timestamp, value, rate) VALUES (?, ?, ?)", now, newValue, latest.Rate)

• _ = blank identifier, discards unwanted return values
• db.Exec() returns (sql.Result, error)
• We only care about the error, so ignore the Result
• Go requires this instead of leaving variables unused

HTTP Handler Pattern

func handler(w http.ResponseWriter, r *http.Request) {
    // w = where response goes
    // r = incoming request data
    fmt.Fprintf(w, "response")
}

• w writes back to client
• r contains request info
• Common to use fmt.Fprintf(w, ...) for simple responses