Go HTTP URL Query Parameters Parsing

In this tutorial, you'll learn about Go HTTP URL Query Parameters Parsing. We cover key concepts, practical examples, and best practices.

URL query parameters -- Parse HTTP query parameters correctly using r.URL.Query() methods to access individual values safely.

The Problem

New Go developers often try to parse query strings manually or use r.URL.RawQuery directly. The Query() method returns a url.Values map that has Get(), Set(), and other helper methods.

Wrong

func handler(w http.ResponseWriter, r *http.Request) {
    name := r.URL.Query()["name"] // Returns []string!
    w.Write([]byte("Hello " + name))
}

Output:

$ curl "http://localhost:8080/hello?name=John"
// Compile error: cannot use name (type []string) as type string

Right

func handler(w http.ResponseWriter, r *http.Request) {
    name := r.URL.Query().Get("name")
    if name == "" { name = "Guest" }
    w.Write([]byte("Hello " + name))
}

Output:

$ curl "http://localhost:8080/hello?name=John"
Hello John

Prevention

• Use Query().Get(key) for single values
• Use Query()[key] when you expect multiple values
• Use r.URL.Query() once and reuse the map
• URL-encoded values are automatically decoded by Query()
• Check with: if v := vals.Get("page"); v != "" { ... }

Common Mistakes with http url params

1. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
2. Using return to exit a function early instead of wrapping a pure value in the monad
3. Mixing let bindings with <- bindings in do notation, producing type errors

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

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