Why Does My Go Code Produce 'Variable Declared and Not Used' Errors Even Though the Variables Are Used?

Go Compiler's "Variable Declared and Not Used" Errors Despite Actual Usage

The following function in Go yields "variable declared and not used" errors:

type Comparison struct {
    Left []byte
    Right []byte
    Name string
}

func img(w http.ResponseWriter, r *http.Request, c appengine.Context, u *user.User) {
    key := datastore.NewKey("Comparison", r.FormValue("id"), 0, nil)
    side := r.FormValue("side")
    comparison := new(Comparison)
    err := datastore.Get(c, key, comparison)
    check(err)

    if side == "left" {
        m, _, err := image.Decode(bytes.NewBuffer(comparison.Left))
    } else {
        m, _, err := image.Decode(bytes.NewBuffer(comparison.Right))
    }
    check(err)

    w.Header().Set("Content-type", "image/jpeg")
    jpeg.Encode(w, m, nil)
}

The errors include:

• dpcompare.go:171: m declared and not used
• dpcompare.go:171: err declared and not used
• dpcompare.go:173: m declared and not used
• dpcompare.go:173: err declared and not used
• dpcompare.go:178: undefined: m
• dpcompare.go:185: key declared and not used

However, upon closer examination, it becomes clear that the variables m, err, and key are indeed being used.

Cause and Solution

As pointed out by @kostix, the issue lies in the scope of m. In the given code, m is declared within the scope of the if and else statements. To fix this, the declaration of m should be moved outside of these blocks:

type Comparison struct {
    Left []byte
    Right []byte
    Name string
}

func img(w http.ResponseWriter, r *http.Request, c appengine.Context, u *user.User) {
    key := datastore.NewKey("Comparison", r.FormValue("id"), 0, nil)
    side := r.FormValue("side")
    comparison := new(Comparison)
    err := datastore.Get(c, key, comparison)
    check(err)

    // NOTE! now m is in the function's scope
    var m Image

    if side == "left" {
        m, _, err = image.Decode(bytes.NewBuffer(comparison.Left))
    } else {
        m, _, err = image.Decode(bytes.NewBuffer(comparison.Right))
    }
    check(err)

    w.Header().Set("Content-type", "image/jpeg")
    jpeg.Encode(w, m, nil)
}

By making m a function-scoped variable, it can be accessed and used throughout the entire function, resolving the "declared and not used" errors.

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