How does Go's short circuit evaluation affect the order of conditions in logical expressions?

Short Circuit Evaluation in Go: Execution Order and Performance

In programming, short circuit evaluation is an optimization technique where a logical expression is evaluated only partially if the result is already known. This can improve performance by avoiding unnecessary calculations.

In Go, short circuit evaluation is implemented for the && (logical AND) and || (logical OR) operators. This means that if the first operand of a logical expression evaluates to false for &&, or true for ||, the second operand will not be evaluated.

Consider the following code snippets:

if !isValidQueryParams(&queries) || r == nil || len(queries) == 0 {
    return "", fmt.Errorf("invalid querystring")
}

if r == nil || len(queries) == 0 || !isValidQueryParams(&queries) {
    return "", fmt.Errorf("invalid querystring")
}

In both cases, if r is nil or len(queries) is 0, the call to isValidQueryParams(&queries) will not be executed, regardless of the order of the conditions. This is because Go follows short circuit evaluation rules.

To demonstrate this, consider the following code:

for i := 0; i < 10; i++ {
    if testFunc(1) || testFunc(2) {
        // do nothing
    }
}

func testFunc(i int) bool {
    fmt.Printf("function %d called\n", i)
    return true
}

Running this code will always print:

$ function 1 called
$ function 1 called
$ function 1 called
$ function 1 called
$ function 1 called
$ function 1 called
$ function 1 called
$ function 1 called
$ function 1 called
$ function 1 called

This demonstrates that Go evaluates the first operand of the logical OR expression (testFunc(1)) and immediately returns true because true || anything is always true. As a result, the second operand (testFunc(2)) is never actually called.

Therefore, the order of conditions in a logical expression in Go can have an impact on performance when short circuit evaluation is applied. The optimal order will depend on the specific circumstances and the likelihood of each condition being true or false.

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