Why Does Go Flag an Unused Variable Error for `append` But Not for Direct Slice Assignment?

Go Compilation Error for Unused “append” Result

The Go compiler may issue an unused variable compilation error when appending to a slice and not using the result. This behavior differs from other languages that raise an unused variable error when declaring a variable without using it.

In Go, the presence of an unused variable is flagged as an error only if the variable is not read. Appending to a slice using append() involves reading the slice as it is passed to the function. Similar behavior is observed when assigning a value to a map key.

Consider the following code:

var mySlice []string
mySlice = append(mySlice, "halo")

In this example, mySlice is read during the append operation, even if its result is not used elsewhere. Therefore, this code will not generate an unused variable error.

However, assigning a value to a slice using the direct assignment operator does not involve reading the slice header. As a result, the following code will produce an unused variable error:

var i = []int{0}
i = []int{1}

To resolve this error, the slice can be used before reassigning it. For example:

var i = []int{0}
i[0] = 1
i = []int{1}

This code will compile successfully because i[0] = 1 reads the slice header.

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