Why Can\'t I Slice the Return Value of a Function in Go?

Variability in Variable and Function Return Value Behavior

In programming, it's essential to understand the distinction in behavior between variables and function return values. Consider the following code snippet:

<code class="go">hash := sha1.Sum([]byte(uf.Pwd))
u.Pwhash = hex.EncodeToString(hash[:])</code>

This code functions as intended, where the hash variable stores the SHA1 hash of a byte array, and the u.Pwhash variable holds the hexadecimal representation of that hash.

However, if we attempt to combine these two lines into one, we encounter an error:

<code class="go">u.Pwhash = hex.EncodeToString(sha1.Sum([]byte(uf.Pwd))[:])</code>

The error message indicates that it's invalid to slice the return value of sha1.Sum(). This difference arises because:

• Variables: Variables like hash are addressable, meaning they represent a location in memory where data can be stored and retrieved.
• Function Return Values: Unlike variables, the return values of function calls (like sha1.Sum()) are not addressable. They are temporary values created and destroyed during function execution.

In the second snippet, we attempt to slice sha1.Sum()[:], which is not permitted because the return value of sha1.Sum() is not addressable. A slice operation requires its operands to be addressable.

Therefore, to correctly combine the lines, we first need to store the return value of sha1.Sum() in an addressable variable like hash before slicing it:

<code class="go">hash := sha1.Sum([]byte(uf.Pwd))
u.Pwhash = hex.EncodeToString(hash[:])</code>

Understanding the addressability and behavior of variables and function return values is crucial for writing correct and efficient Go code.

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