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

Understanding Variable and Return Value Behavior in Go

A common issue arises when attempting to join lines of code involving arrays and function calls. For example, consider the following original code that successfully joins two lines:

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

However, if you attempt to combine the code into a single line, you may encounter an error message:

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

The error message indicates an issue with slicing the return value of a function call. In Go, return values of function calls, like the one from sha1.Sum(), are not addressable.

To understand this behavior, it's important to know what addressable types are in Go. According to the Go specification, only the following are addressable:

• Variables
• Pointer indirections
• Slice indexing operations of addressable structs
• Array indexing operations of addressable arrays
• Composite literals (exempted from the addressability requirement)

Since the return value of sha1.Sum() is not one of these types, it cannot be sliced. Slicing an array, as required here, requires the array to be addressable.

In the first line of the original code, the returned array is stored in a local variable (hash), which is addressable. In the second line, hex.EncodeToString(hash[:]) is computed, which works as intended.

This distinction between variables and return values highlights the importance of understanding Go's addressing rules. By following these rules, you can avoid common errors and write more robust code.

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