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

Differing Behavior between Variables and Return Values in Functions

When utilizing functions in Go, it's crucial to understand the distinction between variables and return values in terms of their addressability and subsequent behavior.

Original Code and Error

Consider the following code snippet:

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

This code functions as expected. However, if we attempt to combine the two lines:

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

we encounter the following error:

models/models.go:104: invalid operation sha1.Sum(([]byte)(uf.Pwd))[:] (slice of unaddressable value)

Reason for Error

The error arises because in the combined line, we try to slice the return value of the function sha1.Sum() directly:

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

Function return values in Go are not addressable. According to the language specification, addressable values include variables, pointer indirections, slice indexing operations, field selectors of addressable structs, array indexing operations of addressable arrays, and, as an exception, composite literals. Slicing an array requires the array to be addressable.

Resolution

The original code works because we first assign the return value of sha1.Sum() to a local variable (hash), which makes the array addressable and thus eligible for slicing. Therefore, it's generally recommended to store return values in variables before performing operations on them to avoid such errors and ensure addressability.

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