Go core · 3 MIN READ

Read a declaration one scope at a time

Follow values through a short declaration, an assignment and a shadowed name without running the program.

A useful way to read Go is to ask three questions at each assignment: which block owns this name, what type does it have, and is a new variable being created? The spelling of a name alone does not answer any of them.

Trace the original variable

Start with a local variable. Its type comes from the expression on the right. An ordinary assignment updates that same variable; it does not create another binding. Read the two lines as a small state table: first count is 2, then count is 5.

count := 2
count = count + 3
fmt.Println(count) // 5

Read the code

The right side is evaluated before the assignment. The old count contributes 2, the expression becomes 5, and only then is count updated. This is useful when stepping through a counter, an index or a retry budget.

A nested block owns a different name

An if body is a new lexical block. A short declaration there may hide an outer variable. This can be intentional for a temporary result, but it becomes confusing when the outer result was supposed to change.

name := "outside"
if ok {
    name := "inside"
    fmt.Println(name) // inside
}
fmt.Println(name) // outside

Read the code

The inner name stops being accessible at the closing brace. Replacing its := with = would instead update the outer string. The difference is scope, not a special rule about if.

Use the rule while reviewing real code

In a multi-name short declaration, at least one non-blank name must be new in the same block. Existing names in that block keep their declared types. Before changing := to =, find every declaration and branch exit; an inner err may be a deliberate short-lived error, or it may hide the value returned later.

TAKE THIS WITH YOU

Read the block before the operator. Prefer a short temporary scope when it clarifies ownership; use assignment when an existing result must change.

Check your understanding

Which statement is correct about nested :=?

Go to the source

go.dev/ref/spec

Progress stays in this browser. No account needed.