var vs := in Go

Go provides var and := because variable declarations occur in different contexts. var is the general declaration form and works both at package scope and inside a function. := is short-declaration syntax and works only inside a function body.

A var declaration can give the type without giving a value. For example, var retries int creates an int with its zero value, which is 0. It can also include a value, as in var name = "Ada", or make the intended type explicit, as in var timeout time.Duration = 5 * time.Second.

A short declaration combines declaration, type inference, and assignment. In name := "Ada", Go creates a new local variable and infers string from the value. It is concise when the value already makes the type obvious, which is why it appears frequently in normal function code.

The assignment operator = is different from both forms because it changes a variable that already exists. After count := 1, the next update is count = 2, not count := 2, because the second statement introduces no new name.

Short declarations have one useful mixed case. If err already exists, value, err := load() is valid when value is new in the same scope. The statement declares value and assigns the new result to err; at least one name on the left must be new.

I normally use := for clear local initialization and var for package variables, deliberate zero values, or cases where an explicit type communicates intent. This is a readability and scope decision, not a runtime performance decision.

I also watch for accidental shadowing. Writing result, err := work() inside a nested block can create a new err that hides the outer one. When the value must survive outside that block, I declare it in the correct scope and use = for the update.