Declaring variables — var, :=, and constants with iota

var is the general-purpose form and works everywhere — inside a function, at package level, with or without an initial value. The short variable declaration (:=) is shorthand available only inside a function body — using := at package level is a compile error, which is the single most common thing that trips someone up moving from var to := for the first time.

:= isn't a different feature so much as a shorter way to write the extremely common case of "declare a variable and immediately give it a value, letting Go infer the type" — which is most of what local variable declarations actually look like in real code. var stays necessary for package-level declarations, and for the less common case of declaring a variable with no initial value (getting its zero value instead, as covered in the types lesson).

const declares a value that can never change after compilation — unlike var, a constant's value must be knowable at compile time, not computed at runtime:

For a sequence of related constants, Go has a specific shorthand: iota, which starts at 0 inside a const block and increments by one for each subsequent line:

This looks unusual coming from languages with explicit enums, but it's solving the same problem: naming a small, fixed set of related integer values without manually writing out 0, 1, 2, 3 and risking a typo if the list ever gets reordered.

Seeing := fail to compile at package level, or seeing iota in someone else's Go code for the first time, are both extremely common early moments — recognizing both immediately as intentional, idiomatic Go (not a bug or unusual code) is exactly the kind of small friction this lesson exists to remove before it costs real debugging time.