Constants and iota in Go

In Go, a constant is a value the compiler can determine and that the program cannot change at runtime. Constants can be strings, booleans, or numeric values. They are useful for names such as MaxRetries, Pi, or a fixed status value because the name explains the meaning and accidental reassignment is impossible.

A constant may be typed, such as const Timeout time.Duration = 5 * time.Second, or untyped, such as const attempts = 3. An untyped numeric constant has more flexibility because Go gives it a concrete type only when the surrounding expression requires one, provided the value can be represented by that type.

iota is a counter available inside a const declaration block. It starts at zero for the first constant specification and increases by one for each following specification. It resets when a new const block begins; it is not a global counter and it does not keep increasing across the package.

For example, type Level int; const (Low Level = iota; Medium; High) produces Low = 0, Medium = 1, and High = 2. The omitted expressions repeat the previous expression, so Medium and High reuse Level = iota with the new value of iota.

This pattern is convenient for small internal enum-like sets and for bit flags such as Read = 1 << iota, Write, and Execute. Giving the constants their own type also prevents unrelated integer values from being passed accidentally, although Go still does not provide a fully closed enum type.

The main risk is treating generated numbers as permanent business data. Inserting a new line in the middle of an iota block changes the values below it, which can break stored database values, API contracts, or file formats. For externally visible values, I normally assign explicit numbers instead.

My practical rule is to use ordinary constants for fixed named values, use iota when the sequence itself is the useful pattern, and use explicit values whenever numeric stability matters outside the program.