Why Can\'t I Initialize a Golang `const` Variable with a Function Call?

Initializing a Const Variable in Golang

When defining constants in Golang, it is important to adhere to the language's strict rules to avoid compilation errors. One such error occurs when attempting to initialize a const variable with a function call.

Consider the following code:

const KILO = math.Pow10(3)

This code produces the error: "const initializer math.Pow10(3) is not a constant."

Reason for the Error:

The reason behind this error is that const variables must be evaluated at compile time, whereas function calls are executed at runtime. Therefore, functions cannot be part of constant declarations. This is to maintain the integrity and predictability of the program's behavior.

Solution:

To initialize a const variable with a fixed value, use a constant expression. A constant expression is an expression that can be fully evaluated at compile time, consisting of constants and predefined functions.

Here are some examples of valid constant expressions:

const Kilo = 1000  // Integer literal
const Kilo = 1e3   // Floating-point literal

Alternatively, if you truly need to calculate a value dynamically, it should be stored in a variable instead of a constant. For example:

var Kilo = math.Pow10(3)

Additional Notes:

There are a few built-in functions that can be used in constant declarations, such as unsafe.Sizeof(), len, and cap. However, these functions must still evaluate to a constant result.

For a comprehensive understanding of constants in Golang, refer to the official Go blog post titled "Constants."

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