Why Can\'t I Initialize a Go Constant with a Function Call?

How to Initialize a Const Variable

In Go, you can declare a constant variable using the const keyword. However, you may encounter an error when attempting to initialize a constant with a function call. This error occurs because constant declarations must be evaluated at compile time, while function calls are executed at runtime.

Constant Declaration Restrictions

According to the Go specification, constant declarations can include:

• Literals (rune, integer, floating-point, imaginary, string)
• Constant identifiers
• Constant expressions
• Conversions with results that are constants
• Results of certain built-in functions (e.g., unsafe.Sizeof(), cap() for some expressions)

Resolving the Initialization Error

To resolve the error, you cannot use a function call to initialize a const variable. Instead, use an integer or floating-point literal, such as:

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

Alternatively, if you require the result of a function call, you must declare it as a variable instead:

var Kilo = math.Pow10(3)

Conclusion

Constant declarations in Go have specific restrictions to ensure that they can be evaluated at compile time. By understanding these limitations and using the appropriate values or variables, you can effectively initialize constant variables in your code.

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