How to Create Your Own Type Constraints in Go

Create Go type constraints by defining an interface with a union of types or underlying types using the ~ prefix, then apply it to generic function parameters.

You create type constraints in Go by defining a type parameter with a ~ prefix for underlying types or a specific interface, then using that constraint in a generic function or type definition. This allows the compiler to enforce that only types matching the constraint (like all integers or all types with a String method) can be used.

// Define a constraint for all integer types
// The ~ prefix matches the underlying type
// This allows int, int32, int64, uint, etc.
type Integer interface {
    ~int | ~int8 | ~int16 | ~int32 | ~int64 | ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64
}

// Use the constraint in a generic function
func Sum[T Integer](values ...T) T {
    var total T
    for _, v := range values {
        total += v
    }
    return total
}

// Usage
result := Sum(1, 2, 3) // Works with int
result64 := Sum[int64](100, 200) // Works with int64

Type constraints act like a rulebook for generic functions, specifying exactly which types are allowed to be used. Instead of writing separate functions for integers, floats, and strings, you write one generic function that accepts any type matching your rule. It's like a vending machine that only accepts specific coins; if you try to use a bill, it won't work, ensuring your code only processes valid data types.