How to assign or return generic T that is constrained by union?

by

The error occurs because operations that involve a type parameter (including assignments and returns) must be valid for all types in its type set.
In case of string | int, there isn’t a common operation to initialize their value from a string.

However you still have a couple options:

Type-switch on T

You use the field with the generic type T in a type-switch, and temporarily set the values with concrete types into an interface{}/any. Then type-assert the interface back to T in order to return it. Beware that this assertion is unchecked, so it may panic if for some reason ret holds something that isn’t in the type set of T. Of course you can check it with comma-ok but it’s still a run-time assertion:

func (f *Field[T]) Get() (T, error) {
    value, ok := os.LookupEnv(f.name)
    if !ok {
        return f.defaultValue, nil
    }
    var ret any
    switch any(f.defaultValue).(type) {
    case string:
        ret = value

    case int:
        // don't actually ignore errors
        i, _ := strconv.ParseInt(value, 10, 64)
        ret = int(i)
    }
    return ret.(T), nil
}

Type-switch on *T

You can further simplify the code above and get rid of the empty interface. In this case you take the address of the T-type variable and switch on the pointer types. This is fully type-checked at compile time:

func (f *Field[T]) Get() (T, error) {
    value, ok := env[f.name]
    if !ok {
        return f.defaultValue, nil
    }

    var ret T
    switch p := any(&ret).(type) {
    case *string:
        *p = value

    case *int:
        i, _ := strconv.ParseInt(value, 10, 64)
        *p = int(i)
    }
    // ret has the zero value if no case matches
    return ret, nil
}

Note that in both cases you must convert the T value to an interface{}/any in order to use it in a type switch. You can’t type-switch directly on T.

Playground with map to simulate os.LookupEnv: https://go.dev/play/p/JVBEZwCXRMW

More Related Contents:

  1. Generic Structs with Go
  2. In Go generics, how to use a common method for types in a union constraint?
  3. generic map value
  4. How can I access a struct field with generics (type T has no field or method)?
  5. How to create generic method in Go? (method must have no type parameters)
  6. Can I construct a slice of a generic type with different type parameters?
  7. Go error: cannot use generic type without instantiation
  8. How to implement generic interfaces?
  9. How can I instantiate a non-nil pointer of type argument with generic Go?
  10. Go Generics – Unions
  11. How to write a generic function that accepts any numerical type?
  12. Difference between any/interface{} as constraint vs. type of argument?
  13. Is there a way to map an array of objects in golang?
  14. How to iterate over a union of slices passed in a generic function? (T has no core type)
  15. Why a generic can’t be assigned to another even if their type arguments can?
  16. Handle maps with same key type but different value type
  17. How to pipe several commands in Go?
  18. slice of struct != slice of interface it implements?
  19. How to dump goroutine stacktraces?
  20. Does Go compiler’s evaluation differ for constant expression and other expression
  21. Why do I get a “cannot assign” error when setting value to a struct as a value in a map? [duplicate]
  22. Organize local code in packages using Go modules
  23. Golang defer behavior
  24. Is unnamed arguments a thing in Go?
  25. How to fix parsing go.mod module declares its path as “x” but was required as “y”
  26. What is the meaning of “dot parenthesis” syntax? [duplicate]
  27. What is the idiomatic Go equivalent of C’s ternary operator?
  28. Does the Go language have function/method overloading?
  29. Go: panic: runtime error: invalid memory address or nil pointer dereference
  30. Build Docker Image From Go Code

Leave a Comment