Understanding the "Cannot Use Variable of Type *T as Type Stringer in Argument" Error in Go Generics
In the code snippet provided, you're attempting to generically invoke a function do that expects a parameter of type stringer, but you're passing a pointer to a type parameter T instead.
The Go compiler is raising an error because:
-
Type Identity and Parameters: T is not inherently equivalent to its constraint FooBar. Therefore, *T is not the same as *FooBar or *bar.
-
Missing Method: The stringer interface requires a method named a(). Currently, your foo and bar types implement this method on pointer receivers, but *T does not inherently possess this method.
Fixing the Issue
There are two main approaches to resolving this issue:
1. Assert Type Safety with Type Assertion (Less Optimal)
- Assert that *T implements stringer using any(t).(stringer) within the do function.
- This approach may lead to panics if T does not actually implement stringer.
2. Redefine Constraint and Customize Generics (Preferred)
- Add stringer as a constraint to the FooBar interface.
- Use pointer receivers for the foo and bar types to align with the constraint.
- Introduce a second type parameter that specifies the constraint within the FooBar interface.
- Pass an instance of the constrained type as an argument to blah.
Revised Code:
type FooBar[T foo | bar] interface {
*T
stringer
}
func blah[T foo | bar, U FooBar[T]]() {
var t T
do(U(&t))
}
func main() {
blah[foo]()
}
Copy after login
In this revised code:
- The FooBar interface includes stringer as a constraint.
- The foo and bar types have pointer receivers for their a() methods.
- U is constrained by FooBar[T], ensuring that it satisfies both FooBar and stringer constraints.
- blah takes an argument of type U, which is an instance of the constrained FooBar interface.
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