GALA-E0100 — Type does not implement the interface it is used as

When it fires. A value of a concrete type fills a place typed by an interface, and the type does not implement that interface under Go’s rules. The place can be an argument, a val with a declared type, a result, a struct field, a collection element or an assignment. The message names the first method that fails and why:

the type has no such method;
the method takes other parameter types than the interface declares;
the method returns another type than the interface declares;
the method has a pointer receiver, so only a pointer to the value has it.

Minimal repro.

package main

type Shape interface {
    Area() float64
    Scale(k float64) Shape
}

struct Square(side float64)

func (s Square) Area() float64 = s.side * s.side
func (s Square) Scale(k float64) Square = Square(s.side * k)

func total(s Shape) float64 = s.Area()

func main() {
    Println(total(Square(2.0)))
}

Error output.

error[GALA-E0100]: Square does not implement Shape: method Scale returns Square, not Shape
  --> main.gala:16:19
   |
16 |     Println(total(Square(2.0)))
   |                   ^^^^^^^^^^^ declare Scale with the interface's exact signature
   |
   = hint: declare Scale with the interface's exact signature

Fix. Declare the method with the exact signature the interface declares:

package main

type Shape interface {
    Area() float64
    Scale(k float64) Shape
}

struct Square(side float64)

func (s Square) Area() float64 = s.side * s.side
func (s Square) Scale(k float64) Shape = Square(s.side * k)

func total(s Shape) float64 = s.Area()

func main() {
    Println(total(Square(2.0)))
}

For a method on a pointer receiver, pass a pointer to a var (&counter), or declare the method on a value receiver.

Rationale. GALA interfaces are Go interfaces, and Go has no covariant result types: Scale(k float64) Square does not implement Scale(k float64) Shape, even though a Square is a Shape. The method’s signature must be identical to the interface’s. This used to pass every GALA check and then fail in go build with a “does not implement” error against the generated code.

GALA-E0100 — Type does not implement the interface it is used as

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