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GALA-E0078 — Invalid interface literal

When it fires. An interface literal, Repo(Find = (id) => ...), is invalid:

• an argument is positional, or names no method of the interface;
• a method is given twice, or left out: a literal gives every method of the interface exactly once;
• the interface is generic, and its type arguments are neither written (Box[int](...)) nor taken from an expected type;
• the interface has a method a function cannot implement: a generic method, or an unexported method of an interface from another package, which only that package can implement; or
• it is a Go interface that is generic, or has a method whose signature uses a type GALA cannot spell exactly: a channel, an anonymous struct, an inline interface or a generic type instance (context.Context’s Done).

Minimal repro.

package main

type Clock interface {
    Now() int
    Zone() string
}

func main() {
    val clock = Clock(Now = () => 12)
    Println(clock.Now())
}

Error output.

error[GALA-E0078]: interface literal of Clock leaves out method Zone
  --> main.gala:9:17
  |
9 |     val clock = Clock(Now = () => 12)
  |                 ^^^^^ give every method of Clock, as in `Zone = ...`
  |
  = hint: give every method of Clock, as in `Zone = ...`

Fix. Give every method, each by its name:

package main

type Clock interface {
    Now() int
    Zone() string
}

func main() {
    val clock = Clock(Now = () => 12, Zone = () => "UTC")
    Println(clock.Now(), clock.Zone())
}

For an interface a literal cannot build, declare a struct that implements it.

Rationale. A literal is a complete implementation of the interface: the compiler checks that it gives every method, so a value of the interface never has a method that does nothing. Naming each method keeps a literal readable and independent of the order the interface declares its methods in. See Interface Literals.