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GALA-E0029 — Interface method redeclared

What it means. An interface declaration lists two method specs with the same name. GALA has no overloading, so the two specs are not distinct methods — the second simply replaces the first in the interface’s method table.

Code that triggers it

package main

type Repo interface {
    Find(id int) string
    Find(name string) string
}

func main() {
    Println("unused")
}

Compiler message

error[GALA-E0029]: method "Find" already declared in interface "Repo"
  --> main.gala:5:5
  |
5 |     Find(name string) string
  |     ^^^^ rename or remove the duplicate method
  |
  = hint: rename or remove the duplicate method

How to fix it

Name each method for what it does. Two lookups that differ by key are two methods:

package main

type Repo interface {
    FindByID(id int) string
    FindByName(name string) string
}

struct MemoryRepo(label string)

func (r MemoryRepo) FindByID(id int) string = s"${r.label}#$id"
func (r MemoryRepo) FindByName(name string) string = s"${r.label}:$name"

func main() {
    val repo = MemoryRepo("users")
    Println(repo.FindByID(1))
    Println(repo.FindByName("ada"))
}

If the two specs really are one operation over different input shapes, model the input as a sealed type and take a single parameter:

sealed type Key {
    case ByID(id int)
    case ByName(name string)
}

type Repo interface {
    Find(key Key) string
}

A type parameter on the method spec is not an option: Go requires interface methods to have no type parameters, so Find[K any](key K) string inside an interface produces Go that does not compile (interface method must have no type parameters). Put the type parameter on the interface itself instead:

type Repo[K any] interface {
    Find(key K) string
}

Why the rule exists

The earlier spec’s metadata was silently overwritten by the later one. That hid a real mistake — the author believed both signatures existed — and pushed the failure to the Go compiler, which reports it against generated code (duplicate method Find) with no pointer back to the GALA line. Rejecting in the analyzer names the interface, the method, and the offending line.

Scope. Method specs inside a single interface declaration. Duplicate implementations on a concrete type are GALA-E0012.

GALA-E0011 types · GALA-E0012 methods · GALA-E0027 functions · GALA-E0028 type aliases · GALA-E0029 interface method specs · GALA-E0030 struct fields · GALA-E0031 sealed cases