On Tue, 4 Apr 2017 02:40:31 -0700 (PDT) Paul Jolly <p...@myitcv.org.uk> wrote:
> *I couldn't find anything previously on the question below, but as > ever would appreciate any pointers to prior questions/issues etc.* > > Consider the following (playground > <https://play.golang.org/p/MQuEfeG5c9>): > > ==== > package main > > import ( > "fmt" > ) > > type S struct { > f func() > } > > func (s S) Impl() {} > > var _ I = S{} > > type I interface { > Impl() > } > > func main() { > m := make(map[I]bool) > m[S{}] = true > fmt.Println(m) > } > ==== > > Per [the spec](https://golang.org/ref/spec#Map_types): > > *If the key type is an interface type, these comparison operators > must be > > defined for the dynamic key values; failure will cause a run-time > > panic.* > > > this program compiles but then fails with a runtime panic. > > However, in this instance, at compile time we know by type-checking > alone the type of the value used to index m (i.e. S{}) and that that > type is not comparable... and so can conclude that if this line > executes it will panic. > > From a spec perspective, is this a case of keeping the implementation > and documentation simple, by treating all cases for interface-keyed > maps the same, i.e. runtime panic? > > Or is there some other reason why this case isn't caught by the > compiler/go vet? In the sequence m := make(map[I]bool) m[S{}] = true the map "m" stores values of type I; that's all it considers when you pass values as keys when performing operations on it. When you insert a value keyed by S{}, the compiler creates a value of type I which has S as its dynamic type and S{} as its value. The program as speficied panics with panic: runtime error: hash of unhashable type main.S because instances of S are indeed not comparable (because S.f is a function type) but you could just as fine do type X struct { x int } func (x X) Impl() {} m[X{}] = true and it would work just fine because values of X are comparable, and still implement I. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
