On Thursday, September 29, 2016 at 8:46:14 PM UTC+8, Konstantin Khomoutov
wrote:
>
> On Thu, 29 Sep 2016 00:25:23 -0700 (PDT)
> T L <tapi...@gmail.com <javascript:>> wrote:
>
> > > > I just want to understand what is the deep reason for the syntax
> > > > inconsistency between map index and type assert.
> > >
> > > a map is fully typed at compile time. even if its key is of type
> > > interface{} the language defines how to compare two interface{}
> > > values using type assertion here:
> > > https://golang.org/ref/spec#Comparison_operators
> > >
> > > "Interface values are comparable. Two interface values are equal if
> > > they have identical dynamic types and equal dynamic values or if
> > > both have value nil."
> > >
> > > Using interface{} in assignments on the other hand is only
> > > type-safe at runtime when used with a type assertion.
> > >
> >
> > Aha, it would be more understandable if you can explain it with code.
> > :), my English is not very good.
>
> "One argument lookup" of a map containing values of type interface{}
> produces the zero value for type interface{}. That's because as Andrey
> said, such map _itself_ is statically typed: the type of its values is
> interface{}.
>
> Conversely, when you type-assert a value of type interface{}, you're
> making a claim the _real dynamic run-time_ type of the value held in
> that value of type interface{} is such and such.
>
> Thus even though both operations superficially look the same, they're
> doing drastically different things, as Andrey and Dave tried to explain.
>
> Here's the code:
>
> package main
>
> import (
> "fmt"
> )
>
> func main() {
> var zvi interface{} // The zero value for type interface{}
>
> m := make(map[string]interface{})
>
> v := m["abc"] // Returns the zero value for type interface{}
> fmt.Println(v == zvi) // Prints "true"
>
> v = zvi.(int64) // panics beause zvi contains no dynamic type
> // information.
> }
>
I'm still not convinced by your explanation.
I never expect the last line should not panic. Type asserting on a nil
interface value is expected to panic (it is a surprise that nil map element
access doesn't panic).
I just expect type asserting on a non-nil interface value shouldn't panic.
>
> Playground link: https://play.golang.org/p/oS7SWHXgJQ
>
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