[go-nuts] Why I can enc/decode []interface{} directly but not interface{}

This call
   enc.Encode(vlToWrite[0])
passes a value of static type interface{} to a function that expects
static type interface{}, which it then unpacks to see what's inside.
The result is _exactly_ the same as calling
    enc.Encode(1)
which "boxes" the 1 into an interface value. Hence you didn't encode
an interface value, you encoded an integer. The behavior is, in fact,
exactly the same as how Printf works. If that is unclear to you, read
this blog post:

   http://blog.golang.org/laws-of-reflection

If you don't want to understand and just want your code to work, put
an & before vlToWrite[0], as in http://play.golang.org/p/nPzVnjG-n6.
That will make sure Encode receives an actual interface value (though
a pointer) rather than an integer.

-rob

Thanks Rod! That's a good workaround.

But is it better dec.Decode can create an instance of the concrete type and
set it to the interface{} variable? It seems it has all the information it
needs.

David

No it does not have all the information it needs. Encode/Decode have
no idea that it was called with a value of type interface{} rather
than int. Again, please read the blog post I cited.

-rob

The Encode doesn't know but the Encode does.

The error message shows Encode receives a *interface{} and it knows the
data to be decoded is a concrete type int. What else information does it
need?

The Encode doesn't know but the Decode does.

The error message shows Decode receives a *interface{} and it knows the
data to be decoded is a concrete type int. What else information does it
need?

David

It's a type mismatch. Yes, it could automatically transcode under some
circumstances but that way leads to madness.

Instead of having the package intuit your meaning, it's better to send
what you expect to receive.

-rob