Json marshal - unsupported type: map[int]string

[bsr]

Hello,

When I marshal the below struct (in fact, part/field of another struct), I get error

json: unsupported type: map[int]string

type Header struct {

M map[int]string

}

But the below works.

type Header struct {

M string

}

I thought JSON can Marshal any exported type.

thanks

[chris dollin]

On 2 July 2012 15:14, bsr <bsr...@gmail.com> wrote:
>
> Hello,
> When I marshal the below struct (in fact, part/field of another struct), I
> get error
>
> json: unsupported type: map[int]string

JSON objects only permit string key: http://www.json.org/

Chris

--
Chris "allusive" Dollin

[DisposaBoy]

Those two structs are entirely different. You get an error on the former because JSON doesn't support integer keys, only strings. see: http://json.org/

 

[bsr]

It is amazing how quickly I got the response from you guys! Fell like SO .. thanks.

I was using map[TMessageType]string

where TMessageType is 

type TMessageType int

const (

_ TMessageType = iota

ALERT

ERROR

SUCCESS

INFO

)

so, I may have to define 4 string constants instead. Thanks.

[chris dollin]

On 2 July 2012 15:33, bsr <bsr...@gmail.com> wrote:
> It is amazing how quickly I got the response from you guys! Fell like SO ..
> thanks.
>
> I was using map[TMessageType]string
>
> where TMessageType is
> type TMessageType int
>
>
> const (
> _ TMessageType = iota
> ALERT
> ERROR
> SUCCESS
> INFO
> )
>
> so, I may have to define 4 string constants instead. Thanks.

Or

type MyMap map[TMessageType]string

and make that type satisfy the json.Marshaler interface.

[bsr]

thanks Chris.

What is the best way to have a set of string constants. This way, I can enforce keys as one of the members of the group. I was thinking something line

const TMessageType = [string]{"ALERT", "ERROR", "SUCCESS", "INFO"}

or

const TMessageType = [4]string{"ALERT", "ERROR", "SUCCESS", "INFO"}

gives error. I am missing on the syntax or slice can't be const?.. thanks

[bsr]

Never mind. I saw a response from Ian about this.

Being precise, passing an array to a function copies the array anyhow. 
Your argument is a reason to not have const slices.  That said, there 
are very similar arguments against const arrays: what happens if you 
take the address?  What happens if you make a slice? 

Spec also says the same.

I guess I may just make it as var instead of const.

[chris dollin]

On 2 July 2012 15:57, bsr <bsr...@gmail.com> wrote:
>
> What is the best way to have a set of string constants. This way, I can
> enforce keys as one of the members of the group. I was thinking something
> line
>
> const TMessageType = [string]{"ALERT", "ERROR", "SUCCESS", "INFO"}

Bad syntax: []string, not [string]. But ...

> or
> const TMessageType = [4]string{"ALERT", "ERROR", "SUCCESS", "INFO"}
> gives error. I am missing on the syntax or slice can't be const?.. thanks

... consts can only be numbers, booleans, and strings, not arrays or slices.

You can name the individual strings.

[bsr]

Sorry for keep coming back to the same subject. 

So how would you enforce a condition, which detects whether a function parameter (or a type) can only have a set of values statically (at compile time.

)

Please see http://play.golang.org/p/Ur75sUvq8b

Only way I could think about defining a struct with unexported field, and having a constructor which validates the constraint. Is there an easier (as the validation method can have ugly  if/else conditions)

thanks.

[DisposaBoy]

On Monday, July 2, 2012 4:28:20 PM UTC+1, bsr wrote:

Sorry for keep coming back to the same subject. 

So how would you enforce a condition, which detects whether a function parameter (or a type) can only have a set of values statically (at compile time.

)

Please see http://play.golang.org/p/Ur75sUvq8b

Only way I could think about defining a struct with unexported field, and having a constructor which validates the constraint. Is there an easier (as the validation method can have ugly  if/else conditions)

thanks.

one simple way is to use a type somewhat like you were doing before but instead of using an int, just use a string as the underlying type

http://play.golang.org/p/YA_7T1QSUT

 

[bsr]

But this works though 

http://play.golang.org/p/tlqO3qwjtt

so the allowed values are not enforced, being the key is in effect a string.

[Xing Xing]

于 Mon, 2 Jul 2012 09:20:22 -0700 (PDT)
bsr <bsr...@gmail.com> 写道:

> But this works though
> http://play.golang.org/p/tlqO3qwjtt
>
> so the allowed values are not enforced, being the key is in effect a
> string.
>

I've committed a patch for this: http://codereview.appspot.com/6338067/
It works fine. But When I was thinking about that, I found some issues
with this patch:

1. The json.Marshal could treat the map[int]T, how about the
json.Unmarshal?

2. json.Marshal treat the map[int]T into json form "{string:T}".
Strange, isn't it?

3. If it count treat a numeric keys of map, shall we treat it in a more
general way? Eg. type of map's key can convert into string, or it has a
method String().