[Haskell-beginners] Deserializing data with ProtocolBuffers

[Raphael Päbst]

Ah, thank you! Now it works.
Now I need only to get rid of the Right from the returned Either value
and I'm done.

Raphael

On 5/12/11, Daniel Fischer <daniel.is.fischer at googlemail.com> wrote:
> On Thursday 12 May 2011 18:21:45, Raphael Päbst wrote:
>> Hey!
>> I am currently working on a project, where I need to serialize data
>> with ProtocolBuffers.
>> I have managed to compile a .proto file and serialize messages with
>> messagePut. However, when I try to deserialize a ByteString again, I
>> get the following error:
>>
>> ambiguous type variable `msg0' in the constraints:
>> (Text.ProtocolBuffers.WireMessage.Wire msg0) arising from a use of
>> "messageGet" at...
>> (Text.ProtocolBuffers.Reflections.ReflectDescriptor msg0) arising from
>> a use of "messageGet" at ...
>> Probable fix: add a type signature that fixes these type variables in
>> the expression "messageGet bs"
>> in an equation for `IT':  "IT = messageGet bs"
>>
>> I'm trying to call messageGet from the prompt in ghc. What exactly am
>> I doing wrong? And how can I fix this?
>
> You have to tell it which type you want, so it can choose the correct
> messageGet.
>
> If you type
>
> ghci> read "True"
>
> it doesn't know either and you have to tell it (and of course the result
> would be different for result type Bool than for Integer).
>
> So you have to type
>
> ghci> messageGet bs :: WhateverWireYouWant
>
> to disambiguate the type.
>
> In programmes, there's rarely a need to provide such expression type
> signatures because the type can usually be inferred from the context.
> If there's not enough context to infer the type - especially if there's no
> context - you have to provide an explicit type.
>
>>
>> Many Thanks
>>
>> Raphael
>