[Haskell-cafe] Is this the "right" way to do polymorphism?

[Luke Palmer]

If you always expect to be passing c as a parameter and never
returned, it is probably better off as a data type.  Eg. HTTPClient
might look like a traditional OO class:

    class HTTPClient c where
        foo :: c -> stuff
        bar :: c -> stuff

I've found that it is easier to work with if you use a data type instead:

    data HTTPClient = HTTPClient {
        foo :: stuff,
        bar :: stuff
    }

But other than that, yeah that's about right.  If you want a function
to be polymorphic in something, take the something as a parameter.
Simple as that.

Luke

On Fri, Jan 7, 2011 at 8:01 PM, Daryoush Mehrtash <dmehrtash at gmail.com> wrote:
> I am trying to evaluate the polymorphism technique used in Hackage library.
> I like to know if the approach taken is "right" or not.
>
> The code in question is in the Hoaut package
> http://hackage.haskell.org/package/hoauth/
>
> As far as I can understand the code wants to have polymorphism on HTTP
> client such that it can use different underlying HTTP.  The package comes
> with Curl and Debug implementation of its HTTPClient class.
>
> My question is with how the polymorphism is used.  In function such as
> "serviceRequest" in the Network.OAuth.Consumer module you have:
>
>
> -- | Performs a signed request with the available token.
> serviceRequest :: (HttpClient c,MonadIO m) => c -> OAuthRequest ->
> OAuthMonadT m Response
>
> serviceRequest c req = do { result <- lift $ runClient c (unpackRq req)
>
>                           ; case (result)
>
>                             of Right rsp -> return rsp
>
>                                Left err  -> fail $ "Failure performing the
> request. [reason=" ++ err ++"]"
>
>                           }
>
>
>
> The function expects the HTTPClient implementation to be the first argument
> to the function.  Is that the right thing to do?   Or should
> the instance of the client be a type parameter in the computation (in this
> case OAuthMonadT)?
>
> thanks,
>
> Daryoush
>
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