Re: [Haskell-cafe] Is this the right way to do polymorphism?
On Fri, 7 Jan 2011, Daryoush Mehrtash wrote:
-- | 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)
I do not see, why different download methods need distinct types at all
(the instances of HttpClient). I would certainly just pass a dictionary of
access methods around.
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[Haskell-cafe] Is this the right way to do polymorphism?
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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Re: [Haskell-cafe] Is this the right way to do polymorphism?
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 dmehrt...@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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