Deriving instance Typeable with context

Question

I am writing function set wor working with HTTP requests and need to create a set of Exceptions for handling failures. Here it is

data HStream ty => ErrorResponse ty = ErrorResponse (Response ty)
data HStream ty => HttpException ty = WrongURIException String | ConnException ConnError | RequestException (ErrorResponse ty)
instance HStream ty => Exception (HttpException ty)

where WrongURIException corresponds to malformed uri, ConnException to errors in TCP stack and RequestException to handle responses with non-2xx response codes. Before declaring instance Exception on line 3 I should derive Typeable but I'm lost in types. How should I it?

daniel gratzer · Accepted Answer · 2014-04-16T11:37:37.877

Might I suggest not doing that. Datatype contexts are bad in pretty much every way possible. There's a reason that they've been deprecated. If you really, really, want them, use GADTs.

If you don't use contexts, this is trivial

 {-# LANGUAGE DeriveDataTypeable #-}
 import Data.Typeable
 import Data.Data


 data ErrorResponse ty = ErrorResponse (Response ty)
                       deriving(Data, Typeable, Show)
 data HttpResponse ty = WrongURIException String 
                      | ConnException ConnError
                      | RequestException (ErrorResponse ty)
                       deriving(Data, Typeable, Show)
 instance (Typeable ty, Show ty) => Exception (HttpException ty)

In particular as of GHC 7.8 you can't make your own instances for Typeable and Data, so deriving is the correct way.

Understood. But how should attach non-2xx responses to exceptions to leave compiler decide which type of response body should be? — Andrey Kuznetsov, Apr 17 '14 at 07:38

Deriving instance Typeable with context

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