[tor-dev] PRELIMINARY: [PATCH 3/3] Add the 'FlexibleInstances' extension.

[Nikita Karetnikov]

I’m attaching three patches.  For an explanation regarding the
‘FlexibleInstances’ extension see [1,2].

As always, I’ve only fixed critical things.  Deprecation warnings can
wait.

Perhaps, I’ll combine all similar patches (like those that deal with
exceptions) later.

I’ve noticed that the author decided to use ‘Dynamic’-related functions
to raise many (all?) exceptions.  If you raise an exception of type
‘Dynamic’, you won’t get a meaningful message.  Consider the following:

{-# LANGUAGE DeriveDataTypeable #-}

import qualified Control.Exception as E
import Data.Dynamic (toDyn)
import Data.Typeable (Typeable)

-- | An exception related to links or monitors.
data LinkException = NonexistentThread -- ^
deriving (Eq, Typeable)

instance Show LinkException where
show NonexistentThread = "Attempt to link to nonexistent thread"

test1 = E.throw . toDyn $ NonexistentThread

instance E.Exception LinkException where

test2 = E.throw NonexistentThread

In GHCi:

*Main> test1
*** Exception: <<LinkException>>
*Main> test2
*** Exception: Attempt to link to nonexistent thread

Note that the first argument of ‘E.throw’ must be an instance of
‘E.Exception’:

E.throw :: E.Exception e => e -> a

‘LinkException’ is not an instance of ‘E.Exception’ in
‘TorDNSEL.Control.Concurrent.Link.Internals’.  It should be easy to
change that.

Later, I’d also like to inspect ‘withLinksDo’, ‘linkTogether’, and
replace ‘$’ with ‘.’ in a couple of places.

The previous set of patches is here [3].

[1] http://www.haskell.org/haskellwiki/List_instance
[2] http://www.haskell.org/ghc/docs/6.8-latest/html/users_guide/type-class-extensions.html#instance-rules
[3] https://lists.torproject.org/pipermail/tor-dev/2013-July/005157.html

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