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    <p>I, er... I almost hate to suggest this, because doing this is kinda horrible, but... doesn't your code work as is?</p> <pre><code>{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} import Data.Maybe(fromJust) lk x = flip lookup x flipPair = uncurry $ flip (,) class (Eq a, Eq b) =&gt; Convert a b where mapping :: [(a, b)] mapping = error "No mapping defined" convert :: a -&gt; b convert = fromJust . lk mapping instance (Convert a b) =&gt; Convert b a where convert = fromJust . lk (map flipPair mapping) data XX = One | Two | Three deriving (Show, Eq) data YY = Eno | Owt | Eerht deriving (Show, Eq) instance Convert XX YY where mapping = [(One, Eno), (Two, Owt), (Three, Eerht)] main = do print $ (convert One :: YY) print $ (convert Owt :: XX) </code></pre> <p>And:</p> <pre><code>[1 of 1] Compiling Main ( GeneralConversion.hs, interpreted ) Ok, modules loaded: Main. *Main&gt; main Eno Two *Main&gt; </code></pre> <p>I'm not sure how useful such a type class is, and all the standard disclaimers about dubious extensions apply, but that much seems to work. Now, if you want to do anything <em>fancier</em>... like <code>Convert a a</code> or <code>(Convert a b, Convert b c) =&gt; Convert a c</code>... things might get awkward.</p> <hr> <p>I suppose I might as well leave a few thoughts about why I doubt the utility of this:</p> <ul> <li><p>In order to use the conversion, both types must be unambiguously known; likewise, the existence of a conversion depends on both types. This limits how useful the class can be for writing very generic code, compared to things such as <code>fromIntegral</code>.</p></li> <li><p>The use of <code>error</code> to handle missing conversions, combined with the above, means that any allegedly generic function using <code>convert</code> will be a seething pit of runtime errors just waiting to happen.</p></li> <li><p>To top it all off, the generic instance being used for the reversed mapping is in fact a <em>universal instance</em>, only being hidden by overlapped, more specific instances. That <code>(Convert a b)</code> in the context? That lets the reversed mapping work, but doesn't restrict it to only reversing instances that are specifically defined.</p></li> </ul>
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    1. POGeneral conversion type class
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    1. COFor simplicity's sake, you could fold all the `LANGUAGE` pragmas into a comma-separated list: `{-# LANGUAGE MultiParamTypeClasses, … #-}`.
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      1. USAntal Spector-Zabusky
    2. CO@Antal S-Z: Yeah, the separate lines thing is just a habit of mine. I use a fairly simpleminded editor for Haskell and adding/removing pragmas while refactoring code is easier if they're self-contained.
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    3. COThat's fair; I've never really had more than one or two in a project, so refactoring them hasn't been a major issue. I can see the advantage, though, now that I think about it.
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