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    <p>I like the record solution, but it is easy to do with typeclasses</p> <pre><code>{-# LANGUAGE ViewPatterns, ScopedTypeVariables #-} import qualified Data.Set as Set data Color = W' | U' | B' | R' | G' deriving (Show, Eq, Ord) data Card = Card (Set.Set Color) newtype W a = W a newtype U a = U a newtype B a = B a newtype R a = R a newtype G a = G a class ToColors x where toColors :: x -&gt; [Color] reify :: x instance ToColors () where toColors _ = [] reify = () instance ToColors a =&gt; ToColors (W a) where toColors (W a) = W':toColors a reify = W reify --other instances members :: Set.Set Color -&gt; [Color] -&gt; Bool members s = foldl (\b e -&gt; b &amp;&amp; (Set.member e s)) True viewColors :: forall a. ToColors a =&gt; Card -&gt; Maybe a viewColors (Card s) = let a = reify :: a in if members s (toColors a) then (Just a) else Nothing foo :: Card -&gt; String foo (viewColors -&gt; Just (W (B ()))) = "card is white and black" foo _ = "whatever" </code></pre> <p>this could easily be reworked to get other syntaxes. Like, you could define the colors to be types that don't take parameters, and then use an infix heterogeneous list constructor. Either way it does not care about order.</p> <p>Edit: if you want to match exact sets that is easy also--just replace the <code>members</code> function like so</p> <pre><code>viewColors :: forall a. ToColors a =&gt; Card -&gt; Maybe a viewColors (Card s) = let a = reify :: a in if s == (Set.fromList . toColors $ a) then (Just a) else Nothing </code></pre>
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    1. COAwesome, this satisfies my wants exactly. That said, ScopedTypeVariables is completely unneeded to get this to compile ;) (Well at least if monomorphism restriction is not disabled.)
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      1. USThomas Eding
    2. CO@ThomasEding. I should have thought of that. You can also get it to typecheck even with `NoMonomorphismRestriction` enabled if you use abstraction instead of `let` to avoid generalization `viewColors (Card s) = flip ($) reify (\a -> if s == (Set.fromList . toColors $ a) then (Just a) else Nothing)` has the infered type `viewColors :: ToColors a => Card -> Maybe a` which is exactly what you want
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    3. CO@ThomasEding This solution has same power as "record solution". `Just (W (B ())))` is both `[W',B',U']` and `[W',B']`, but not `[W']`
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