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copied!<p>If you are planning to copy <code>Reifies</code> methods in a wrapper, then you could write something like this:</p> <pre><code>class R s a | s -> a where r :: s -> a -- etc. class R s a => RW s a where type RT s r' :: a ~ RT s => s -> RT s r' = r -- etc. instance R Bool String where r = show -- etc. instance RW Bool String where type RT Bool = String -- no need to write r' = r, etc. -- > :t r -- r :: R s a => s -> a -- > :t r' -- r' :: RW s (RT s) => s -> RT s -- > :t r True -- r True :: [Char] -- > :t r' True -- r' True :: RT Bool -- > r True -- "True" -- > r' True -- "True" </code></pre> <p>so that an association between <code>a</code> and <code>RT s</code> can be expressed as an <a href="http://www.haskell.org/haskellwiki/GHC/Type_families#Equality_constraints" rel="nofollow">equality constraint</a>.</p> <hr> <p><strong>UPDATE</strong>: for 1-parametric wrapper with type family over 2-parametric class with fundep:</p> <pre><code>class R s a | s -> a where r :: s -> a class R' s where type RT s a r' :: (R s a, a ~ RT s a) => s -> RT s a r' = r instance R Bool String where r = show instance R' Bool where type RT Bool a = a </code></pre> <p>so that:</p> <pre><code>> :t r r :: R s a => s -> a > :t r' r' :: (R' s, R s a, RT s a ~ a) => s -> RT s a > :t r True r True :: [Char] > :t r' True r' True :: RT Bool [Char] > r True "True" > r' True "True" </code></pre> <p>For example:</p> <pre><code>import qualified Data.Reflection as Reflection ( reflect ) import Data.Reflection hiding ( reflect ) class Reified s where type ReifiedType s a reflect :: (Reifies s a, a ~ ReifiedType s a) => proxy s -> ReifiedType s a reflect = Reflection.reflect </code></pre>

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