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POHaskell: type classes question
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  1. POHaskell: type classes question
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    copied!<p>I wish to define the following typeclass <code>Mapping</code>:</p> <pre><code>{-# LANGUAGE MultiParamTypeClasses #-} class Mapping k v m where empty :: m v insert :: k -&gt; v -&gt; m v -&gt; m v search :: k -&gt; m v -&gt; Maybe v delete :: k -&gt; m v -&gt; m v </code></pre> <p>One instance of <code>Mapping</code> is <code>Data.Map.Map</code></p> <pre><code>{-# LANGUAGE ..., FlexibleInstances #-} instance Ord k =&gt; Mapping k v (Map.Map k) where empty = Map.empty search = Map.lookup insert = Map.insert delete = Map.delete </code></pre> <p>And now I want to create a type <code>Trie :: * -&gt; * -&gt; * -&gt; *</code> such as</p> <pre><code>{-# LANGUAGE ..., UndecidableInstances #-} data Trie m k v = Trie { trValue :: Maybe v, trChildren :: m (Trie m k v) } instance Mapping k (Trie m k v) m =&gt; Mapping [k] v (Trie m k) where search [] tree = trValue tree search (x:xs) tree = search xs =&lt;&lt; search x (trChildren tree) </code></pre> <p>So far so good, now I also want to define <code>Trie</code>'s <code>insert</code> and <code>empty</code>, and that's where I get into problems.</p> <p>I will discuss <code>empty</code> because it's simpler and <code>insert</code> needs it anyhow.. If I try this:</p> <pre><code>instance Mapping k (Trie m k v) m =&gt; Mapping [k] v (Trie m k) where empty = Trie { trValue = Nothing, trChildren = empty } ... </code></pre> <p>and that makes me get the following error:</p> <pre><code>Could not deduce (Mapping k (Trie m k1 v) (m k1)) from the context (Mapping [k1] v (Trie m k1), Mapping k1 (Trie m k1 v) (m k1)) arising from a use of `empty' at test.hs:27:49-53 Possible fix: add (Mapping k (Trie m k1 v) (m k1)) to the context of the instance declaration or add an instance declaration for (Mapping k (Trie m k1 v) (m k1)) In the `trChildren' field of a record In the expression: Trie {trValue = Nothing, trChildren = empty} In the definition of `empty': empty = Trie {trValue = Nothing, trChildren = empty} </code></pre> <p>I've tried and tried to solve it but failed.</p> <p>Does anyone know how to make it work? Is it even possible?</p>
 

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