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POFundeps for constraint families
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  1. POFundeps for constraint families
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    copied!<p>A lot of constraints seem to come together. Let's abstract these away.</p> <pre><code>type MonadNumState a m = (MonadState a m, Num a) </code></pre> <p><code>MonadNumState</code> is just a constraint synonym, so I get the benefit of functional dependencies at every use, and can easily throw a <code>MonadNumState a m</code> into a context. Now, suppose I wish to abstract this into a constraint family:</p> <pre><code>class Iterator t where type MonadIter t a m :: Constraint next :: (MonadIter t a m) =&gt; m t ... instance Iterator Foo where type MonadIter Foo a m = (MonadState a m, Num a) ... instance Iterator Bar where type MonadIter Bar a m = (MonadRandom m, MonadSplit a m, RandomGen a) ... </code></pre> <p>But now <code>a</code> is not a functional dependency. <code>next</code> is virtually unusable since <code>a</code> cannot be inferred. What can I do? Well, I could, of course, use a type family instead. <code>MonadState</code> is written using fundeps, but it should be easy to convert the fundeps to type families.</p> <pre><code>instance (MonadState s m) =&gt; MonadStateFamily m where type St m = s get' = get ... instance (MonadStateFamily m) =&gt; MonadState (St m) m where get = get' ... </code></pre> <p>Guess not.</p> <pre><code>Foo.hs:25:3: The RHS of an associated type declaration mentions type variable `s' All such variables must be bound on the LHS </code></pre> <p>What else might I be able to do? What I really want is to existentially quantify away <code>s</code>. I've not found any way to do that without explicit dictionary passing.</p> <p>So, how do I get the benefit of fundeps for constraint families?</p>
 

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