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POUsing constraint kinds and type families with 'limited' constraints
 

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    <p>I'm working on an applicative functor that contains a monoid to "view" the execution. However, sometimes I don't care about this part at all, so the choice of monoid is irrelevant as it will never be consumed. I've simplified what I have into:</p> <pre><code>{-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} import GHC.Exts class Render a b where render :: a -&gt; b instance Render a () where render = const () class Merge a where type Renderer a b :: Constraint merge :: Renderer a b =&gt; a -&gt; b data Foo = Foo Bool instance Merge Foo where type (Renderer Foo) m = (Render Bool m) merge (Foo b) = render b </code></pre> <p><code>Render</code> is used to transform various <code>a</code>s into a single <code>b</code>. <code>Merge</code> is a big simplification of my actual functor, but the point is it contains a type family/constraint and my intention of that is to specify exactly what <code>Render</code>ers a <code>Merge</code> requires.</p> <p>Now, I might want to "run" the <code>Merge</code>, but discard the view, which is akin to something like:</p> <pre><code>runFoo :: Merge a =&gt; a -&gt; Int runFoo x = case merge x of () -&gt; 5 </code></pre> <p>But this will fail because:</p> <blockquote> <p>Could not deduce <code>(Renderer a ())</code> arising from a use of <code>merge</code></p> </blockquote> <p>I chose <code>()</code> as my monoid because forall <code>a</code>, we have an instance of <code>Render a ()</code>. So if there was a way to say that <code>Merge a</code> just means a collection <code>Render</code> constraints then this would work fine. Of course, <code>Merge a</code> is more general than that - it could add arbitrary constraints, which explains the compilation error.</p> <p>Is there anyway to achieve what I want <em>without</em> changing the signature of <code>runFoo</code>?</p>
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    1. CODoes `Renderer` always include exactly one `Render`?
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    2. CO@Tinctorius - no, generally depends on the amount of distinct types in the fields of `Foo`
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    3. COIs there a reason why `Merge` doesn't have `b` as a parameter as well?
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