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    <h2>Use data and parameters, not existentials to get contexts</h2> <p>I think if you wanted</p> <pre><code>data Thing = Good [(Char,Int)] | Bad String | Indifferent Leg </code></pre> <p>but also sometimes</p> <pre><code>data Thing = Good [(Char,Float)] | Bad String | Indifferent Arm </code></pre> <p>You could define</p> <pre><code>data Thing num bodypart = Good [(Char,num)] | Bad String | Indifferent bodypart </code></pre> <p>or if you want to make sure <code>num</code> is always numeric, you could do</p> <pre><code>data Num num =&gt; Thing num bodypart = Good [(Char,num)] | Bad String | Indifferent bodypart </code></pre> <p>and finally, you could restrict the types allowed in <code>bodypart</code> by defining your own class:</p> <pre><code>class Body a where -- some useful function(s) here instance Body Leg where -- define useful function(s) on Legs instance Body Arm -- define useful function(s) on Arms data (Num num,Body bodypart) =&gt; Thing num bodypart = Good [(Char,num)] | Bad String | Indifferent bodypart </code></pre> <p>I'd like to discourage you from using existential types via the forall constructor or via GADTs because adding the <code>num</code> parameter to your data type is <em>considerably</em> more useful in practice, even though it takes more typing.</p> <h2>Constraints on type synonyms?</h2> <p>Note that when you use a constraint like </p> <pre><code>data (Num num) =&gt; Thing num = T [(Char,num)] </code></pre> <p>really only changes the type of the constructor <code>T</code> to </p> <pre><code>T :: (Num num) =&gt; [(Char,num)] -&gt; Thing num </code></pre> <p>instead of <code>T :: [(Char,num)] -&gt; Thing num</code>. This means every time you use <code>T</code>, there needs to be a context <code>(Num num)</code>, but that's really what you wanted - to stop people putting data in your data type that isn't numeric.</p> <p>A consequence of this fact is that you <em>can't</em> write</p> <pre><code>type Num num =&gt; [(Char,num)] </code></pre> <p>because there's no data constructor function <code>T</code> for the context <code>(Num num)</code> to be required on; if I have a [('4',False)], it automatically matches the type <code>[(Char,num)]</code> because it's a <em>synonym</em>. The compiler can't be going running around your code looking for instances before it decides what type something is. In the <code>data</code> case, it's got a constructor <code>T</code> that tells it the type, and it can guarantee there's a <code>Num num</code> instance because it checked your use of the function <code>T</code>. No <code>T</code>, no context.</p>
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    1. COThanks for a thorough but still possible to understand explanation!
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