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POWriting A Function Polymorphic In A Type Family
 

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    <p>I was experimenting with type families yesterday and ran into an obstacle with the following code:</p> <pre><code> {-# LANGUAGE TypeFamilies #-} class C a where type A a myLength :: A a -&gt; Int instance C String where type A String = [String] myLength = length instance C Int where type A Int = [Int] myLength = length main = let a1 = [1,2,3] a2 = ["hello","world"] in print (myLength a1) &gt;&gt; print (myLength a2) </code></pre> <p>Here I have a type associated with class C and a function that calculates the length of the associated type. However the above code gives me this error:</p> <pre><code> /tmp/type-families.hs:18:30: Couldn't match type `A a1' with `[a]' In the first argument of `myLength', namely `a1' In the first argument of `print', namely `(myLength a1)' In the first argument of `(&gt;&gt;)', namely `print (myLength a1)' /tmp/type-families.hs:19:30: Couldn't match type `A a2' with `[[Char]]' In the first argument of `myLength', namely `a2' In the first argument of `print', namely `(myLength a2)' In the second argument of `(&gt;&gt;)', namely `print (myLength a2)' Failed, modules loaded: none. </code></pre> <p>If, however I change "type" to "data" the code compiles and works:</p> <pre><code> {-# LANGUAGE TypeFamilies #-} class C a where data A a myLength :: A a -&gt; Int instance C String where data A String = S [String] myLength (S a) = length a instance C Int where data A Int = I [Int] myLength (I a) = length a main = let a1 = I [1,2,3] a2 = S ["hello","world"] in print (myLength a1) &gt;&gt; print (myLength a2) </code></pre> <p>Why does "length" not work as expected in the first case? The lines "type A String ..." and "type A Int ..." specify that the type "A a" is a list so myLength should have the following types respectively : "myLength :: [String] -> Int" or "myLength :: [Int] -> Int".</p>
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    1. COIt looks like you may need a `{-# LANGUAGE TypeSynonymInstances -#}` in there too, as `String` is a type synonym for `[Char]`, and without the flag GHC expects instance heads to be built up of primitive type variables.
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