Why does this Show instance throw "Count not match a0 with type String" (Purescript)

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Why does this Show instance throw "Count not match a0 with type String" (Purescript)

Question

I'm working with this custom data type, and I would to be able to show it if the type of a is showable.

data NonEmpty a = NonEmpty a (Array a)

So I created this type class instance with the dependency that a be showable...

instance showNonEmpty :: (Show a) => Show (NonEmpty a) where
show (NonEmpty c d) = c

But when I try and run this, the compiler says "Could not match type a0 with type String." I'm not sure why this wouldn't work, because if a has an instance of Show and the first argument to NonEmpty (pattern matched to c) is of type a, then c should be a string...

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Answer

c's kind is Show, meaning that is can be shown with the show function. c being of kind Show should not be confused with saying that c is a string. It's not a string. In order for c to become a string, you have to pass it into the show function. So what you're looking for is...

instance showNonEmpty :: (Show a) => Show (NonEmpty a) where
show (NonEmpty c d) = show c

To further illustrate, here is an instance of Show that will show NonEmpty, and combine the values contained within it.

instance showNonEmpty :: (Monoid a, Show a) => Show (NonEmpty a) where
show (NonEmpty c d) = show $ append c (foldl (\e v -> append e v) mempty d)