Haskell Constraint is no smaller than the instance head

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hammar’s already provided a solution; I’d like to point out another problem with this example. What you want to express is “Whenever a type is an instance of Eq and EuclideanDomain, use this rule to make an instance for Ord.” But this is inexpressible in Haskell. The line

instance (EuclideanDomain a, Eq a) => Ord a where

actually means, “Use this rule to make an Ord instance for any type. It’s an error if instances of EuclideanDomain and Eq aren’t in scope”. That’s not good, because this rule will overlap with every other Ord instance.

Basically any time you want to write an instance Class typevar, you’re going to need a newtype.

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