[Haskell-cafe] Explicitly calling syntactic equality on datatypes

MarLinn monkleyon at gmail.com
Wed Sep 18 12:53:32 UTC 2019

What about phantom types?

	{-# LANGUAGE KindSignatures, DataKinds #-}
	data IsNormalized = Normalized | NotNormalized
	data Sum (n :: IsNormalized) = Value Int | Sum (Sum n) (Sum n)

You could still say

	instance MyClass (Sum n) where…

but you could also write

	normalizeSum :: Sum NotNormalized -> Sum Normalized -- or even create a class with two inhabitants for this
	instance Eq (Sum NotNormalized) where (==) = (==) `on` normalizeSum
	instance Eq (Sum Normalized) where … -- real implementation

It's not ideal. For example if you want to sort a list, it would still 
be better to normalize the whole list before sorting, but at least you 
could still use the other operators afterwards without a 
"denormalization" step. So maybe it would help reduce some boilerplate?


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