Nice! Moreover the "real implementation" can use standalone deriving: ``` {-# LANGUAGE DataKinds #-}{-# LANGUAGE KindSignatures #-}{-# LANGUAGE StandaloneDeriving #-}{-# LANGUAGE FlexibleInstances #-}import Data.Functiondata IsNormalized = Normalized | NotNormalizeddata Sum (e :: IsNormalized) = Value Int | Sum (Sum e) (Sum e) deriving (Show)normalize :: Sum a -> Sum Normalizednormalize (Value i) = Value inormalize (Sum (Value i) b) = Sum (Value i) (normalize b)normalize (Sum (Sum x y) b) = normalize (Sum x (Sum y b))instance Eq (Sum NotNormalized) where (==) = (==) `on` normalizederiving instance Eq (Sum Normalized)sample1 :: Sum NotNormalizedsample1 = Value 10 `Sum` (Value 11 `Sum` Value 12)sample2 :: Sum NotNormalizedsample2 = (Value 10 `Sum` Value 11) `Sum` Value 12test = sample1 == sample2-- True ``` On 18/09/2019 14:53, MarLinn wrote:
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?
Cheers, MarLinn
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