[Haskell-cafe] Explicitly calling syntactic equality on datatypes

[Sylvain Henry]

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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