Quoting MarLinn
What about phantom types?
{-# LANGUAGE KindSignatures, DataKinds #-} data IsNormalized = Normalized | NotNormalized data Sum (n :: IsNormalized) = Value Int | Sum (Sum n) (Sum n)
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
That looks quite good tbh. I did not know you could do that. Care to explain, are "Normalized" and "NotNormalized" types, data constructors, type families? A combination of them? Of course you still have to write a fair amount of code, but it does look a lot better than what I've been using and what I've seen suggested so far. At least to me. Thanks, Juan. -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.