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haskellnewtypederiving

Haskell instance read for newtype that is just an int

I am new to Haskell and I want to be able to a newtype so I can tell what is what, but I also have to read it from a string. I have 

newtype SpecialId Int
    deriving (Eq, Ord, Show)

I want to be able to read "5" :: SpecialId if I derive Read in the newtype it doesn't work it only works on read "SpecialId 5" :: SpecialId. I have tried

instance Read SpecialId where
    readsPrec _ s = read s

But this gives me 

SpecialId *** Exception: Prelude.read: no parse
Solution

  • This is possible since GHC 8.2 using -XDerivingStrategies:

    {-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE DerivingStrategies         #-}

newtype SpecialId = SpecialId Int
    deriving stock   (Eq, Ord, Show)
    deriving newtype Read

    In ghci:

    ghci> read "5" :: SpecialId 
SpecialId 5