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Deriving can't find an instance that holds, standalone deriving works

    • Closed Issue created by Icelandjack @Icelandjack

    This works:

    {-# Language ScopedTypeVariables, GeneralizedNewtypeDeriving, DeriveGeneric, UndecidableInstances, StandaloneDeriving, FlexibleContexts #-}

import GHC.Generics
-- https://hackage.haskell.org/package/generic-deriving-1.11.2/docs/Generics-Deriving-Monoid.html
import Generics.Deriving.Monoid hiding (GMonoid)
import Data.Coerce
import Data.Constraint

import Data.Semigroup
newtype GenericMonoid a = GenericMonoid a

instance (Generic a, Monoid' (Rep a)) => Semigroup (GenericMonoid a) where
  (<>) = coerce (mappenddefault :: a -> a -> a)

instance (Generic a, Monoid' (Rep a)) => Monoid (GenericMonoid a) where
  mempty  = coerce (memptydefault  :: a)
  mappend = coerce (mappenddefault :: a -> a -> a)

data Urls = Urls String String String
  deriving (Show, Generic)

newtype UrlsDeriv = UD (GenericMonoid Urls)

deriving instance Semigroup UrlsDeriv
deriving instance Monoid    UrlsDeriv

    but changing that to

    newtype UrlsDeriv = UD (GenericMonoid Urls)
  deriving (Semigroup, Monoid)

    fails

    $ ghci -ignore-dot-ghci tWqD.hs
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( tWqD.hs, interpreted )
Ok, modules loaded: Main.
*Main> :r
[1 of 1] Compiling Main             ( tWqD.hs, interpreted )

tWqD.hs:26:13: error:
    • No instance for (Monoid' (Rep Urls))
        arising from the 'deriving' clause of a data type declaration
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    • When deriving the instance for (Semigroup UrlsDeriv)

tWqD.hs:26:24: error:
    • No instance for (Monoid' (Rep Urls))
        arising from the 'deriving' clause of a data type declaration
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    • When deriving the instance for (Monoid UrlsDeriv)
Failed, modules loaded: none.

    This feels familiar but I couldn't quickly. I can't recall if this behavior is intended so I'm filing a ticket just in case.

    Magically having pure [] separate them, works:

    {-# Language ..., TemplateHaskell #-}

...

doo :: Dict (Monoid' (Rep Urls))
doo = Dict

pure []

newtype UrlsDeriv = UD (GenericMonoid Urls)
  deriving (Semigroup, Monoid)
    Edited
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