#13758: Deriving can't find an instance that holds, standalone deriving works -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by Iceland_jack: @@ -0,0 +1,2 @@ + This works: + @@ -70,2 +72,2 @@ - It even following proof of `Monoid' (Rep Urls)` and a dummy quasiquote - `pure []` separating it them, + ---- + '''Magically''' having `pure []` separate them, works: @@ -84,1 +86,1 @@ - deriving Monoid + deriving (Semigroup, Monoid) New description: This works: {{{#!hs {-# 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 {{{#!hs 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: {{{#!hs {-# Language ..., TemplateHaskell #-} ... doo :: Dict (Monoid' (Rep Urls)) doo = Dict pure [] newtype UrlsDeriv = UD (GenericMonoid Urls) deriving (Semigroup, Monoid) }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13758#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler