[Haskell-cafe] <<loop>> as result of SemigroupMonoid changes

[Joey Hess]

I tried to follow the instructions at 
https://prime.haskell.org/wiki/Libraries/Proposals/SemigroupMonoid
and I got a <<loop>> when my code runs with ghc 8.2.2.

My code looked like this:

import Data.Monoid
import Prelude

data InfoVal v = NoInfoVal | InfoVal v
	deriving (Show)

instance Monoid (InfoVal v) where
	mempty = NoInfoVal
	mappend _ v@(InfoVal _) = v
	mappend v NoInfoVal = v

Following the "recommended variant" for compatible code, I changed that
too:

{-# LANGUAGE CPP #-}

import Data.Monoid
import qualified Data.Semigroup as Sem
import Prelude

data InfoVal v = NoInfoVal | InfoVal v
	deriving (Show)

instance Monoid (InfoVal v) where
	mempty = NoInfoVal
#if !(MIN_VERSION_base(4,11,0))
	mappend = (<>)
#endif

instance Sem.Semigroup (InfoVal v) where
	_ <> v@(InfoVal _) = v
	v <> NoInfoVal = v

This loops because <> comes from Data.Monoid not from Data.Semigroup, 
so mappend = mappend.

Note that I diverged slightly from the instructions to get to this wrong
code; I imported Data.Semigroup qualified. Without the qualification,
the above code fails to compile, with Ambiguous occurrence ‘<>’

So, the transition instructions don't produce code that ghc 8.2.2 can build, 
and when the obvious fix is made to get it to compile, it compiles into a
loop, that is *not* a loop when compiled with newer versions of ghc.

-- 
see shy jo
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