fix broken SemigroupMonoid transition <<loop>>

Turns out that with ghc 8.2.2, the instructions given on the page don't
work. And the cppless variant that I had compiles, but into effectively
mappend = mappend so it loops.

The only way I can see to make it work without cpp is to use
mappend = (Sem.<>)
which is ugly and a land mine waiting to explode if someone changes it
to a nicer mappend = (<>) with a newer version of ghc which will compile
it and work ok, while breaking it with 8.2.2. Sigh.

I posted to haskell-cafe about this.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/Propellor/Property/Ssh.hs b/src/Propellor/Property/Ssh.hs
index 05098983..c23a121b 100644
--- a/src/Propellor/Property/Ssh.hs
+++ b/src/Propellor/Property/Ssh.hs
@@ -238,7 +238,7 @@ instance Sem.Semigroup HostKeyInfo where
 
 instance Monoid HostKeyInfo where
 	mempty = HostKeyInfo M.empty
-	mappend = (<>)
+	mappend = (Sem.<>)
 
 userPubKeys :: User -> [(SshKeyType, PubKeyText)] -> Property (HasInfo + UnixLike)
 userPubKeys u@(User n) l = pureInfoProperty ("ssh pubkey for " ++ n) $
@@ -260,7 +260,7 @@ instance Sem.Semigroup UserKeyInfo where
 
 instance Monoid UserKeyInfo where
 	mempty = UserKeyInfo M.empty
-	mappend = (<>)
+	mappend = (Sem.<>)
 
 -- | Sets up a user with the specified public keys, and the corresponding
 -- private keys from the privdata.