[Haskell-cafe] Applicative Monoid instance

Brian Hurt bhurt at spnz.org
Mon Sep 5 18:54:20 UTC 2016

This may be a stupid question, but I'm wondering why the following instance
isn't in prelude:

instance (Applicative m, Monoid a) => Monoid (m a) where
    mempty = pure mempty
    mappend a b = mappend <$> a <*> b


I ask, because being able to use a WriterT (IO ()) STM a to debug complex
STM expressions would be really nice.  But it depends upon IO () being a

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