I'm not sure how I feel about the proposal, but I'd just like to add that in fact the laws for Monoid can also be expressed in terms of mconcat: mconcat [x] = x mconcat . concat = mconcat . map mconcat (for those who appreciate that sort of thing, these are the laws for mconcat to be a monad algebra for the list monad). On Fri, Jan 24, 2014 at 10:42:41PM +0100, Niklas Haas wrote:
Hello,
I noticed that the current Monoid class doesn't actually provide default implementations of mappend/mempty in terms of mconcat, even though this is technically very simple:
mempty = mconcat [] mappend a b = mconcat [a, b]
This would be a rather small and non-invasive change that shouldn't break any existing programs. The main downside is that an empty Monoid declaration would produce no warnings, but #7633 gives us the ability to solve this.
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