[Haskell-beginners] duplicate monoids...
mike_k_houghton at yahoo.co.uk
Tue Oct 20 21:24:10 UTC 2015
I’m looking at a blog post on Monoids and finger trees at http://apfelmus.nfshost.com/articles/monoid-fingertree.html <http://apfelmus.nfshost.com/articles/monoid-fingertree.html>
and would appreciate a bit of advice
type Size = Int
type Priority = Int
instance Monoid Size where
mempty = 0
mappend = (+)
instance Monoid Priority where
mempty = maxBound
mappend = min
and I get compiler error
Duplicate instance declarations:
instance Monoid Size
-- Defined at /Users/mike/haskell/FingerTrees/Ftree.hs:60:10
instance Monoid Priority
-- Defined at /Users/mike/haskell/FingerTrees/Ftree.hs:64:10
Which I can sort of understand as Size and Priority are both Int but on the other hand, internally, the monoids are different.
Is this genuinely incorrect code or is there a language extension to get around this?
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