A type can only have a single instance of a given class. Imagine if this weren't true. The compiler would have to guess which of the instances you meant to use. The solution is to use newtype. That will introduce a different type, allowing separate instances, but is optimised out so it carries no runtime cost.

On Tue, 20 Oct 2015, 17:24 Mike Houghton <mike_k_houghton@yahoo.co.uk> wrote:

Hi,

I’m looking at a blog post on Monoids and finger trees at http://apfelmus.nfshost.com/articles/monoid-fingertree.html
and would appreciate a bit of advice

I have

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?

Thanks

Mike

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