Ahh! Nice. Thanks

On 20 Oct 2015, at 22:40, Joel Williamson wrote:

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 mailto: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 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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