gistfile1.txt

-- src/Data/Sum.purs

module Data.Sum where

import Data.Monoid

newtype Sum = Sum {getSum :: Number}

instance monoidSum :: Monoid Sum where
  mempty               = Sum 0
  (<>) (Sum m) (Sum n) = Sum $ m + n
  
==========================================
  
$ grunt
Running "psc:all" (psc) task
>> Error creating file dist/Main.js
>>
Warning: Error at src/Data/Sum.purs line 7, column 1 - line 9, column 36:
  Error in type class instance Data.Monoid.Monoid Data.Sum.Sum:
  Error at src/Data/Sum.purs line 9, column 3 - line 9, column 36:
    Member (<>) has not been implemented
See https://github.com/purescript/purescript/wiki/Error-Code-MissingClassMember for more information, or to contribute content related to this error. Use --force to continue.

Aborted due to warnings.

==========================================

It's complaining about (<>) not being implemented, but the location it gives (L9, 3-36) is exactly where I implement it!