<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi,<div class=""><br class=""></div><div class="">I’m looking at a blog post on Monoids and finger trees at <a href="http://apfelmus.nfshost.com/articles/monoid-fingertree.html" class="">http://apfelmus.nfshost.com/articles/monoid-fingertree.html</a></div><div class="">and would appreciate a bit of advice</div><div class=""><br class=""></div><div class="">I have </div><div class=""><br class=""></div><div class=""><div class="">type Size = Int</div><div class="">type Priority = Int </div></div><div class=""><br class=""></div><div class=""><div class="">instance Monoid Size where</div><div class=""> mempty = 0</div><div class=""> mappend = (+)</div></div><div class=""><br class=""></div><div class=""><div class="">instance Monoid Priority where</div><div class=""> mempty = maxBound</div><div class=""> mappend = min</div></div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">and I get compiler error</div><div class=""><br class=""></div><div class=""><div class=""> Duplicate instance declarations:</div><div class=""> instance Monoid Size</div><div class=""> -- Defined at /Users/mike/haskell/FingerTrees/Ftree.hs:60:10</div><div class=""> instance Monoid Priority</div><div class=""> -- Defined at /Users/mike/haskell/FingerTrees/Ftree.hs:64:10</div></div><div class=""><br class=""></div><div class="">Which I can sort of understand as Size and Priority are both Int but on the other hand, internally, the monoids are different.</div><div class=""><br class=""></div><div class="">Is this genuinely incorrect code or is there a language extension to get around this?</div><div class=""><br class=""></div><div class="">Thanks</div><div class=""><br class=""></div><div class="">Mike</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div></body></html>