Hi, Is there a way rewriting the definition of (+) so that testPlusArg returns a (Moving Double). My current intuition is that the signature [(+) :: a -> a -> a] says that the type should be the same as the arguments. And indeed (:t testPlus) confirms this. But the type of testPlusArg is a Double. Can I make it (Moving Double) ? Thanks, Pat {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE TypeSynonymInstances #-} module Moving where data Time = Time Double type Moving v = Time -> v class Number a where (+) :: a -> a -> a instance Number (Moving Double) where (+) a b = \t -> ((a t) Prelude.+ (b t)) a,b :: Moving Double a (Time x) = 2.0 b (Time x) = 2.0 testPlus ::(Moving Double) testPlus = (a Moving.+ b) testPlusArg = (a Moving.+ b) (Time 2.0) -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman http://www.dit.ie/grangegorman