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Today's Topics:
1. Return type from class method (PATRICK BROWNE)
----------------------------------------------------------------------
Message: 1
Date: Sat, 7 Oct 2017 13:07:43 +0100
From: PATRICK BROWNE <patrick.bro...@dit.ie>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] Return type from class method
Message-ID:
<cagflrke17atqurpquygh1czzz5rwnwpo1wy4h7fqye2he_0...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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)
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