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Today's Topics:
1. Re: Return type from class method (Thomas Jakway)
2. Re: Return type from class method (PATRICK BROWNE)
----------------------------------------------------------------------
Message: 1
Date: Sat, 7 Oct 2017 15:30:27 -0700
From: Thomas Jakway <tjak...@nyu.edu>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Return type from class method
Message-ID: <b49ce16b-8ed7-ae50-846f-0219230cd...@nyu.edu>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
You could hide Prelude and define it yourself in a different module but
that would be a pretty bad idea. Everyone who wanted to use it would
have to import your module qualified and refer to it as MyModule.+,
which defeats the point of making it (+) and not `myAdditionFunction` in
the first place.
Haskell deliberately doesn't allow overloading. Having (+) return
something other than Num would be extremely confusing.
On 10/07/2017 05:07 AM, PATRICK BROWNE wrote:
> 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 <http://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
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> <http://www.dit.ie/>
>
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>
>
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Message: 2
Date: Sun, 8 Oct 2017 06:24:06 +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: Re: [Haskell-beginners] Return type from class method
Message-ID:
<cagflrkeejbmvdrq46yjcfcoyog6-u5jn_h1jq_hnrumxf48...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Thomas,
Thanks for your response. I agree that using (+) in this manner leads to
name clashes.
My question is prompted by a research paper [1] where a form of lifting is
attempted using type classes.
I think that the original code in paper (below) is intended to be
illustrative rather than practical.
I have edited the code to compile and run (after a fashion, see below). But
I cannot get the functions to return the lifted type.
Also I have a problem compiling the lines with two lambdas, e.g. (np1 = xy
(\t -> 4.0 + 0.5 * t) (\t -> 4.0 - 0.5 * t))
Regards,
Pat
---------------------------------------------------------------------------------------------
Original code [1]:
------------------------------------------------------------------------------------------
class Number a where
(+), (-), (*) :: a -> a -> a
sqr, sqrt :: a -> a
sqr a = a * a
type Moving v = Time -> v
instance Number v => Number (Moving v) where
(+) a b = \t -> (a t) + (b t)
(-) a b = \t -> (a t) - (b t)
(*) a b = \t -> (a t) * (b t)
sqrt a = \t -> sqrt (a t)
class Number s => Points p s where
x, y :: p s -> s
xy :: s -> s -> p s
dist :: p s -> p s -> s
dist a b = sqrt (sqr ((x a) - (x b)) +
sqr ((y a) - (y b)))
data Point f = Point f f
instance Number v => Points Point v where
x (Point x1 y1) = x1
y (Point x1 y1) = y1
xy x1 y1 = Point x1 y1
instance Number v => (Point v) where
(+) a b = xy (x a + x b) (y a + y b)
(-) a b = xy (x a - x b) (y a - y b)
np1, np2 :: Point (Moving Float)
np1 = xy (\t -> 4.0 + 0.5 * t) (\t -> 4.0 - 0.5 * t)
np2 = xy (\t -> 0.0 + 1.0 * t) (\t -> 0.0 - 1.0 * t)
movingDist_1_2 = dist np1 np2
dist_at_1 = movingDist_1_2 1.0
----------------------------------------------------------------------------------------------
My attempt at getting above code to run:
-----------------------------------------------------------------------------------------------
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeSynonymInstances #-}
module Moving where
data Time = Time Double
type Moving v = Time -> v
data Point v = Point v v deriving Show
class Number a where
(+),(-),(*) :: a -> a -> a
sqrt :: a -> a
instance (Fractional a,Floating a) => Number (Moving a) where
(+) a b = \t -> ((a t) Prelude.+ (b t))
(-) a b = \t -> ((a t) Prelude.- (b t))
(*) a b = \t -> ((a t) Prelude.* (b t))
sqrt a = \t -> Prelude.sqrt (a t)
a,b :: Moving Double
a (Time x) = 4.0
b (Time x) = 4.0
testPlus ::(Moving Double)
testPlus = (a Moving.+ b)
testPlusArg = (a Moving.+ b) (Time 2.0)
testSqrt = (Moving.sqrt a) (Time 2.0)
class (Number s) => Points p s where
x, y :: p s -> s
xy :: s -> s -> p s
dist :: p s -> p s -> s
dist a b = Moving.sqrt (sqr ((x a) Moving.- (x b)) Moving.+ sqr ((y a)
Moving.- (y b)))
where sqr z = z Moving.* z
-- instance (Floating v,Number v) => Points Point v where
instance (Number s) => Points Point s where
x (Point x1 y1) = x1
y (Point x1 y1) = y1
xy x1 y1 = Point x1 y1
instance Number v => Number (Point v) where
(+) a b = xy (x a Moving.+ x b) (y a Moving.+ y b)
(-) a b = xy (x a Moving.- x b) (y a Moving.- y b)
md1,md2,md3,md4 :: Moving Double
md1 (Time x) = 0.0
md2 (Time x) = 0.0
md3 (Time x) = 10.0
md4 (Time x) = 10.0
testMD1 = (md1 (Time 2.0))
testX = x (Point md1 md2) (Time 2.0)
testY = y (Point md1 md2) (Time 2.0)
testXY = (xy md1 md2)::(Point (Moving Double))
testX' = x testXY (Time 1.0)
testD = dist (Point md1 md2) (Point md3 md4) (Time 1.0)
--- I cannot get the rest to work
[1] Ontology for Spatio-temporal Databases
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.113.9804&rep=rep1&type=pdf
On 7 October 2017 at 23:30, Thomas Jakway <tjak...@nyu.edu> wrote:
> You could hide Prelude and define it yourself in a different module but
> that would be a pretty bad idea. Everyone who wanted to use it would have
> to import your module qualified and refer to it as MyModule.+, which
> defeats the point of making it (+) and not `myAdditionFunction` in the
> first place.
>
> Haskell deliberately doesn't allow overloading. Having (+) return
> something other than Num would be extremely confusing.
>
> On 10/07/2017 05:07 AM, PATRICK BROWNE wrote:
>
> 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>
>
>
> _______________________________________________
> Beginners mailing
> listBeginners@haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
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>
--
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>
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