[Haskell-beginners] Beginners Digest, Vol 99, Issue 15
Lai Boon Hui
laiboonh at gmail.com
Fri Sep 23 12:53:04 UTC 2016
Hi,
still does not compile
sumList :: (Fractional a) => [a] -> a
sumList [] = 0
sumList (x:xs) = x + (sumList xs)
lengthList :: (Fractional a) => [t] -> a
lengthList [] = 0
lengthList (_:xs) = 1 + (lengthList xs)
meanList :: (Num a, Fractional b) => [a] -> b
meanList xs = (sumList xs) / (lengthList xs)
i know
meanList :: (Fractional b) => [b] -> b
meanList xs = (sumList xs) / (lengthList xs)
compiles. But why do i have to restrict the inputs to be a list of
Fractionals and not Nums??
On Fri, Sep 23, 2016 at 11:18 AM, <beginners-request at haskell.org> wrote:
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> 1. Re: Beginners Digest, Vol 99, Issue 13 (Tushar Tyagi)
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>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 23 Sep 2016 09:01:18 +0530
> From: Tushar Tyagi <tushar4r at gmail.com>
> To: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell <beginners at haskell.org>
> Subject: Re: [Haskell-beginners] Beginners Digest, Vol 99, Issue 13
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>
> You can read more about Numbers here:
> https://www.haskell.org/tutorial/numbers.html
>
> In your implementation sumList and lengthList both return 'Num' which
> doesn't define a division operator. So you have to convert them into
> fractional by either changing the signatures of these 2 functions from Num
> to Fractional, or use fromIntegral function, (or something else) . Two of
> these approaches have been suggested by people here. :)
>
> Typed using my phone, so excuse my brevity.
>
> On 23 Sep 2016 6:14 a.m., "Lai Boon Hui" <laiboonh at gmail.com> wrote:
>
> > Hi All,
> >
> > i am overwhelmed by all the helpful responses. Thanks guys.
> >
> > I am more curious about why
> >
> > meanList :: (Num a, Fractional b) => [a] -> b
> > meanList xs = (sumList xs) / (lengthList xs)
> >
> > does not compile.
> >
> > 'a' being a Num type seems perfectly fine, (/) returns a Fractional type
> > hence 'b' being Fractional seems also fine.
> >
> >
> >
> > On Fri, Sep 23, 2016 at 7:13 AM, <beginners-request at haskell.org> wrote:
> >
> >> Send Beginners mailing list submissions to
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> >> Today's Topics:
> >>
> >> 1. Newbie question about function type constraints (Lai Boon Hui)
> >> 2. Re: Newbie question about function type constraints
> >> (Tushar Tyagi)
> >> 3. Re: Newbie question about function type constraints
> >> (Imants Cekusins)
> >> 4. Re: Newbie question about function type constraints
> >> (Harald Bögeholz)
> >> 5. Re: Newbie question about function type constraints
> >> (Sylvain Henry)
> >> 6. Re: Newbie question about function type constraints
> >> (Sylvain Henry)
> >> 7. The meaning of categories constructed from HASK
> >> (Dimitri DeFigueiredo)
> >>
> >>
> >> ----------------------------------------------------------------------
> >>
> >> Message: 1
> >> Date: Thu, 22 Sep 2016 21:19:12 +0800
> >> From: Lai Boon Hui <laiboonh at gmail.com>
> >> To: beginners at haskell.org
> >> Subject: [Haskell-beginners] Newbie question about function type
> >> constraints
> >> Message-ID:
> >> <CAJdQgg=jJYfdRsq+QfBr1aC-hS1ft9+4Sp4jopoktg=JnDMirg at mail.
> >> gmail.com>
> >> Content-Type: text/plain; charset="utf-8"
> >>
> >> Hi, can someone explain to me why i cannot define meanList as:
> >>
> >> meanList :: (Integral a, Fractional b) => [a] -> a
> >> meanList xs = (sumList xs) / (lengthList xs)
> >>
> >> I want to restrict the function to only accept lists like [1,2,3] and
> >> return answer 2.0
> >>
> >>
> >> sumList :: (Num a) => [a] -> a
> >> sumList [] = 0
> >> sumList (x:xs) = x + (sumList xs)
> >>
> >> lengthList :: (Num a) => [t] -> a
> >> lengthList [] = 0
> >> lengthList (_:xs) = 1 + (lengthList xs)
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> >>
> >> ------------------------------
> >>
> >> Message: 2
> >> Date: Thu, 22 Sep 2016 19:36:04 +0530
> >> From: Tushar Tyagi <tushar4r at gmail.com>
> >> To: The Haskell-Beginners Mailing List - Discussion of primarily
> >> beginner-level topics related to Haskell <beginners at haskell.org
> >
> >> Subject: Re: [Haskell-beginners] Newbie question about function type
> >> constraints
> >> Message-ID:
> >> <CAEDPzin_vcZA+hh9ZLFo-k91Bn63vaemgQJKqN_P4hsAk+ALzQ at mail.
> >> gmail.com>
> >> Content-Type: text/plain; charset="utf-8"
> >>
> >> What happens if you change the signature of meanList to
> >>
> >> meanList :: ( Fractional b) => [b] ->b
> >>
> >> The integrals in [1,2,3] would be converted to [1.0, 2.0, 3.0] before
> you
> >> act upon them.
> >>
> >> On 22 Sep 2016 6:49 p.m., "Lai Boon Hui" <laiboonh at gmail.com> wrote:
> >>
> >> Hi, can someone explain to me why i cannot define meanList as:
> >>
> >> meanList :: (Integral a, Fractional b) => [a] -> a
> >> meanList xs = (sumList xs) / (lengthList xs)
> >>
> >> I want to restrict the function to only accept lists like [1,2,3] and
> >> return answer 2.0
> >>
> >>
> >> sumList :: (Num a) => [a] -> a
> >> sumList [] = 0
> >> sumList (x:xs) = x + (sumList xs)
> >>
> >> lengthList :: (Num a) => [t] -> a
> >> lengthList [] = 0
> >> lengthList (_:xs) = 1 + (lengthList xs)
> >>
> >>
> >>
> >> _______________________________________________
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> >> Beginners at haskell.org
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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> >>
> >> ------------------------------
> >>
> >> Message: 3
> >> Date: Thu, 22 Sep 2016 16:06:52 +0200
> >> From: Imants Cekusins <imantc at gmail.com>
> >> To: The Haskell-Beginners Mailing List - Discussion of primarily
> >> beginner-level topics related to Haskell <beginners at haskell.org
> >
> >> Subject: Re: [Haskell-beginners] Newbie question about function type
> >> constraints
> >> Message-ID:
> >> <CAP1qinZcONa4X3nozriFYnbkkguRfBmcFp2r3wMoZ6H3aLGGSQ at mail.gm
> >> ail.com>
> >> Content-Type: text/plain; charset="utf-8"
> >>
> >> Hello,
> >>
> >> this works too:
> >>
> >> meanList :: (Fractional a) => [a] -> a
> >> meanList xs = (sumList xs) / (lengthList xs)
> >>
> >>
> >> sumList :: (Fractional a) => [a] -> a
> >> sumList [] = 0
> >> sumList (x:xs) = x + (sumList xs)
> >>
> >>
> >> lengthList :: (Fractional a) => [t] -> a
> >> lengthList [] = 0
> >> lengthList (_:xs) = 1 + (lengthList xs)
> >> -------------- next part --------------
> >> An HTML attachment was scrubbed...
> >> URL: <http://mail.haskell.org/pipermail/beginners/attachments/
> >> 20160922/4c967a0d/attachment-0001.html>
> >>
> >> ------------------------------
> >>
> >> Message: 4
> >> Date: Thu, 22 Sep 2016 16:08:56 +0200
> >> From: Harald Bögeholz <bo at ct.de>
> >> To: The Haskell-Beginners Mailing List - Discussion of primarily
> >> beginner-level topics related to Haskell <beginners at haskell.org
> >
> >> Subject: Re: [Haskell-beginners] Newbie question about function type
> >> constraints
> >> Message-ID: <a134a975-6b0f-b364-7db9-6b6a0d99759a at ct.de>
> >> Content-Type: text/plain; charset=utf-8
> >>
> >> Am 22.09.16 um 15:19 schrieb Lai Boon Hui:
> >> > Hi, can someone explain to me why i cannot define meanList as:
> >> >
> >> > meanList :: (Integral a, Fractional b) => [a] -> a
> >> > meanList xs = (sumList xs) / (lengthList xs)
> >> >
> >> > I want to restrict the function to only accept lists like [1,2,3] and
> >> > return answer 2.0
> >>
> >> It will work like this:
> >>
> >> meanList :: (Integral a, Fractional b) => [a] -> b
> >> meanList xs = fromIntegral (sumList xs) / (lengthList xs)
> >>
> >> You probably meant -> b in the type signature, that was a typo.
> >>
> >> And you need to insert fromIntegral to convert to Fractional before you
> >> can divide. Now that I see it I am beginning to wonder why it works,
> >> though, because I was just about to insert another fromIntegral before
> >> lengthList ...
> >>
> >>
> >> > sumList :: (Num a) => [a] -> a
> >> > sumList [] = 0
> >> > sumList (x:xs) = x + (sumList xs)
> >> >
> >> > lengthList :: (Num a) => [t] -> a
> >> > lengthList [] = 0
> >> > lengthList (_:xs) = 1 + (lengthList xs)
> >>
> >> Hope this helps
> >>
> >>
> >> --
> >> Harald Bögeholz <bo at ct.de> (PGP key available from servers)
> >> Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417
> >>
> >> int f[9814],b,c=9814,g,i;long a=1e4,d,e,h;
> >> main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a)
> >> while(g=--b*2)d=h*b+a*(i?f[b]:
> a/5),h=d/--g,f[b]=d%g;}
> >> (Arndt/Haenel)
> >>
> >> Affe Apfel Vergaser
> >>
> >> /*Heise Medien GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover
> >> Registergericht: Amtsgericht Hannover HRA 26709
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> >>
> >>
> >> ------------------------------
> >>
> >> Message: 5
> >> Date: Thu, 22 Sep 2016 16:10:17 +0200
> >> From: Sylvain Henry <sylvain at haskus.fr>
> >> To: beginners at haskell.org
> >> Subject: Re: [Haskell-beginners] Newbie question about function type
> >> constraints
> >> Message-ID: <66b119b3-ff51-74d7-80c5-d19450164508 at haskus.fr>
> >> Content-Type: text/plain; charset="utf-8"; Format="flowed"
> >>
> >> Hi,
> >>
> >> You can define it, but in practice there is no instance of "a" that
> >> satisfies both constraints: Integral a and Fractional a
> >> > meanList ([1,2,3] :: [Int])
> >>
> >> <interactive>:4:1: error:
> >> • No instance for (Fractional Int) arising from a use of ‘meanList’
> >>
> >> > meanList ([1,2,3] :: [Float])
> >>
> >> <interactive>:5:1: error:
> >> • No instance for (Integral Float) arising from a use of ‘meanList’
> >>
> >> What you probably want is:
> >> meanList :: (Integral a, Fractional b) => [a] -> b
> >> meanList xs = fromIntegral (sumList xs) / fromIntegral (lengthList xs)
> >>
> >> Where we convert from the integral type "a" to the fractional type "b"
> >> before performing the division.
> >>
> >> > meanList ([1,2,3] :: [Int])
> >> 2.0
> >>
> >> Cheers
> >> Sylvain
> >>
> >>
> >> On 22/09/2016 15:19, Lai Boon Hui wrote:
> >> > Hi, can someone explain to me why i cannot define meanList as:
> >> >
> >> > meanList :: (Integral a, Fractional b) => [a] -> a
> >> > meanList xs = (sumList xs) / (lengthList xs)
> >> >
> >> > I want to restrict the function to only accept lists like [1,2,3] and
> >> > return answer 2.0
> >> >
> >> >
> >> > sumList :: (Num a) => [a] -> a
> >> > sumList [] = 0
> >> > sumList (x:xs) = x + (sumList xs)
> >> >
> >> > lengthList :: (Num a) => [t] -> a
> >> > lengthList [] = 0
> >> > lengthList (_:xs) = 1 + (lengthList xs)
> >> >
> >> >
> >> >
> >> >
> >> > _______________________________________________
> >> > Beginners mailing list
> >> > Beginners at haskell.org
> >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >>
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> >>
> >> ------------------------------
> >>
> >> Message: 6
> >> Date: Thu, 22 Sep 2016 16:14:48 +0200
> >> From: Sylvain Henry <sylvain at haskus.fr>
> >> To: beginners at haskell.org
> >> Subject: Re: [Haskell-beginners] Newbie question about function type
> >> constraints
> >> Message-ID: <074cbf97-2316-9d70-3fb8-7c8c9904c602 at haskus.fr>
> >> Content-Type: text/plain; charset=utf-8; format=flowed
> >>
> >> On 22/09/2016 16:08, Harald Bögeholz wrote:
> >> > It will work like this:
> >> > meanList :: (Integral a, Fractional b) => [a] -> b
> >> > meanList xs = fromIntegral (sumList xs) / (lengthList xs)
> >> >
> >> > You probably meant -> b in the type signature, that was a typo.
> >> >
> >> > And you need to insert fromIntegral to convert to Fractional before
> you
> >> > can divide. Now that I see it I am beginning to wonder why it works,
> >> > though, because I was just about to insert another fromIntegral before
> >> > lengthList ...
> >> It works because in this case lengthList uses the fractional type b to
> >> perfom its summation (it doesn't care about the type of the elements in
> >> xs).
> >>
> >> Cheers
> >> Sylvain
> >>
> >>
> >> ------------------------------
> >>
> >> Message: 7
> >> Date: Thu, 22 Sep 2016 17:26:36 -0600
> >> From: Dimitri DeFigueiredo <defigueiredo at ucdavis.edu>
> >> To: Haskell Cafe <haskell-cafe at haskell.org>, The Haskell-Beginners
> >> Mailing List - Discussion of primarily beginner-level topics
> >> related
> >> to Haskell <beginners at haskell.org>
> >> Subject: [Haskell-beginners] The meaning of categories constructed
> >> from HASK
> >> Message-ID: <36383e22-0b32-426d-7c9f-5e611bbca233 at ucdavis.edu>
> >> Content-Type: text/plain; charset=utf-8
> >>
> >> In category theory, there are many ways one can make new categories out
> >> of an old one.
> >>
> >> In particular, given a category C one can construct:
> >>
> >> 1. The arrows category of C:
> >> arrows in C become objects and
> >> commutative squares in C become arrows
> >> 2. The slice category of C given an object A:
> >> arrows into a distinguished object A become objects in the slice
> >> commutative triangles become arrows
> >>
> >> There are also functors going from C to these new categories (and back).
> >>
> >> Are these constructed categories useful when C = `Hask` (the category of
> >> haskell types and functions)?
> >> What do they represent in programming terms?
> >>
> >> In other words, is there intuition for what the arrows category of Hask
> >> is?
> >> What about the slice category of Hask over a specific type?
> >> Do the functors between these match some programming abstractions?
> >>
> >> Any pointers are much appreciated.
> >>
> >> Thanks,
> >>
> >> Dimitri
> >>
> >>
> >>
> >>
> >> ------------------------------
> >>
> >> Subject: Digest Footer
> >>
> >> _______________________________________________
> >> Beginners mailing list
> >> Beginners at haskell.org
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >>
> >>
> >> ------------------------------
> >>
> >> End of Beginners Digest, Vol 99, Issue 13
> >> *****************************************
> >>
> >
> >
> >
> > --
> > Best Regards,
> > Boon Hui
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
> >
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>
> End of Beginners Digest, Vol 99, Issue 15
> *****************************************
>
--
Best Regards,
Boon Hui
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