[Haskell-beginners] Newbie question about function type constraints
ARJANEN Loïc Jean David
arjanen.loic at gmail.com
Fri Sep 23 14:00:45 UTC 2016
Because if you say that your function accept all types which are
instances of Num, then it must accept *all* of them. Even those who do
not define a division, or whose division has a different behaviour than
the fractional one.
Le 23/09/2016 à 19:53, Lai Boon Hui a écrit :
> 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
> <mailto:beginners-request at haskell.org>> wrote:
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> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 23 Sep 2016 09:01:18 +0530
> From: Tushar Tyagi <tushar4r at gmail.com <mailto:tushar4r at gmail.com>>
> To: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell
> <beginners at haskell.org <mailto:beginners at haskell.org>>
> Subject: Re: [Haskell-beginners] Beginners Digest, Vol 99, Issue 13
> Message-ID:
>
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> Content-Type: text/plain; charset="utf-8"
>
> You can read more about Numbers here:
> https://www.haskell.org/tutorial/numbers.html
> <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
> <mailto: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
> <mailto:beginners-request at haskell.org>> wrote:
> >
> >> Send Beginners mailing list submissions to
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> >>
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> <http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners>
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> >> than "Re: Contents of Beginners digest..."
> >>
> >>
> >> 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 <mailto:laiboonh at gmail.com>>
> >> To: beginners at haskell.org <mailto: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 <http://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)
> >> -------------- next part --------------
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> >>
> >> ------------------------------
> >>
> >> Message: 2
> >> Date: Thu, 22 Sep 2016 19:36:04 +0530
> >> From: Tushar Tyagi <tushar4r at gmail.com <mailto:tushar4r at gmail.com>>
> >> To: The Haskell-Beginners Mailing List - Discussion of primarily
> >> beginner-level topics related to Haskell
> <beginners at haskell.org <mailto: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 <http://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
> <mailto: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)
> >>
> >>
> >>
> >> _______________________________________________
> >> Beginners mailing list
> >> Beginners at haskell.org <mailto:Beginners at haskell.org>
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> <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 <mailto:imantc at gmail.com>>
> >> To: The Haskell-Beginners Mailing List - Discussion of primarily
> >> beginner-level topics related to Haskell
> <beginners at haskell.org <mailto:beginners at haskell.org>>
> >> Subject: Re: [Haskell-beginners] Newbie question about function
> type
> >> constraints
> >> Message-ID:
> >>
> <CAP1qinZcONa4X3nozriFYnbkkguRfBmcFp2r3wMoZ6H3aLGGSQ at mail.gm
> <mailto:CAP1qinZcONa4X3nozriFYnbkkguRfBmcFp2r3wMoZ6H3aLGGSQ at mail.gm>
> >> ail.com <http://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 --------------
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> >>
> >> ------------------------------
> >>
> >> Message: 4
> >> Date: Thu, 22 Sep 2016 16:08:56 +0200
> >> From: Harald Bögeholz <bo at ct.de <mailto:bo at ct.de>>
> >> To: The Haskell-Beginners Mailing List - Discussion of primarily
> >> beginner-level topics related to Haskell
> <beginners at haskell.org <mailto:beginners at haskell.org>>
> >> Subject: Re: [Haskell-beginners] Newbie question about function
> type
> >> constraints
> >> Message-ID: <a134a975-6b0f-b364-7db9-6b6a0d99759a at ct.de
> <mailto: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 <mailto:bo at ct.de>> (PGP key
> available from servers)
> >> Redaktion c't Tel.: +49 511 5352-300
> <tel:%2B49%20511%205352-300> Fax: +49 511 5352-417
> <tel:%2B49%20511%205352-417>
> >>
> >> 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
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> >>
> >>
> >> ------------------------------
> >>
> >> Message: 5
> >> Date: Thu, 22 Sep 2016 16:10:17 +0200
> >> From: Sylvain Henry <sylvain at haskus.fr <mailto:sylvain at haskus.fr>>
> >> To: beginners at haskell.org <mailto:beginners at haskell.org>
> >> Subject: Re: [Haskell-beginners] Newbie question about function
> type
> >> constraints
> >> Message-ID: <66b119b3-ff51-74d7-80c5-d19450164508 at haskus.fr
> <mailto: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 <mailto:Beginners at haskell.org>
> >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> <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 <mailto:sylvain at haskus.fr>>
> >> To: beginners at haskell.org <mailto:beginners at haskell.org>
> >> Subject: Re: [Haskell-beginners] Newbie question about function
> type
> >> constraints
> >> Message-ID: <074cbf97-2316-9d70-3fb8-7c8c9904c602 at haskus.fr
> <mailto: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
> <mailto:defigueiredo at ucdavis.edu>>
> >> To: Haskell Cafe <haskell-cafe at haskell.org
> <mailto:haskell-cafe at haskell.org>>, The Haskell-Beginners
> >> Mailing List - Discussion of primarily beginner-level
> topics
> >> related
> >> to Haskell <beginners at haskell.org
> <mailto:beginners at haskell.org>>
> >> Subject: [Haskell-beginners] The meaning of categories constructed
> >> from HASK
> >> Message-ID: <36383e22-0b32-426d-7c9f-5e611bbca233 at ucdavis.edu
> <mailto: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 <mailto:Beginners at haskell.org>
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> <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 <mailto:Beginners at haskell.org>
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> <http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners>
> >
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>
> Subject: Digest Footer
>
> _______________________________________________
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> ------------------------------
>
> End of Beginners Digest, Vol 99, Issue 15
> *****************************************
>
>
>
>
> --
> Best Regards,
> Boon Hui
--
ARJANEN Loïc Jean David
http://blog.luigiscorner.com
---
“Computer science is no more about computers than astronomy is about telescopes, biology is about microscopes, or chemistry is about beakers and test tubes. Science is not about tools. It is about how we use them, and what we find out when we do.”
Michael R. Fellows and Ian Parberry
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