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.
Hi,
still does not compile
sumList :: (Fractional a) => [a] -> asumList [] = 0sumList (x:xs) = x + (sumList xs)
lengthList :: (Fractional a) => [t] -> alengthList [] = 0lengthList (_:xs) = 1 + (lengthList xs)
meanList :: (Num a, Fractional b) => [a] -> bmeanList xs = (sumList xs) / (lengthList xs)
i know
meanList :: (Fractional b) => [b] -> bmeanList 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@haskell.org> wrote:
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Message: 1
Date: Fri, 23 Sep 2016 09:01:18 +0530
From: Tushar Tyagi <tushar4r@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Beginners Digest, Vol 99, Issue 13
Message-ID:
<CAEDPzikCmLEpiop-5BGbDaKZK1TifAu6ymF+jmJq_ >rEuMNN_Kg@mail.gmail.com
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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@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@haskell.org> wrote:
>
>> Send Beginners mailing list submissions to
>> beginners@haskell.org
>>
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>> or, via email, send a message with subject or body 'help' to
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>>
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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@gmail.com>
>> To: beginners@haskell.org
>> Subject: [Haskell-beginners] Newbie question about function type
>> constraints
>> Message-ID:
>> <CAJdQgg=jJYfdRsq+QfBr1aC-hS1ft9+4Sp4jopoktg=JnDMirg@ 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@gmail.com>
>> To: The Haskell-Beginners Mailing List - Discussion of primarily
>> beginner-level topics related to Haskell <beginners@haskell.org>
>> Subject: Re: [Haskell-beginners] Newbie question about function type
>> constraints
>> Message-ID:
>> <CAEDPzin_vcZA+hh9ZLFo-k91Bn63vaemgQJKqN_P4hsAk+ALzQ@ 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@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@haskell.org
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>> ------------------------------
>>
>> Message: 3
>> Date: Thu, 22 Sep 2016 16:06:52 +0200
>> From: Imants Cekusins <imantc@gmail.com>
>> To: The Haskell-Beginners Mailing List - Discussion of primarily
>> beginner-level topics related to Haskell <beginners@haskell.org>
>> Subject: Re: [Haskell-beginners] Newbie question about function type
>> constraints
>> Message-ID:
>> <CAP1qinZcONa4X3nozriFYnbkkguRfBmcFp2r3wMoZ6H3aLGGSQ@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 --------------
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>>
>> ------------------------------
>>
>> Message: 4
>> Date: Thu, 22 Sep 2016 16:08:56 +0200
>> From: Harald Bögeholz <bo@ct.de>
>> To: The Haskell-Beginners Mailing List - Discussion of primarily
>> beginner-level topics related to Haskell <beginners@haskell.org>
>> Subject: Re: [Haskell-beginners] Newbie question about function type
>> constraints
>> Message-ID: <a134a975-6b0f-b364-7db9-6b6a0d99759a@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@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@haskus.fr>
>> To: beginners@haskell.org
>> Subject: Re: [Haskell-beginners] Newbie question about function type
>> constraints
>> Message-ID: <66b119b3-ff51-74d7-80c5-d19450164508@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@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@haskus.fr>
>> To: beginners@haskell.org
>> Subject: Re: [Haskell-beginners] Newbie question about function type
>> constraints
>> Message-ID: <074cbf97-2316-9d70-3fb8-7c8c9904c602@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@ucdavis.edu>
>> To: Haskell Cafe <haskell-cafe@haskell.org>, The Haskell-Beginners
>> Mailing List - Discussion of primarily beginner-level topics
>> related
>> to Haskell <beginners@haskell.org>
>> Subject: [Haskell-beginners] The meaning of categories constructed
>> from HASK
>> Message-ID: <36383e22-0b32-426d-7c9f-5e611bbca233@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
>>
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>> End of Beginners Digest, Vol 99, Issue 13
>> *****************************************
>>
>
>
>
> --
> Best Regards,
> Boon Hui
>
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--
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