Re: [Haskell-beginners] Beginners Digest, Vol 99, Issue 15

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, 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 To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: Re: [Haskell-beginners] Beginners Digest, Vol 99, Issue 13 Message-ID: Content-Type: text/plain; charset="utf-8"

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" wrote:

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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, wrote:

...

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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)

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Message: 1 Date: Thu, 22 Sep 2016 21:19:12 +0800 From: Lai Boon Hui To: beginners@haskell.org Subject: [Haskell-beginners] Newbie question about function type constraints Message-ID: 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)