Re: [Haskell-beginners] Beginners Digest, Vol 99, Issue 13

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)