[Haskell-beginners] Newbie question about function type constraints

[Harald Bögeholz]

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


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Harald Bögeholz    <bo at ct.de> (PGP key available from servers)
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