[Haskell-beginners] Get max element of a list using foldl or foldr

[jamb at hinojosa.com]

El 2015-09-25 18:57, Francesco Ariis escribió:
> On Fri, Sep 25, 2015 at 06:18:08PM -0500, jamb at hinojosa.com wrote:
>> Hello,
>> 
>> I am a complete Haskell beginner and I am doing some exercises of my 
>> book
>> but I am stuck with the following:
>> 
>> Define
>> myMax :: Ord a => [a] -> a
>> which returns the maximum element of a list.
>> 
>> I must use foldl1 or foldr1 and I am given the hint to use max which 
>> gets
>> the maximum of 2 elements.
>> 
>> I will very much appreciate if you help me solve it.
> 
> Let's say you have
> 
>     foldl1 f [1,7,2,5]
> 
> where f is a binary operator (a function that 'takes two parameters').
> `foldl1` will apply 1 and 7 to f, obtaining X, so we have:
> 
>     X and [2,5]
> 
> then it will apply X and 2 to f, obtaining Y, so we're left with
> 
>     Y and 5
> 
> and finally `f Y 5`, leading to your final result Z.
> 
> Now, if `f a b = a + b`, we would have
> 
>        [1,7,2,5]  -- 1+7
>     8  [2,5]      -- 8+2
>     10 [5]        -- 10+5
>     15 <-- final result
> 
> But you aren't interested in the sum of the list, but its maximum.
> Which operation could you use instead of (+) to achieve your goal?
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I´ve got it!
I tried the following successfully:

myMax [] = error "List is empty."
myMax xs = foldl1 (max) xs

I see that my problem was in understanding the way max was applied to a 
list.


Thanks for your kind support.