foldl does a loop, foldr does constructor replacement

https://www.youtube.com/watch?v=GPwtT31zKRY

On 5/28/20 4:55 AM, Alexander Chen wrote:
> Hi,
> 
> I guess it's time to get acquainted with foldr and foldl.
> 
> *prelude>xs = [1..5] ++ undefined*
> *prelude> foldr const 0 xs*
> 1
> 
> I assume it goes something like this:
> 
> ( 1 `const`(2 `const`(3 `const`(4 `const`(5 `const`(undefined `const`
> 0))))))
>                                                               (
> 5 `const` undefined)
>                                                (4 `const` 5)    
>                                 (3 `const` 4)
>                 ( 2 `const` 3)
> (1 `const`2)
> = 1
> 
> ========================================================================
> 
> 
> What i don't get is the opposite:
> 
> *prelude> foldl const 0 xs*
> error
> 
> 
> in my mind this should go like this: 
> ((((((0`const`1)`const` 2) `const` 3 )`const` 4)`const`  5)
> `const` undefined)
>                     (0 `const`2)
>                                       (0`const` 3)            
>                                                       ( 0`const`4)
>                                                                      
>  (0`const` 5)
>                                                                        
>               (0 `const` undefined )
> = 0
> 
> 
> 
> I have been told that the main difference between foldl and foldr is
> that foldl needs to evaluate the whole spline before it continues. And i
> guess that has something to do with it. What I don't understand is WHY
> foldl need to do this and foldr doesn't.
> 
> 
> thanks in advance!
> 
> best,
> 
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