[Haskell-beginners] Foldl

[Alexander Chen]

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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