Sage-Tips

Should I use foldl or foldr?

Another difference is that because it matches the structure of the list, foldr is often more efficient for lazy evaluation, so can be used with an infinite list as long as f is non-strict in its second argument (like (:) or (++) ). foldl is only rarely the better choice.

Is foldr or foldl more efficient?

Haskell Wiki compares foldr , foldl and foldl’ and recommends using either foldr or foldl’ . foldl’ is the more efficient way to arrive at that result because it doesn’t build a huge thunk.

How does foldr work in Haskell?

Haskell : foldr. Description: it takes the second argument and the last item of the list and applies the function, then it takes the penultimate item from the end and the result, and so on.

What is foldl and foldr in Haskell?

I like to call foldr as “fold from the right”, while foldl is “fold from the left”. As you can see, for foldl the position of the seed value is on the left hand side of the tree, while foldr seed value is on the right hand side of the tree. …

Is foldr strict?

The problem is that (+) is strict in both of its arguments. This means that both arguments must be fully evaluated before (+) can return a result.

Why does foldr work on infinite lists?

So we can see why foldr sometimes works on infinite lists when foldl doesn’t: The former can lazily convert an infinite list into another lazy infinite data structure, whereas the latter must inspect the entire list to generate any part of the result.

How do you access the Foldr?

To access Foldr simply type the following web address into your browser and login with your college credentials (username and password). The ‘My Files’ page is shown and this provides access to college files. It is necessary to link your ‘OneDrive/Office 365’ files to foldr.

Why are folders lazy?

On the other hand, consider foldr . It’s also lazy, but because it runs forwards, each application of f is added to the inside of the result. So, to compute the result, Haskell constructs a single function application, the second argument of which is the rest of the folded list.

What does Scanl do in Haskell?

Haskell : scanl. Description: it takes the second argument and the first item of the list and applies the function to them, then feeds the function with this result and the second argument and so on. It returns the list of intermediate and final results.