Re: [Haskell-beginners] Why this order of parameters

Harald and Kim-Ee, thanks for your explanations. So in other words the order of parameters naturally follows from the corresponding equations and is not driven by technical need. A follow up question.. what do you mean by elements being "naturally foldable"? Is it that you could just cons them together without any transformation? Martin Kim-Ee Yeoh:

On Fri, Nov 13, 2015 at 12:49 AM, Martin Vlk wrote:

...

I'm curious if this could be for the sake of making the types of the two functions (foldr/foldl) different? E.g. so that you can tell from the type what function it is.

No, that would be foolish. One doesn't simply make the types different to distinguish them.

Suppose the elements are already naturally foldable. Then foldr and foldl have the same type signature:

foldl, foldr :: (a -> a -> a) -> a -> [a] -> a

But sometimes they are only foldable qua another type. Call it r. If you work through Harald's reasoning, you'll arrive at:

foldr :: (a -> r -> r) -> r -> [a] -> r foldl :: (r -> a -> r) -> r -> [a] -> r

-- Kim-Ee

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