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Comment by Y_Y | Hacker News Reader

What I learned, many years ago, is that foldr was a footgun, and that foldl1' was almost certainly what I wanted.

Don't you mean foldl (non-strict version) is a footgun? I can't think of how foldr is a footgun, at least not in a lazy language.

    print (foldl const 0 [undefined, undefined])   -- 0
    print (foldr const 0 [undefined, undefined])   -- Exception: Prelude.undefined

Oh haha. I was thinking performance-wise, but yeah if you want to pass around undefineds..

I'm not sure what the point you're trying to illustrate is, but I think you forgot a `flip` (unless the point is that you always mix up the argument order of the foldr reducing function, in which case fair enough).

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