[Haskell-beginners] foldr on infinite list to decide prime number

Francesco Ariis fa-ml at ariis.it
Wed Feb 3 01:55:09 UTC 2016

On Tue, Feb 02, 2016 at 05:52:08PM -0700, derek riemer wrote:
> Doesn't foldr have to "reach" to the far right of the list of infinite
> primes to get the last number since it consumes from the right?

foldl is /left/ associative, foldr /right/ associative.

λ> foldl1 (-) [1..3]
-- which is: ((1-2)-3)

λ> foldr1 (-) [1..3]
-- which is (1-(2-3))

Haskell being non strict (i.e. "proceeding from the outside"), it will
immediately short circuit on foldr but have to build the whole list first
on foldl.

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