Is reduce() foldl() or foldr()?

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Sun Jun 7 12:42:20 CEST 2009


Calling all functional programming fans... is Python's built-in reduce() 
a left-fold or a right-fold?

Wikipedia says it's a left-fold:

http://en.wikipedia.org/wiki/Fold_(higher-order_function)

but other people say it's a right-fold, e.g.:

"... there is a `foldr` in Haskell that just works like `reduce()`"
http://mail.python.org/pipermail/python-list/2007-November/638647.html


and 

"Note that Python already has a variation of foldr, called reduce."
http://blog.sigfpe.com/2008/02/purely-functional-recursive-types-in.html


So which is correct? Or is it that different people have different 
definitions of foldl() and foldr()?


-- 
Steven



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