Nicer way to write folding? (list reversal?)

[andrew at acooke.org]

Hi,

In the code below, foldr2 is clearly uglier than the rest.  Is there a
nicer way (without destructive reversal - yuck - of the list
argument)?  Also, are these things built-in (can't find them)?  Is
recursion any faster than it used to be (I've not been following
Python development so am wondering if anything in 2 (or 2.2) would
help; foldr2 and fold2 are expected to be faster than foldl and foldr,
although I've not done any timing).

Thanks,
Andrew

# foldr: f(x1, f(x2, ... f(xn-1, f(xn, c))...)) 

# foldl: f(xn, f(xn-1, ... f(x2, f(x1, c))...)) 

def foldr(action, start, list):
        if not list: return start
        else: return action(list[0], foldr(action, start, list[1:]))

def foldl(action, start, list):
        if not list: return start
        else: return foldl(action, action(list[0], start), list[1:])

def foldr2(action, start, list):
        for i in range(len(list), 0, -1): start = action(list[i-1], start)
        return start

def foldl2(action, start, list):
        for x in list: start = action(x, start)
        return start            

print foldr(operator.add, "x", ["a", "b", "c"])
print foldl(operator.add, "x", ["a", "b", "c"])
print foldr2(operator.add, "x", ["a", "b", "c"])
print foldl2(operator.add, "x", ["a", "b", "c"])

abcx
cbax
abcx
cbax

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