Thank you for finding MapCar. That does a lot of what I need.

So yeah, I just realized that fold_left can be written by simply calling reduce with the same arguments (so that fold_left's initial becomes the first element of reduce's input). But fold_right does require two reverses, in the following sense: suppose I wanted to compare left- and right-associative subtraction. Then, I have to reverse once to change a call of fold_right into fold_left but then I also have to change the order of arguments into the block.

Well, I really want sum to be reduce { $a + $b } 0, @input, so that would make multiplication reduce { $a * $b }, 1, @input. Note that sum is not reduce internally. Here's the code:

sum(...)
PROTOTYPE: @
CODE:
{
    SV *sv;
    int index;
    if(!items) {
        XSRETURN_UNDEF;
    }
    sv = ST(0);
    RETVAL = slu_sv_value(sv);
    for(index = 1 ; index < items ; index++) {
        sv = ST(index);
        RETVAL += slu_sv_value(sv);
    }
}
OUTPUT:
    RETVAL
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