Terminating sum early

General Usage
7

EDIT: This is probably not what you want, give me a sec.

How about

function mapreduceuntil(op, unit, xs; sentinel=(x->false))
    u = iterate(xs)
    u === nothing && error("empty reduction")
    y = unit(u[1])
    while u != nothing
        x, s = u
        sentinel(x) && break
        y = op(y, x)
        u = iterate(xs, s)
    end
    y
end

mapreduceuntil(+, zero, [1.0,2.0,Inf], sentinel=isinf)
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