Map (running) pairs with f and reduce with r

mappairreduce.jl

function mappairreduce(r, f, itr, dec)
    ϕ  = iterate(itr)
    if ϕ === nothing
        return dec
    end
    xlast, state = ϕ
    while true
        ϕ = iterate(itr, state)
        if ϕ === nothing 
            return dec
        end
        x, state = ϕ
        dec = r(dec, f(xlast, x))
        xlast = x
    end
end
mappairreduce(&, <, 1:10, true)