I have a solution :
(defn unfold [f & as]
(if-let [[hd new-as] (apply f as)]
(lazy-seq
(cons hd (apply unfold f new-as)))
()))
unfold could be called co-reduce or coreduce. It is the dual operation
of reduce, in category theory.
Instead of reducing a seq to create a value, it generates a seq from a
value. (generate is sometimes a name for that, too)
I don't know if it is somewhere in core or contrib.
With this function, it is very simple:
(defn nil-coalesce [s1 s2]
(unfold #(and %1
(if (first %1)
[(first %1) [(next %1) %2]]
[(first %2) [(next %1) (next %2)]])) s1 s2))
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