Hi, folks.
Just a quick newbie question.
One can create lazy sequences by wrapping each sequence element inside a
'lazy-seq' macro while constructing the sequence. Usually it's done with
recursion. But what if I need a tail recursive construction of the sequence
(using recur) accumulating results by appending them to the end of an
accumulator. It just works using 'conj' and vector until 'lazy-seq' is
added to the picture. It then breaks append to the end vector semantics and
the result is now reversed. Here is an example (just for illustration - no
real world value):
(defn list-set
"Returns a seq like s, except that the n-th element, using zero-based
index, is x."
[s n x]
{:pre [(not (neg? n))]}
(loop [s s, index 0, acc []]
(if-let [s' (seq s)]
(recur
(next s')
(inc index)
(conj acc
(if (= index n) x (first s'))))
acc)))
(list-set [:a :b :c :d] 1 :zzz)
=> [:a :zzz :c :d]
Now with 'lazy-seq':
(defn list-set
"Returns a seq like s, except that the n-th element, using zero-based
index, is x."
[s n x]
{:pre [(not (neg? n))]}
(loop [s s, index 0, acc []]
(if-let [s' (seq s)]
(recur
(next s')
(inc index)
(lazy-seq (conj acc
(if (= index n) x (first s')))))
acc)))
(list-set [:a :b :c :d] 1 :zzz)
=> (:d :c :zzz :a)
Is there a workaround or some idiomatic approach to this?
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