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clojure

Loop endlessly on lazy-seq

Why does the second evaluation with butlast loop endlessly in Clojure?

user=> (->> (range) butlast lazy-seq (take 0))
()
user=> (->> (range) butlast lazy-seq first) ; ...

The equivalent in Haskell can be reasonably lazy-eval'ed.

ghci> take 0 . init $ [0..]
[]
ghci> head . init $ [0..]
0

Edit

take 0 is no-op as being pointed out below.

Solution

  • If you look at the implementation of butlast, you can see that it's eager (using loop instead of lazy-seq). An alternate lazy implementation would do what you want:

    (defn butlast'
  "Like clojure.core/butlast but lazy."
  [xs]
  (when (seq xs)
    ((fn f [[x & xs]]
       (if (seq xs)
         (lazy-seq (cons x (f xs)))
         ()))
     xs)))