Adding metadata to a lazy sequence

Question

When I try to add metadata to an endless lazy sequence in Clojure, I get a stack overflow, and if I delete the metadata, it will be fine. Why with-metadoes adding a macro interrupt lazy seq?

First create an infinite seq of a very nice number:

(defn good []
  (lazy-seq 
    (cons 42
      (good))))

user> (take 5 (good))
(42 42 42 42 42)

Then add some metadata to each of the lazy-seq instances:

(defn bad []
  (lazy-seq 
    (cons 42
      (with-meta 
       (bad)
       {: padding 4}))))


user> (take 5 (bad))
java.lang.StackOverflowError (NO_SOURCE_FILE: 0)
  [Thrown class clojure.lang.Compiler $ CompilerException]

Try moving the metadata one level:

(defn also-bad []
  (with-meta 
   (lazy-seq 
     (cons 42
       (also-bad)))
   {:padding 4}))

user> (take 5 (foo))
java.lang.StackOverflowError (NO_SOURCE_FILE:0)
  [Thrown class clojure.lang.Compiler$CompilerException]

:

(defn also-works []
     (lazy-seq 
       (cons 4 
         (with-meta 
          () 
          {:a 5}))))

user> (also-works)
(4)
user> (meta (rest (also-works)))
{:a 5}
user> 

Answer 1

LazySeq , withMeta LazySeq. .

public final class LazySeq extends Obj implements ISeq, List{
    ...
    public Obj withMeta(IPersistentMap meta){
        return new LazySeq(meta, seq());
    }
    ...
}

seq() seq, . with-meta , , . , seq, .