Question

I am looking through some example Fibonacci sequence clojure code:

 (def fibs (lazy-cat [1 2] (map + fibs (rest fibs))))

I generally understand what is going on, but don’t get the point of lazy-cat.
I know that lazy-cat is a macro that is translating to something like this:

(def fibs (concat (lazy-seq [1 2]) (lazy-seq (map + fibs (rest fibs))))) 

What exactly is lazy-seq accomplishing? It would still be evaluated lazily even without lazy-seq? Is this strictly for caching purposes?

EDIT: Thanks for the answers. My confusion was that it worked with a plain concat from the REPL because I had a previous binding to fibs in scope.

Answer 1

The lazy-seq on [1 2] is not needed, but doesn’t really hurt.

The lazy-seq on (map + fibs (rest fibs)) is essential; without it, the function call will be evaluated before fibs is bound to a value, which will cause an exception. By wrapping it in lazy-seq, the call will be deferred until the value is needed, and fibs will have a value at that point.