What happened with lazy evaluation of Clojure

Question

I'm twisting my old java/python head the clojure way. Please help me to understand the lazy feature of clojure.

=> (def myvar (lazy-seq [1 2 (prn "abc")]))
#'user/myvar

The above is easy to understand. Since it's a lazy sequence, the (prn "abc") will not be evaluated, hence nothing printed.

=> (def myvar (lazy-seq [1 2 (prn undefined-var)]))
CompilerException java.lang.RuntimeException: Unable to resolve symbol: undefined-var in this context, compiling:(NO_SOURCE_PATH:1) 

The above will raise an error as you can see. Why ?

My (wrong)understanding is, since it's lazy, the (prn undefined-var) could be legally here even the "undefined-var" has not been defined yet.

Please anyone point my understanding to the correct way.

Answer 1

When the clojure reader finds

 (def myvar (lazy-seq [1 2 (prn undefined-var)]))

It needs to compile it, that is the reason why it throws the error, as the undefined-var is not defined. In the first case, it compiles ok, but it is not executed until you consume the seq.