Forgetting to force evaluation of lazy seqs
Lazy seqs aren't evaluated unless you ask them to be evaluated. You might expect this to print something, but it doesn't.
user=> (defn foo [] (map println [:foo :bar]) nil)
#'user/foo
user=> (foo)
nil
The map is never evaluated, it's silently discarded, because it's lazy. You have to use one of doseq, dorun, doall etc. to force evaluation of lazy sequences for side-effects.
user=> (defn foo [] (doseq [x [:foo :bar]] (println x)) nil)
#'user/foo
user=> (foo)
:foo
:bar
nil
user=> (defn foo [] (dorun (map println [:foo :bar])) nil)
#'user/foo
user=> (foo)
:foo
:bar
nil
Using a bare map at the REPL kind of looks like it works, but it only works because the REPL forces evaluation of lazy seqs itself. This can make the bug even harder to notice, because your code works at the REPL and doesn't work from a source file or inside a function.
user=> (map println [:foo :bar])
(:foo
:bar
nil nil)
