Clojure apply vs map

Question

I have a sequence (foundApps) returned from a function and I want to map a function to all it's elements. For some reason, `apply` and `count` work for the sequnece but `map` doesn't: ``` (apply println foundApps) (map println rest foundApps) (map (fn [app] (println app)) foundApps) (println (str "Found " (count foundApps) " apps to delete")))) ``` Prints: ``` {:description another descr, :title apptwo, :owner jim, :appstoreid 1235, :kind App, :key #} {:description another descr, :title apptwo, :owner jim, :appstoreid 1235, :kind App, :key #} Found 2 apps to delete for id 1235 ``` So `apply` seems to happily work for the sequence, but `map` doesn't. Where am I being stupid?

Accepted Answer

Most likely you're being hit by `map`'s laziness. (`map` produces a lazy sequence which is only realised when some code actually uses its elements. And even then the realisation happens in chunks, so that you have to walk the whole sequence to make sure it all got realised.) Try wrapping the `map` expression in a `dorun`:

```
(dorun (map println foundApps))
```

Also, since you're doing it just for the side effects, it might be cleaner to use `doseq` instead:

```
(doseq [fa foundApps]
  (println fa))
```

Note that `(map println foundApps)` should work just fine at the REPL; I'm assuming you've extracted it from somewhere in your code where it's not being forced. There's no such difference with `doseq` which is strict (i.e. not lazy) and will walk its argument sequences for you under any circumstances. Also note that `doseq` returns `nil` as its value; it's only good for side-effects. Finally I've skipped the `rest` from your code; you might have meant `(rest foundApps)` (unless it's just a typo).

Also note that `(apply println foundApps)` will print all the `foundApps` on one line, whereas `(dorun (map println foundApps))` will print each member of `foundApps` on its own line.

Clojure apply vs map

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