"apply map vector" idiom - How happens to be 2 functions?

Question

Here is a sample from my mini code where I copied from clojure docs site. ``` (apply map vector (vec jpgList)) ``` I guess map and vector are both functions, but apply takes only one function. How come in here apply takes two functions?

Accepted Answer

Read the documentation of `apply`:

```
user=> (doc apply)
-------------------------
clojure.core/apply
([f args] [f x args] [f x y args] [f x y z args] [f a b c d & args])
  Applies fn f to the argument list formed by prepending intervening arguments to args.
nil
```

So, `(apply map vector (vec jpgList))` corresponds to `f x args`, so `map` will be applied to the the function `vector`, followed by the elements of `(vec jpgList)`. Unlike Haskell, Clojure's `map` supports multiple collections to operate on. `(vec jpgList)` presumably is a nested vector, or list, like in the following example:

```
user=> (apply map vector [[1 2 3] [4 5 6]])
([1 4] [2 5] [3 6])  
```

What happened is, every element produced by `map` is the vector of each nth element of the elements of the nested vector. This function is also known as `transpose` in matrix operations.

"apply map vector" idiom - How happens to be 2 functions?

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