Question

I’m trying to do something similar to Python’s func(*lst) but with Clojure, and without using the apply function. My admittedly silly use case is:

{:k1 v1 (cond exp '(:k2 v2) :else '(:k3 v3))}

So if exp was true, the dict would contain {:k1 v1 :k2 v2}, otherwise {:k1 v1 :k3 v3}. I basically want a Python-esque * applied to the return value of cond. I tried playing around with data/code modes with ‘, `, and ~, though didn’t find a solution. I can repeat the cond for the individual parameters to the underlying hash-map but that kind of defeats the point.

Why? I just think it’d be cool if Clojure can do it easily. 🙂

Answer 1

No. A single form can only be a single form: it cannot magically be two of them. If this were possible, all kinds of things would break.

In your particular example, the easy answer is

(apply hash-map :k1 v1 
                (cond exp   '(:k2 v2)
                      :else '(:k3 v3))

The only way to do this is apply, which turns a single function parameter into zero or more function parameters, by expanding them as a list. It cannot work at the source-code level for use in things like a hash literal.

Edit: I don’t know much Python, but I’m pretty sure Python can’t do this either. You can splat things into function calls, but not straight into source. You can’t write

test_expr = ((x == 2), return x)
if *test_expr

or anything like it – it just isn’t possible because the compiler has to analyze the if before it can understand what to do with test_expr. Likewise in Clojure, the compiler has to analyze the hash literal before it can understand what to do with the objects inside – it can’t know that you “want” them expanded into the map expression somehow.