Why do we have to use funcall to call higher order functions in Common Lisp? For example, why do we have to use:
(defun foo (test-func args)
(funcall test-func args))
instead of the simpler:
(defun bar (test-func args)
(test-func args))
Coming from a procedural background, I'm a bit surprised by that since the languages I'm more used to (e.g. Python, C#) don't need the distinction. In particular, on the source level at least, the C# compiler transforms it to something like func.invoke().
The only problem I see is that this would mean we couldn't call a global function test-func anymore because it'd be shadowed, but that's hardly a problem.
Strictly speaking, funcall would not be needed, but there are some Lisps (Lisp-2 variants, such as Common Lisp) that separate the variable name space and the function name space. Lisp-1 variants (e.g. Scheme) do not make this distinction.
More specifically, in your case, test-func is in the variable name space.
(defun foo (test-func args)
(funcall test-func args))
Therefore you need a construct that actually searches the function object associated with this variable in the variable name space. In Common Lisp this construct is funcall.
See also this answer.