Common Lisp: Beginner's trouble with funcall

I'm trying to pass a function as an argument and call that function within another function.

A piece of my code looks like this:

(defun getmove(strategy player board printflag)
(setq move (funcall strategy player board))
(if printflag
    (printboard board))

str开发者_JAVA百科ategy is passed as a symbol represented in a two dimensional list as something such as 'randomstrategy

I keep getting the error: "FUNCALL: 'RANDOMSTRATEGY is not a function name; try using a symbol instead...

When I replace strategy with 'randomstrategy it works fine. I can also call randomstrategy independently. What is the problem?

The problem is that the variable strategy does not contain the symbol randomstrategy but rather the list (!) 'randomstrategy (which is a shorthand notation for (quote randomstrategy)).

Now, you could, of course, extract the symbol from the list by way of the function second, but that would only cover the real problem up, which is probably somewhere up the call chain. Try to determine why the argument that is passed to function getmove is 'randomstrategy, not randomstrategy as it should be. (Maybe you erroneously used a quote inside of a quoted list?)

Oh, and don't let yourself be confused by the fact that (funcall 'randomstrategy ...) works: the expression 'randomstrategy does not, after all, evaluate to itself, but to the symbol randomstrategy.

Is strategy a variable with a functional value? If not, then use the #' syntax macro before it, i.e. #'strategy, or just (if the function is global) 'strategy.

WHY? Because arguments of a funcall call are evaluated. And your strategy symbol is just a variable name in this case. Variable this value 'RANDOMSTRATEGY. But you should give to funcall a function. How to access function if we have a symbol?

Three cases:

1. Symbol may denote a variable with functional value.
2. Symbol may denote a global function (symbol-function is the accessor in this case.
3. Symbol may denote a local function (flet, labels and so on).

It looks like you forgot to define RANDOMSTRATEGY function.

(defun RANDOMSTRATEGY …)

Hmm

FUNCALL: 'RANDOMSTRATEGY

Maybe you have (setq strategy ''RANDOMSTRATEGY)?

Then strategy will evaluate to 'RANDOMSTRATEGY. Did you notice ' before the symbol name? 'RANDOMSTRATEGY <=> (quote RANDOMSTRATEGY); it is not a proper function name.

Have you set strategy anywhere? It looks like a scoping issue.

Try this

(setq strategy 'randomstrategy)
(setq move (funcall strategy player board))

Not seeing the code, I'm imagining you're doing something like this:

(defun randomstrategy (a b c) ...)

and then doing this:

(getmove 'randomstrategy x y z)

What you want to do is pass the function "randomstrategy" to getmove using #':

(getmove #'randomstrategy x y z)

In CommonLisp, #' yields the function bound to the symbol, which is what you want to pass to getmove.