Understanding How the Compile Function Works in SBCL Common Lisp

I was hoping someone can explain why the compile function is not working as I would expect.

First question:

* (compile 'square (lambda (x) (* x x)))
SQUARE
NIL
NIL

But then:

* (square 3)
; in: SQUARE 3
;     (SQUARE 3)
;
; caught STYLE-WARNING:
;   undefined function: COMMON-LISP-USER::SQUARE
;
; compilation unit finished
;   Undefined function:
;     SQUARE
;   caught 1 STYLE-WARNING condition

* (describe 'square)
COMMON-LISP-USER::SQUARE
  [symbol]

SQUARE names an undefined function
  Assumed type: FUNCTION

What does this mean?

Second question, starting over:

* (defun square (x) (* x x x))  ;faulty definition
    SQUARE
 * (compile 'square (lambda (x) (* x x)))  ;attempted correction
    SQUARE
    NIL
    NIL
    * (square 3)
    27

Here the old definition is still in place. However, the hyperspec says about compile: "If a non-nil name is given, then the resulting compiled function replaces the existing function definition of name and the name is returned as the primary value;".

(ps: my goal is to define (or update) a function programmatically, but have it be recognized by the sbcl static profiler--otherwise, it is ignored. An alternative is to use the statistical profiler, but it doesn't seem to work in windows-64.)

Edit 5/26/24 & 5/27/24: Thanks for the enlightening discussions. I tend to agree with ignis volens that sbcl's implementation of compile could be improved. Maybe someone with a deeper understanding of the details could discuss this with the maintainers. Re my initial objective of building lambda expressions that are recognized by the sbcl static profiler, I have a simplified example that for better or worse seems to work and readily integrates into existing structures, so I'll use this unless there's a downside:

* (defmacro init-fns (name1 name2)
  `(progn (defun ,name1 ())  ;define dummy fns at top-level
          (defun ,name2 ())))
INIT-FNS
* (init-fns square cube)
CUBE
* (defun build-fns ()
  (setf (symbol-function 'square)
        (compile nil '(lambda (x) (* x x))))
  (setf (symbol-function 'cube)
        (compile nil '(lambda (x) (* x x x)))))
BUILD-FNS
* (build-fns)
#<FUNCTION (LAMBDA (X)) {2479F75B}>
* (square 3)
9
* (cube 3)
27

Solution

Compiling an already compiled function in SBCL

The problem is very subtle:

(compile 'square (lambda (x) (* x x)))

COMPILE is a function. Now look at (lambda (x) (* x x)). What is it evaluated?

> (compiled-function-p (lambda (x) (* x x)))
T

It is a compiled function!

In SBCL all functions are by default compiled.

Now we call COMPILE on an already compiled function.

What's going on?

we call COMPILE with a name and a compiled function
SBCL does not compile the function, it is already compiled
SBCL does not set the name, since it hasn't compiled the already compiled function

One can argue that it nevertheless should set the function. Best to discuss that with the maintainers

Compiling source code

This works:

CL-USER> (compile 'square '(lambda (x) (* x x)))
SQUARE
NIL
NIL

CL-USER> #'square
#<FUNCTION SQUARE>

It works because '(lambda (x) (* x x)) evaluates to a list, which is a valid LAMBDA macro form. The compiler will compile this source successfully and then set the symbol function of SQUARE to the resulting compiled function.

Compare with another CL implementation, here LispWorks

Compare with LispWorks, which does not compile functions by default.

CL-USER 134 > (let ((f (compile nil '(lambda (x) (* x x)))))
                (compile 'square f))
;;;*** Warning in SQUARE: The definition supplied for SQUARE is already compiled.
SQUARE
((SQUARE #<CONDITIONS::SIMPLE-STYLE-WARNING 80101F2C9B>))
NIL

CL-USER 135 > (symbol-function 'square)
#<Function 14 8020000CB9>

LispWorks warns that the function is already compiled and sets the symbol function of SQUARE anyway.