0057-pretty-function.org

pretty-function

This small library makes debugging easier when you are using many anonymous functions. With pretty-functions you can give names to your lambdas.

Here is a traceback you’ll have with ordinal lambda:

POFTHEDAY> (defun bar ()
             (error "Hello World!"))

POFTHEDAY> (defun foo (func)
             (funcall func))

POFTHEDAY> (foo (lambda ()
                  (bar)))

->
Hello World!
   [Condition of type SIMPLE-ERROR]

Restarts:
 0: [RETRY] Retry SLY mREPL evaluation request.
 1: [*ABORT] Return to SLY's top level.
 2: [ABORT] abort thread (#<THREAD "sly-channel-1-mrepl-remote-1" RUNNING {1003785853}>)

Backtrace:
 0: (BAR)
 1: ((LAMBDA ()))
 2: (SB-INT:SIMPLE-EVAL-IN-LEXENV (FOO (LAMBDA NIL (BAR))) #A<NULL-LEXENV>)
 3: (EVAL (FOO (LAMBDA NIL (BAR))))

With pretty-function we should see the name instead of LAMBDA:

POFTHEDAY> (pretty-function:enable-pretty-function-printing)

POFTHEDAY> (foo (pretty-function:named-lambda bar-caller ()
                  (bar)))

->
Hello World!
   [Condition of type SIMPLE-ERROR]

Restarts:
 0: [RETRY] Retry SLY mREPL evaluation request.
 1: [*ABORT] Return to SLY's top level.
 2: [ABORT] abort thread (#<THREAD "sly-channel-1-mrepl-remote-1" RUNNING {100394D853}>)

Backtrace:
 0: (BAR)
 1: ((LAMBDA ()))
 2: (FOO #<FUNCTION (LAMBDA NIL) {2253906B}>)
 3: (SB-INT:SIMPLE-EVAL-IN-LEXENV (FOO (PRETTY-FUNCTION:NAMED-LAMBDA BAR-CALLER NIL (BAR))) #<NULL-LEXENV>)
 4: (EVAL (FOO (PRETTY-FUNCTION:NAMED-LAMBDA BAR-CALLER NIL (BAR))))

However, it does not work. Probably, because SLY turns off *pretty-print* when rendering the traceback. At least, it does not work in SBCL.

Almost the same result we’ll get if request a backtrace as a list:

POFTHEDAY> (block test
               (handler-bind ((error (lambda (c)
                                       (return-from test
                                         (sb-debug:backtrace-as-list 7)))))
                 (foo (pretty-function:named-lambda bar-caller ()
                        (bar)))))

(((FLET "H0" :IN TEST) #<unused argument>)
 (SB-KERNEL::%SIGNAL #<SIMPLE-ERROR "Hello World!" {10050B8763}>)
 (ERROR "Hello World!")
 (BAR)
 ((LAMBDA () :IN TEST))
 (FOO #<named-lambda BAR-CALLER>)
 ((LAMBDA ())))

Named function has its name only when it is rendered as function’s argument. But anyway, it is useful.

Probably, it will work better on other supported implementations: Allegro, Clisp, CMU, Lispworks or MCL.

More complex example uses another macro - named-lambda*. It allows to use of arbitrary form to return a current name of a lambda function:

POFTHEDAY> (defparameter *f*
             (let ((n 0))
               (pretty-function:named-lambda*
                   ;; a form to return a description
                   (format nil "counter=~A" n)
                   () ;; arguments
                 (incf n))))

POFTHEDAY> *f*
#<named-lambda counter=0>

POFTHEDAY> (funcall *f*)
1

POFTHEDAY> *f*
#<named-lambda counter=1>

POFTHEDAY> (funcall *f*)
2

POFTHEDAY> *f*
#<named-lambda counter=2>

You might want to make a shorter name like <counter=1>, then you need to use lower-level macro with-function-printer:

POFTHEDAY> (defparameter *f*
             (let ((n 0))
               (pretty-function:with-function-printer
                   ;; A lambda to wriite a description
                   (lambda (s) (format s "#<counter=~A>" n))
                 ;; A lambda to do a real job
                 (lambda ()
                   (incf n)))))

POFTHEDAY> *f*
#<counter=0>

POFTHEDAY> (funcall *f*)
1

POFTHEDAY> *f*
#<counter=1>

Of cause, with Lisp, you always can write your own syntax sugar around this macro.