Can somebody explain why the function eval behaves like this?
(eval (list 'cons t nil)) returns (T)
(eval (list 'cons 'a nil)) causes an error
(eval (list 'cons ''a nil)) returns (A)
Thanks a lot.
First:
(CONS T NIL)
T is a constant and returns T when evaluated. NIL is also a constant and evaluates to NIL. (CONS T NIL) then returns (T . NIL), which is shorter written as (T).
Second:
(CONS A NIL)
A is a variable. It is possibly undefined. Evaluating it will lead to an error when A is undefined.
Third:
Now you should think about the third form...
One more thing that you may want to note is that third form is embedding the symbol A in the list. Usually this is the form which is mostly taught in Lisp books for learning by experimenting on REPL. However in actual programs / functions, you may be using initially more of putting value or list represented by A in the list and not the symbol A.
e.g.
(setf a 2)
(eval (list 'cons a nil)) => (2) [A is evaluated before list is evaluated; (eval '(cons 2 nil))]
(eval (list 'cons 'a nil)) => (2) [A is evaluated when (eval '(cons a nil)) is evaluated]
(eval (list 'cons ''a nil)) => (A) [A does not get evaluated as call is (eval '(cons 'a nil)); 'a is symbol A]
If you don't do (setf a 2) in the beginning, 1st and 2nd both forms will give error. This is because when a is evaluated, it is not bounded (i.e. crudely, it does not have any value associated with it)
Related
I want some reader macros to print as as shortened expression that the macro understands. Lets say I want to extend the #' macro to take #'~[rest-of-symbol] and turn that into (complement #'rest-of-symbol).
What controls how that is printed? On SBCL, for instance, '(function +) prints as #'+. How do i make '(complement #'listp) print as #~listp?
My first thought was
(defmethod print-object :around ((obj cons) stream)
;; if #'~fn-name / (complement (function fn-name))
;; => fn-name otherwise NIL
(let ((fn-name
(ignore-errors
(destructuring-bind (complement (function fn-name))
obj
(when (and (eq complement 'complement)
(eq function 'function))
fn-name)))))
(if fn-name
(format stream "#'~~~S" fn-name)
(call-next-method))))
This works insofar as (print-object '(complement #'evenp) *standard-output*) prints it the way I want, but the REPL doesn't. Also (print-object '#'+ *standard-output*) prints it as (function +) so the REPL isn't using print-object. With defining the print-object method for user defined classes the REPL always picks up on the new definition.
This is my first post and I'm sorry I can't get the code to format properly. If someone can put a link on how to do that I would appreciate it.
Evaluation time
You are mixing code with data in your example:
(function +)
Is a special form that evaluates to a function object, which admits a shorter syntax:
#'+
But when you are writing:
'(function +)
or
'(complement fn)
Then in both cases you are writing quoted, literal lists, which evaluates to themselves (namely a list starting with symbol function or complement, followed respectively by symbol + and fn).
However, you want the code to be evaluated at runtime to actual function objects; if you type this in the REPL:
(complement #'alpha-char-p)
The result is a value that is printed as follows:
#<FUNCTION (LAMBDA (&REST SB-IMPL::ARGUMENTS) :IN COMPLEMENT) {101AAC8D9B}>
You have an actual function object that you can funcall. In other words, by the time you reach print-object, you no longer have access to source code, you are manipulating data at runtime which happens to be functions. So you cannot use destructuring-bind to get the complement symbol that was present in the source code.
What you need to do instead is to attach metadata to your function. There is a way to do that in Common Lisp by defining a new type of function, thanks to the Meta-Object Protocol.
Funcallable objects
I'm relying on Closer-MOP for all the symbols prefixed with c2cl: below. I define a new class of functions, annotated-fn, which is a function with addditional data:
(defclass annotated-fn (c2cl:funcallable-standard-object)
((data :initform :data :initarg :data :reader annotated-fn-data))
(:metaclass c2cl:funcallable-standard-class))
Notice that this class is a funcallable-standard-object (like the usual functions), and its metaclass is funcallable-standard-class. Such an object has an additional implicit slot that is a function to call.
More precisely, you have to call c2cl:set-funcallable-instance-function to set a function associated with the object, and when later you use funcall or apply with the object, then the wrapped function is called instead. So you can transparently use this class of functions wherever you usually use a function. It just has additional slots (here data).
For example, here is how I instantiate it, with a function to wrap additional data:
(defun annotate-fn (function data)
(let ((object (make-instance 'annotated-fn :data data)))
(prog1 object
(c2cl:set-funcallable-instance-function object function))))
Let's try it:
(describe
(annotate-fn (constantly 3)
'(:category :constantly)))
#<ANNOTATED-FN {1006275C7B}>
[funcallable-instance]
Lambda-list: UNKNOWN
Derived type: FUNCTION
Documentation:
T
Source file: SYS:SRC;CODE;FUNUTILS.LISP
Slots with :INSTANCE allocation:
DATA = (:CATEGORY :CONSTANTLY)
You can also use this object like any other function.
Now, your reader macros can expand into calls to annotate-fn, and add any kind of additional metadata you need to the function.
Reader macro
For our example, imagine you define a reader macros for constant functions:
(set-macro-character #\[ 'read-constantly t)
(set-macro-character #\] (get-macro-character #\) nil))
(defun read-constantly (stream char)
(declare (ignore char))
(let* ((list (read-delimited-list #\] stream t))
(value (if (rest list) list (first list)))
(var (gensym)))
`(let ((,var ,value))
(annotate-fn (constantly ,var)
(list :category :constantly
:constant ,var)))))
Using this syntax:
> [(+ 8 5)]
=> #<ANNOTATED-FN ...>
By the way, the syntax I defined also allows the following:
> [+ 8 5]
Pretty-printing
Let's define a generic function that prints an annotated function given its :category field:
(defgeneric print-for-category (category data object stream))
(defmethod print-object ((o annotated-fn) s)
(let* ((data (annotated-fn-data o))
(category (getf data :category)))
(print-for-category category data o s)))
Then, we can specialize it for :constantly, and here we assume also that the data associated with the function contains a :constant field:
(defmethod print-for-category ((_ (eql :constantly)) data o s)
(format s "[~s]" (getf data :constant)))
For example:
(let ((value (+ 8 6)))
(annotate-fn (constantly value)
`(:constant ,value
:category :constantly)))
This above is printed as:
[14]
Which would be the same as your hypothetical reader macro.
To do this you need to understand the pretty printer. I have understood it in the past but no longer do completely. It dispatches on type and the trick for things like this is that you can specify very specific types for trees of conses, although doing so is verbose.
Here is an example which is almost certainly not completely correct, but does achieve what you want in this case:
(defparameter *ppd* (copy-pprint-dispatch))
(defun pprint-complement-function (s form)
;; This is the thing that the pretty printer will call. It can
;; assume that the form it wants to print is already correct.
(destructuring-bind (complement (function name)) form
(declare (ignore complement function))
(format s "#'~~~W" name)))
;;; Now set this in the table with a suitable hairy type specification
;;;
(set-pprint-dispatch '(cons (eql complement)
(cons (cons (eql function)
(cons t null))
null))
'pprint-complement-function
0
*ppd*)
And now
> (let ((*print-pprint-dispatch* *ppd*))
(pprint '(complement (function foo)))
(pprint '((complement (function foo)) (function foo))))
#'~foo
(#'~foo #'foo)
You can make the awful nested cons type specifier easier by defining this (which, perhaps, should be the compound type specifier for list except you can't do that):
(deftype list-of-types (&rest types)
(labels ((lot (tt)
(if (null tt)
'null
`(cons ,(first tt) ,(lot (rest tt))))))
(lot types)))
And then
(set-pprint-dispatch '(list-of-types (eql complement)
(list-of-types (eql function)
*))
'pprint-complement-function
0
*ppd*)
is perhaps easier to read.
I am trying to use generic functions' ability to specify behaviour based on the first argument of a list.
In other words, I want the list (atypelist 1 2 3) and the list (btypelist 1 2 3) to have their individual behaviour when passed to foo. So far, this is what I came up with:
(deftype atypelist (lst)
`(eq (car ,lst) 'atypelist))
(deftype btypelist (lst)
`(eq (car ,lst) 'btypelist))
(defmethod foo ((lst atypelist))
(format nil "success atypelist: ~S" lst))
(defmethod foo ((lst btypelist))
(format nil "success btypelist: ~S" lst))
However, when I call (typep (list 'atypelist 1 2 3) 'atypelist) I get the following error:
error while parsing arguments to DEFTYPE ATYPELIST:
too few elements in
()
to satisfy lambda list
(LST):
exactly 1 expected, but got 0
I am guessing the error is in my definition of atypelist.
Questions:
Is there a better way to get the functionality I am looking for?
If yes - what is the way?
If not - how to properly define a type on a list/cons that has a particular symbol in the car?
Before I start: what you want to do can't work, and is confused in two ways.
Firstly deftype defines a type in terms of other type specifiers: the body of a deftype form must expand into a type specifier, not an expression, as yours does. And deftype's arguments are not the thing you want to check the type for, they are parts of the type specification.
In this case you want to specify that the thing is a cons, and that its car is eql to something. Fortunately there are specializing type specifiers for both of these things, and you end up with something like this:
(deftype cons-with-specified-car (x)
`(cons (eql ,x) t))
And now
> (typep '(1) '(cons-with-specified-car 1))
t
> (typep '(a) '(cons-with-specified-car a))
t
> (typep '() '(cons-with-specified-car a))
nil
And if you want:
(deftype cons-with-a ()
'(cons-with-specified-car a))
and now
> (typep '(a) 'cons-with-a)
t
Secondly none of this will work because this it not how CLOS works. CLOS dispatches on classes not types, and you have merely defined a type, not a class: your method definitions simply cannot work, since classes cannot be parametrized in this way like types can.
Some ways you might achieve what you want.
If what you want to do is to dispatch on the first element of a list, then the obvious approach, if you want to use CLOS, is to use a two-level approach where you first dispatch on the class of the thing (cons is a class), and then use eql specializers to pick out the things you want.
(defgeneric select (it)
(:method ((it cons))
(select* (car it) it))
(:method (it)
nil))
(defgeneric select* (key it)
(:method (key it)
(format t "~&unknown key ~S in ~S~%" key it)))
(defmethod select* ((key (eql 'a)) it)
(format t "~&~S begins with a~%" it))
However in a case like this, unless you very much want the extensibility that CLOS gets you (which is a good reason to use CLOS here), I'd just use typecase. You could do this using the type defined above:
(defun select (it)
(typecase it
((cons-with-specified-car a)
'(cons a))
(cons
'cons)
(t
nil)))
or, probably simpler, just use what the deftype expands into:
(defun select (it)
(typecase it
((cons (eql a) t)
'(cons a))
(cons
'cons)
(t
nil)))
Finally probably what anyone doing this would actually write (again, assuming you do not want the extensibility CLOS gets you) is:
(defun select (it)
(typecase it
(cons
(case (car it)
...))
(t
...)))
Here is a possible solution, using the type specifier satisfies:
CL-USER> (defun is-atypelist (list)
(eq (car list) 'atypelist))
IS-ATYPELIST
CL-USER> (defun is-btypelist (list)
(eq (car list) 'btypelist))
IS-BTYPELIST
CL-USER> (deftype atypelist ()
`(satisfies is-atypelist))
ATYPELIST
CL-USER> (deftype btypelist ()
`(satisfies is-btypelist))
BTYPELIST
CL-USER> (typep (list 'atypelist 1 2 3) 'atypelist)
T
CL-USER> (typep (list 'atypelist 1 2 3) 'btypelist)
NIL
Note that this does not define a class, but a type, if this is what you need.
Is there a better way to get the functionality I am looking for?
1. Wrap your lists in container types
(defclass lst () ((items :initarg :items :reader items)))
(defclass alst (lst) ())
(defclass blst (lst) ())
It may be a little bit more cumbersome to work with but this is pretty much straightforward and not too suprising.
2. Douple-dispatch
(defgeneric foo (val))
(defgeneric foo/tag (tag val))
For example:
(defmethod foo ((c cons))
(destructuring-bind (tag . list) c
(foo/tag tag list)))
3. Define a custom method combination
It should be possible to hack the meta-object protocol dispatch mechanism to dispatch on the first item of a list. I wouldn't recommend it however.
4. Use a different dispatch mechanism
Use a completely different dispatching mechanism outside of CLOS, like pprint-dispatch does. For example you may want to use trivia or optima pattern-matching libraries, or cl-algebraic-data-type. This may be more useful if you are dealing with trees of symbols.
I'm trying to understand dynamical and lexical bindings of a variable.
I was browsing SO and some links, when I encountered a problem on this one :
https://www.emacswiki.org/emacs/DynamicBindingVsLexicalBinding#toc2
I ran this code :
(let ((a 1))
(let ((f (lambda () (print a))))
(let ((a 2))
(funcall f))))
With expected result : 1 (and a warning which says that the second a variable is unused, which is normal).
Next, I tried :
(defvar a 99)
And reran the first code. Result is 2, like the tutorial says. Then, to experiment further, I tried to remove the dynamically binding variable a, to get again 1.
I tried to (makunbound 'a) or even (setq a 55) (I tried this by default, I think global lexical binding depends on implementation if I understand correctly...). makunbound seems to remove the symbol, but the "dynamically binding state" seems to be saved anyway. The result is still 2.
How can I reset Common Lisp to the previous state (before I dynamically bind the a variable) ? Restart SLIME do the trick, but I would rather have a way to do it programatically...
Thank you for your answer.
There is no portable way to revert the
special
proclaimation.
(CLISP offers
notspecial).
However, you can use unintern
to some extent: it will make new code treat your symbol as not
special because it is now a different symbol:
(defun test-a ()
(let ((a 1))
(let ((f (lambda () (print a))))
(let ((a 2))
(funcall f)))))
(test-a)
==> 1
(defvar a)
(test-a)
==> 2
now, let us try to "revert" defvar:
(unintern 'a)
(test-a)
==> 2
oops! Let us see:
(fdefinition 'test-a)
#<FUNCTION TEST-A NIL (DECLARE (SYSTEM::IN-DEFUN TEST-A))
(BLOCK TEST-A
(LET ((#:A 1)) (LET ((F (LAMBDA NIL (PRINT #:A)))) (LET ((#:A 2)) (FUNCALL F)))))>
you see, test-a is still using the old symbol a which is now uninterned (so printed as #:A). To get back, you need to re-eval the defun above and then you get
(test-a)
==> 1
again!
I am working through the MP3 database example in Peter Seibel's Practical Common Lisp. Seibel demonstrates how macros can be used to shorten the code for the where function; so now, I am trying to use a macro to shorten the code for the update function. (The original version of the update function is included for reference.) When I run my code, the following error originates from the second-to-last line --
*** - CAR: TERMS is not a list
What am I doing wrong? Here is my code.
(defvar *db* nil)
(defun add-record (cd)
(push cd *db*))
(defun dump-db ()
(dolist (cd *db*)
(format t "~{~a:~10t~a~%~}~%" cd)))
(defun make-cd (title artist rating ripped)
(list :title title :artist artist :rating rating :ripped ripped))
(defun prompt-read (prompt)
(format *query-io* "~a: " prompt)
(force-output *query-io*)
(read-line *query-io*))
(defun prompt-for-cd ()
(make-cd
(prompt-read "Title")
(prompt-read "Artist")
(or (parse-integer (prompt-read "Rating") :junk-allowed t) 0)
(y-or-n-p "Ripped [y/n]: ")))
(defun add-cds ()
(loop (add-record (prompt-for-cd) )
(if (not (y-or-n-p "Another? [y/n]: ")) (return) )))
(defun save-db (filename)
(with-open-file (out filename
:direction :output
:if-exists :supersede)
(with-standard-io-syntax
(print *db* out))))
(defun load-db (filename)
(with-open-file (in filename)
(with-standard-io-syntax
(setf *db* (read in) ))))
(defun select (selector-fn)
(remove-if-not selector-fn *db*))
(defun make-comparison-expr (field value)
`(equal (getf cd ,field) ,value))
(defun make-comparison-list (func fields)
(loop while fields
collecting (funcall func (pop fields) (pop fields))))
(defmacro where (&rest clauses)
`#'(lambda (cd) (and ,#(make-comparison-list 'make-comparison-expr clauses))))
(defun make-update-expr (field value)
`(setf (getf row ,field) ,value))
(defmacro make-update-list (fields)
(make-comparison-list 'make-update-expr fields))
(defun update (selector-fn &rest terms)
(print (type-of terms))
(setf *db*
(mapcar
#'(lambda (row)
(when (funcall selector-fn row)
(make-update-list terms))
row)
*db*)))
;(defun update (selector-fn &key title artist rating (ripped nil ripped-p))
; (setf *db*
; (mapcar
; #'(lambda (row)
; (when (funcall selector-fn row)
; (if title (setf (getf row :title) title) )
; (if artist (setf (getf row :artist) artist) )
; (if rating (setf (getf row :rating) rating) )
; (if ripped-p (setf (getf row :ripped) ripped) ))
; row)
; *db*)))
(defun delete-rows (selector-fn)
(setf *db* (remove-if selector-fn *db*)))
;(loop (print (eval (read))))
(add-record (make-cd "Be" "Common" 9 nil))
(add-record (make-cd "Like Water for Chocolate" "Common" 9 nil))
(add-record (make-cd "Be" "Beatles" 9 nil))
(dump-db)
(update (where :artist "Common" :title "Be") :rating 8)
(dump-db)
-----Edit-----
I figured it out. The solution was to make update a macro and to make make-update-list a function. This way, make-update-list could evaluate fields at run-time and update can still abstract away some tedious if statements. Here is the updated update and make-update-list below:
(defun make-update-list (fields)
(make-comparison-list 'make-update-expr fields))
(defmacro update (selector-fn &rest terms)
`(setf *db*
(mapcar
#'(lambda (row)
(when (funcall ,selector-fn row)
,#(make-update-list terms))
row)
*db*)))
Macroexpansion of that make-update-list is done in a separate phase (called "macroexpansion phase") - which occurs around the time a piece of code is compiled or loaded; in this case we're talking about compilation / loading of update. The macro gets expanded with fields bound to the symbol terms, which (the symbol itself) is used as a value in make-comparison-list; I suppose that was not what you expected.
Note, if you go and compile the file line-by-line (C-c C-c in Emacs + SLIME), it'll tell you right during compilation of update that the macro expansion fails because "the value TERMS is not of type LIST".
Generally, think of macros as functions that take in their arguments unevaluated - i.e. a form (make-update-list foo) will get expanded with the macro parameter's fields value bound to foo. What you're trying to achieve here - code generation based on run-time values - is a bit more difficult to do.
You are trying to take the car of a symbol!
> (car 'terms)
*** - CAR: TERMS is not a list
Think of macros as a function that, when used, replaces the code with the result of the macro function everywhere it's used. At this time variables are just symbols and have no meaning besides that.
When you do (make-update-list terms) it will call the macro function with the argument fields being the symbol you passed, which is terms. Since it's a symbol it cannot be iterated like you are trying. You may iterate it at runtime when it surely is a list, but as a macro it isn't a list until you are passing it a list like (make-update-list (title artist rating ripped)).
If it is dynamic in runtime then your macro needs to expand to code that does most of its magic at runtime. Thus a macro is just a source rewriting service and should not have anything to do with what variable might be at runtime since then it has already done its thing.
I just started writing this function and I was wondering if there was a way, that if just the &key argument was entered, the &optional list could be ignored.
(defun test (&optional arg (i 0) &key (size s))
...)
I would like to be able to run
(test arg)
or
(test arg i)
but also
(test :size)
Now this is a better mock up but I don't know where to put :size in params list
(defun test (&optional arg (i 0))
(cond ((eq arg nil) (return-from test (test-1)))
((listp arg)
(return-from test (test-2 arg)))
((pointerp arg) (mem-aref (test-3 arg) :int i))
(:size (size-test arg))
(t nil)))
so i can run (test) and get:
<output of (test-1)>
I can run (test '(1 2 3)) and get:
<output of (test-2 arg)>
I can run (test <pointer> 0)
and output is:
<output of (mem-aref (test-3 arg) :int i)>
I can run (test :size) and get:
<output of (test-size arg)>
Mixing optional and keyword arguments
Mixing optional and keyword arguments is still something that's not all that easy to do. If a function accepts an optional argument, then you won't be able to use the keyword arguments unless the optional argument is also provided. Otherwise, the first keyword would be interpreted as the optional argument, and so on. See, for instance, this Stack Overflow question: How can I have optional arguments AND keyword arguments to the same function?. As the answer to that question points out, it's usually a bug-prone practice to mix optional and keyword arguments. Common Lisp does it with read-from-string, and it often leads people into trouble.
What you're proposing, though, isn't just having a function that uses both keyword and optional arguments, but, from the sounds of it, is actually doing some checking of the types of arguments, and taking one behavior in one case, and another in another. In this case, if i is supposed to be a number, then you could check the first argument, and if it's a number, then treat it as the optional argument, and the rest as keyword arguments, and if it's not a number, then treat the whole list as keyword arguments. You can do that with an &rest argument that you destructure in different ways:
(defun frob (&rest args)
(flet ((frob-driver (i size)
(list i size)))
(if (or (endp args) (numberp (first args)))
;; no args, or the first argument is a number (and thus
;; not a keyword argument)...
(destructuring-bind (&optional (i 'default-i) &key (size 'default-size)) args
(frob-driver i size))
;; otherwise, there are some non-numeric arguments at
;; beginning, so it must be the keyword list, and that the
;; "optional" wasn't provided.
(destructuring-bind (&key (size 'default-size) &aux (i 'default-i)) args
(frob-driver i size)))))
(frob 10 :size 50) ; give i and size
;=> (10 50)
(frob :size 60) ; give size, but not i
;=> (default-i 60)
(frob 40) ; give i, but not size
;=> (40 default-size)
(frob) ; give neither
;=> (default-i default-size)
Keyword arguments without keyword symbols
In the comments, you mentioned that you'd like to be able to use non-keyword symbols as keywords in argument lists. This is easy enough. In the HyperSpec, ยง3.4.1 Ordinary Lambda Lists describes the syntax for keyword arguments:
[&key {var | ({var | (keyword-name var)} [init-form [supplied-p-parameter]])}* [&allow-other-keys]]
This means that you can define functions like this:
(defun frob (&key foo ((bar-keyword bar-variable) 'default-baz))
(list foo bar-variable))
(frob :foo 1 'bar-keyword 2)
;=> (1 2)
(frob :foo 3)
;=> (3 default-baz)
(frob 'bar-keyword 2)
;=> (nil 2)
You have to use a &rest argument list and process it in the function.
Mixing optional and keyword arguments should be avoided. It is BAD style to use optional and keyword arguments. This has been the source for countless errors with the few functions which use that (like READ-FROM-STRING).
In short, "no". There's no way of making that happen. As a general rule, try to stay away from mixing &rest, &optional and &key arguments, since their interactions are subtle and will more frequently trip you up than actually be useful.
Furthermore, if :size is a keyword argument, hen (test :size) is missing one argument (the value to bind size to). Your best bet is probably to look a arg to see if it is :size or something else.
Posting as answer because I solved the main issue here is the resultant code I came up with, The cond statements are what matter. The &args usage created another issue and that post is being discussed here. The ((symbolp (cadr args)) (%vector-float-size (first args))) line is what I came up with from Joshua Taylors kindly written and extremely informative answer.
(defun vector-float (&rest args)
(cond ((eq (first args) nil) (return-from vector-float (%vector-float)))
((listp (first args))
(c-arr-to-vector-float (first args)))
((symbolp (cadr args)) (%vector-float-size (first args)))
((pointerp (first args)) (mem-aref (%vector-float-to-c-array (first args)) :float (second args)))
(t nil)))