My program creates dummy data. Part of these dummy data are keys in the form of symbols: (lambda (i) (make-symbol(format nil"~#r"i))) (fixnum values with their Roman numeral as symbol as key). The program stores these dummy key value pairs in a data structure i'm developing.
I tried to use these symbols from the REPL to test retrieving the values by their key. However:
(equal (make-symbol "IX") (make-symbol "IX"))
NIL
(equal (make-symbol "IX") 'IX))
NIL
Is there a way to enter a symbol previously created with make-symbol in the REPL so that both are equal?
As #coredump suggests, use intern to add these symbols into some package, like that:
CL-USER> (defpackage :my-symbols)
#<Package "MY-SYMBOLS">
CL-USER> (intern "IX" :my-symbols)
MY-SYMBOLS::IX
NIL
CL-USER> (intern "IX" :my-symbols)
MY-SYMBOLS::IX
:INTERNAL
CL-USER> (eql (intern "IX" :my-symbols)
(intern "IX" :my-symbols))
T
CL-USER>
One can use string= to compare symbols by name:
CL-USER 29 > (string= (make-symbol "IX") (make-symbol "IX"))
T
CL-USER 30 > (string= (make-symbol "IX") (make-symbol "Ix"))
NIL
string-equal is the case insensitive variant.
Related
I am using SBCL, Emacs, and Slime. Hence, I can do:
CL-USER> (defvar example #'(lambda (x) (* x 20)))
EXAMPLE
CL-USER> (funcall example 10)
200
Ok. It works as expected. Using the library Dexador, I can also so:
CL-USER> (ql:quickload :dexador)
To load "dexador":
Load 1 ASDF system:
dexador
; Loading "dexador"
.......
(:DEXADOR)
CL-USER> (dex:get "http://www.paulgraham.com")
"big HTML ommited"
200
#<HASH-TABLE :TEST EQUAL :COUNT 11 {10029F1443}>
#<QURI.URI.HTTP:URI-HTTP http://www.paulgraham.com>
#<SB-SYS:FD-STREAM for "socket 10.0.0.193:44936, peer: 74.6.52.135:80" {1002681F73}>
Now, I am trying to make the argument to be passed be a function! More specifically, the dex:get function. I tried different approaches, but none of them worked out:
CL-USER> (defvar example-failing #'(lambda (x) (x "http://www.paulgraham.com")))
; in: DEFVAR EXAMPLE-FAILING
; (LAMBDA (X) (X "http://www.paulgraham.com"))
;
; caught STYLE-WARNING:
; The variable X is defined but never used.
; in: DEFVAR EXAMPLE-FAILING
; (X "http://www.paulgraham.com")
;
; caught STYLE-WARNING:
; undefined function: COMMON-LISP-USER::X
;
; compilation unit finished
; Undefined function:
; X
; caught 2 STYLE-WARNING conditions
EXAMPLE-FAILING
CL-USER> (funcall example-failing dex:get)
; Evaluation aborted on #<UNBOUND-VARIABLE GET {1002C57103}>.
CL-USER> (funcall example-failing 'dex:get)
; Evaluation aborted on #<UNDEFINED-FUNCTION X {1002DEA263}>.
CL-USER> (funcall example-failing #'dex:get)
; Evaluation aborted on #<UNDEFINED-FUNCTION X {1002F906C3}>.
CL-USER> (funcall example-failing (function dex:get))
; Evaluation aborted on #<UNDEFINED-FUNCTION X {1003147F83}>.
I managed to do it with:
CL-USER> (defvar hacky-eval #'(lambda (x) (eval x)))
HACKY-EVAL
CL-USER> (funcall hacky-eval (dex:get "http://www.paulgraham.com"))
"big html omitted"
But, this feels as bad practice. Is there another way to fix this?
Thanks
I am confused by your question although not as confused as you seem to be. You already seem to know that, to call a function which is the value of a variable, you need either
funcall if you have all the arguments as individual things;
apply if you have only a list of arguments;
and that to get the function value of something you need (function thing) or equivalently #'thing[1].
But then you forget that in your function, and pay no attention to the copious warnings from SBCL.
So
(defvar *example* (lambda (f) (funcall f "http://www.paulgraham.com")))
...
(funcall *example* #'dex:get)
Note that none of this (and nothing in your question) relies on lexical scope: this would all have worked in any historical Lisp.
[1]: you don't need #' for (lambda ...) only because lambda is a macro which expands into (function (lambda ...)). Very old code sometimes uses the explicit #'(lambda ...) form since this macro did not always exist in CL.
Your code:
(defvar example-failing
#'(lambda (x)
(x "http://www.paulgraham.com")))
This makes no sense in Common Lisp. x is a variable. You can't use a variable as a function as in (x arg). In Common Lisp there are different namespaces for functions and variables. For example LET introduces a local variable and FLET introduces a local function.
The ways to call a function bound to a variable are:
(funcall x arg)
(apply x (list arg))
Thus correct examples would be:
(defvar example-failing
#'(lambda (x)
(apply x (list "http://www.paulgraham.com"))))
or
(defvar example-failing
#'(lambda (x)
(funcall x "http://www.paulgraham.com")))
Your solution is no solution
This is your example:
CL-USER> (defvar hacky-eval #'(lambda (x) (eval x)))
HACKY-EVAL
CL-USER> (funcall hacky-eval (dex:get "http://www.paulgraham.com"))
"big html omitted"
This does not work as you think.
(funcall hacky-eval (dex:get "http://www.paulgraham.com"))
is just the same as
(funcall hacky-eval "big html omitted")
and then
(eval "big html omitted")
and then
"big html omitted"
All your call to eval does is to evaluate a string to itself.
You really need to understand basic evaluation rules in Lisp:
(defun foo (arg)
(eval arg))
(foo (+ 3 4))
is simply the same as:
(defun foo (arg)
arg)
(foo (+ 3 4))
which is the same as
(identity (+ 3 4))
Note: if you pass just self evaluating data to EVAL, then all it does is to return the data
A function call (foo (+ 1 2)) works like this:
Lisp sees that FOO is a function
Lisp evaluates the arguments. (+ 1 2) -> 3
Lisp calls the function FOO with the evaluated argument: (funcall #'foo 3)
Lisp computes the function FOO: (EVAL 3) -> 3
Lisp returns the value(s) from FOO -> 3
Start by using a proper defun:
(defun request (url)
(dex:get url))
CL-USER> (request "http://…")
Now you want to use something else than dex:get? Well… write another function, because their argument handling, headers, return values… might be different.
(defun request-drakma (url)
(drakma:… url))
Maybe in later code you want to reference to either function?
(defun do-request (url &optional (get-fn #'request))
(funcall get-fn url))
(defvar example #'(lambda (x) (* x 20)))
Here you are giving a name to an anonymous function… just use defun^^
I'm trying to write a macro that defines some helpers for struct-of-arrays data structure (based on this snippet). Inside that macro I define another macro that helps with traversing all of the slot values in struct. The thing is I can't make double unquoting work properly. Here's the code:
(defmacro defcomponent (name-and-options &body slots)
(setf name-and-options (ensure-list name-and-options))
(let ((struct (first name-and-options))
(slot-names (iter (for s in slots)
(collecting
(ematch s
((or (and name (symbol)
(<> _ '*)
(<> _ nil))
(list* name _ (plist :type _ :read-only _)))
name))))))
`(progn (defstruct ,name-and-options
;; some task-specific stuff omitted here
)
(defmacro ,(symbolicate 'with- struct) (components &rest body)
`(loop
,#',(iter (for s in slot-names)
(appending `(for ,s across (,(symbolicate struct '- s) components))))
do ,#body)))))
So for instance (defcomponent buzz x y) macroexpands to
(PROGN
(DEFSTRUCT (BUZZ)
X Y) ;; details omitted
(DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
`(LOOP ,#'(FOR X ACROSS (BUZZ-X COMPONENTS) FOR Y ACROSS (BUZZ-Y COMPONENTS))
DO ,#BODY))
which kinda works, but I want to access components parameter of the internal with-buzz macro, i.e. something like this
(DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
`(LOOP FOR X ACROSS (BUZZ-X ,COMPONENTS) FOR Y ACROSS (BUZZ-Y ,COMPONENTS)
DO ,#BODY))
How do I possibly acheive that? I've tried a lot of the combinations of , and ,# to no avail.
Sometimes it helps not to work with backquote patterns. Then scope problems can be easier understood with the help of a compiler, which would warn about the usual variable scope problems.
As a slightly simplified exercise, we will write a function, which generates code. The generated code is a macro definition, which itself generates code.
(defun makeit (name slots)
(labels ((symbolicate (pattern &rest things)
(intern (apply #'format nil pattern things)))
(compute-for-clauses (slots)
(loop for s in slots
append (list ''for (list 'quote s)
''across (list 'list
(list 'quote
(symbolicate "~a-~a" name s))
'components)))))
(list 'progn
(list 'defmacro
(symbolicate "WITH-~a" name)
'(components &rest body)
(append '(list* 'loop)
(compute-for-clauses slots)
(list ''do 'body))))))
Example
CL-USER 51 > (pprint (makeit 'buzz '(x y)))
(PROGN
(DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
(LIST* 'LOOP
'FOR
'X
'ACROSS
(LIST 'BUZZ-X COMPONENTS)
'FOR
'Y
'ACROSS
(LIST 'BUZZ-Y COMPONENTS)
'DO
BODY)))
CL-USER 52 > (eval *)
NIL
CL-USER 53 > (macroexpand-1 '(with-buzz a (+ 12) (+ 30)))
(LOOP FOR X ACROSS (BUZZ-X A) FOR Y ACROSS (BUZZ-Y A) DO (+ 12) (+ 30))
T
All right, I've managed to do it by resorting to manual list construction + eval instead of quasiquoting, but sweet mother of god it looks so hakish.
;; skip
(defmacro ,(symbolicate 'with- struct) (components &rest body)
(append
'(loop)
(eval
`(iter (for s in ',',slot-names)
(appending `(for ,s across (,(symbolicate ',',struct '- ,'s) ,,components)))))
'(do)
body))
I'll gladly accept any other answer solving the problem more idiomatically.
Task is to read N string like "name phone" and store in. Then find stored data with requests like "name".
My code stores names and numbers in hashtable, but after it doesn't find any values. Stored values checks with maphash (it shows all pairs key-value).
Function split-by-one-space is just utility.
(defparameter data (make-hash-table))
(defun split-by-one-space (string) ; to split string: "aaa bbb" -> (aaa bbb)
(loop for i = 0 then (1+ j)
as j = (position #\Space string :start i)
collect (subseq string i j)
while j))
(dotimes (i (read)) ; input data
(let* ((inp (read-line))
(raw (split-by-one-space inp))
(name (string (car raw)))
(phone (cadr raw)))
(format t "Adding: ~W ~W~%" name phone) ; debug
(setf (gethash name data) phone)))
(maphash #'(lambda (k v) (format t "~a => ~a~%" k v)) data) ; this show all stored data
(loop for line = (read-line *standard-input* nil :eof)
until (or (eq line :eof) (eq line nil))
do
(let ((key (gethash line data))) ; it cannot find anything. Why?
(format t "Searching: ~W~%" line) ; debug
(if (null key)
(format t "Not found~%")
(format t "~A=~A~%" (car key) (cdr key)))))
Sample input:
3
sam 99912222
tom 11122222
harry 12299933
sam
edward
harry
Unless you specify a test function, hash tables will use eql to determine "is this key identical to that key".
(defvar *s1* "a string")
(defvar *s2* "a string")
(loop for pred in '(eq eql equal equalp)
do (format t "Using ~a, the result is ~a~%"
pred (funcall pred *s1* *s2*)))
This generates the output:
Using EQ, the result is NIL
Using EQL, the result is NIL
Using EQUAL, the result is T
Using EQUALP, the result is T
In this case, the main difference between equal and equalp is that the latter is case-insensitive, while the former is not. To use another test function, use the :test keyword and one of the found "standard" test functions. If you don't need case-insensitive matches, you would simply create your hash table like this: (make-hash-table :test #'equal).
I've been struggling with this for two days now, and I can't find the answer.
What I want is to define three variables, a, b, and c each with a value of 0.
Naive:
(dolist (lbl '(a b c)) (defvar lbl 0))
Doesn't do what I want. a, b, and c remain undefined, and lbl now has a value of 0.
I think I may understand why this can't work: defvar is a macro, not a function, and as such I am passing it the form lbl, not the current value of label (which is a, b, c in turn). I think.
But in the resulting macroexpansion, shouldn't lbl eventually be linked-up(?) or evaluated(?) to the value I'm intending? Obviously not, either because it can't be done or I'm doing it wrong.
I want to understand:
How to make this work: (dolist (lbl '(a b c)) (defvar lbl 0))
What's going wrong under the hood. I have a feeling it has something to do with symbols or the mechanics of the quote operator.
Here are a few options:
With eval, by building a defvar expression:
(dolist (lbl '(a b c))
(eval `(defvar ,lbl 0))
With proclaim and setf of symbol-value (note: set is deprecated, since 1994 for what it's worth):
(dolist (lbl '(a b c))
(proclaim `(special ,lbl))
(setf (symbol-value lbl) 0))
This is actually mostly what defvar does (see notes in the linked page), but each Lisp implementation usually also records source file location, as they do for other defining macros.
Under the hood, defvar is a macro that makes the variable special (i.e. with dynamic extent bindings in the current dynamic environment; note: there's no portable undoing for this!), and optionally initializes it if it's not yet bound.
The fact that it's a macro means it doesn't evaluate its arguments, so it can take the variable name literally, and it does so. As such, (defvar lbl 0) will define the variable lbl, not the symbol stored in a lbl variable.
The fact that it optionally initializes the variable means that the initializing expression will not even be evaluated if the variable is boundp. So, its secondary effects won't happen if the variable is already initialized. This might or might not be expected.
Note that this expression isn't actually evaluated at macro-expansion time, it's left for evaluation when the expansion is evaluated, which in a REPL means right after macro expansion (and possibly after compilation, depending on the Lisp implementation; read more about evaluation and compilation, it's quite interesting).
Similar:
(dolist (lbl '(a b c))
(let ((lbl 0))
(print lbl)))
Why is lbl 0 and not some of a, b, c?
Because LET binds the symbol lbl and not its value.
Similar with (DEFVAR FOO 3).
Imagine following code:
(DEFVAR FOO 3)
(LET ((FOO 3)) ...)
Now, if we compile this code, the Lisp compiler recognizes the DEFVAR declaration and now knows that FOO is a special global variable. Thus in the let form FOO will be dynamically bound.
Compare this code:
(dolist (v '(FOO)) (eval `(DEFVAR ,v 3)))
(LET ((FOO 3)) ...)
The compiler won't see the DEFVAR and does not know that it should be a global special variable. In the LET form, FOO will have a lexical binding.
Thus DEFVAR needs to be a macro which knows the symbol at compile time (!) and which expands into a form that informs the compiler that the symbol is a special global variable. The form also sets the value when executed.
Thus the best way to create multiple DEFVAR declarations from a list of variables is to write a macro, which expands into a PROGN form with multiple DEFVARs. Inside the PROGN, the compiler will still recognize them.
CL-USER 21 > (pprint (macroexpand '(defvar* (a b c) 0)))
(PROGN (DEFVAR A 0) (DEFVAR B 0) (DEFVAR C 0))
Implemented as:
(defmacro defvar* (vars initial-value)
`(progn
,#(loop for var in vars
do (check-type var symbol)
collect `(defvar ,var ,initial-value))))
Note that it makes sense to check that the variables are really provided as symbols.
defvar is a special form which makes sure the symbol of it's first argument is a bound variable. If the variable is not bound the evaluated expression of the second argument becomes the bound variables value. Thus:
(defvar *x* 10) ; if *x* was not bound it's now 10
(defvar *x* 20) ; since *x* is defined nothing happens
Notice that *x* is not evaluated but is used unevaluated. In order to get the same functionality by using a variable that evaluates to a symbol which you want to exist as a variable in global scope you need to do something like this:
(defvar b 10)
(dolist (lbl '(a b c))
(when (not (boundp lbl))
(setf (symbol-value lbl) 0)))
Still, neither of the ones not already bound becomes special like with defvar, but at least you get the same behaviour:
(list a b c) ; => (0 10 0)
Perhaps you should just do:
(defvar *a* 0)
(defvar *b* 0)
(defvar *c* 0)
If you have a lot of variables you need to do this with you can do:
(defmacro defvars (lst value)
(loop :for e :in lst
:collect `(defvar ,e ,value) :into result
:finally (return (cons 'progn result))))
(defparameter *w* 10)
(defvars (*q* *w* *e*) 1)
(list *q* *w* *e* ; ==> (1 10 1)
Also, it's really important to earmuff your global variables. Once special it will follow dynamic binding. eg.
(defun test ()
(let ((*b* 15))
(test2)))
(defun test2 ()
*b*)
(test) ; ==> 15
Reimplementing DEFVAR
You can approximate the behavior of defvar with a function like this:
(defun %defvar (symbol value documentation)
"Define a global special variable.
symbol---a symbol
value---nil or a function of zero arguments
documentation---nil or a documentation string
returns symbol
Proclaim SYMBOL globally as a special variable. If VALUE is non-nil,
then if SYMBOL is not already bound, SYMBOL is assigned the value
returned by calling VALUE. DOCUMENATION is assigned as the
documentation of type variable to for SYMBOL."
(prog1 symbol
;; make it globally special
(proclaim (list 'special symbol))
;; if a value is provided, and symbol isn't
;; already bound, set its value to the result
;; of calling the value-function
(when (not (null value))
(unless (boundp symbol)
(setf (symbol-value symbol)
(funcall value))))
;; set the documentation
(setf (documentation symbol 'variable) documentation)))
Then you can do, e.g.,
CL-USER> (%defvar '*the-answer* (lambda () 42) "the answer")
*THE-ANSWER*
CL-USER> *the-answer*
42
CL-USER> (documentation '*the-answer* 'variable)
"the answer"
And with your original code, you could do something like:
(dolist (lbl '(a b c)) (%defvar lbl (lambda () 0)))
Now, how does this relate to what defvar actually does? Well, you could now implement a defvar like macro by doing:
(defmacro define-var (symbol &optional (value nil value-p) documentation)
`(%defvar
',symbol
,(if value-p `(lambda () ,value) 'nil)
,documentation))
This expands as we'd expect:
CL-USER> (macroexpand-1 '(define-var *the-answer* 42 "the answer"))
(%DEFVAR '*THE-ANSWER* (LAMBDA () 42) "the answer")
You can actually use macroexpand to look at what an implementation does, too. E.g., in SBCL:
CL-USER> (macroexpand-1 '(defvar *the-answer* 42 "the answer"))
(PROGN
(EVAL-WHEN (:COMPILE-TOPLEVEL) (SB-IMPL::%COMPILER-DEFVAR '*THE-ANSWER*))
(SB-IMPL::%DEFVAR '*THE-ANSWER* (UNLESS (BOUNDP '*THE-ANSWER*) 42) 'T
"the answer" 'T (SB-C:SOURCE-LOCATION)))
This isn't too much different from what we wrote above, though it's handling the non-evaluation of the form when the variable is already bound in a slightly different way, and it's also got some handling for recording a source location. The general idea is the same, though.
Why things don't get "linked up"
But in the resulting macroexpansion, shouldn't lbl eventually be
linked-up(?) or evaluated(?) to the value I'm intending?
The original code is:
(dolist (lbl '(a b c)) (defvar lbl 0))
We can macroexpand this to see what it becomes (in SBCL):
CL-USER> (macroexpand '(dolist (lbl '(a b c)) (defvar lbl 0)))
(BLOCK NIL
(LET ((#:N-LIST1022 '(A B C)))
(TAGBODY
#:START1023
(UNLESS (ENDP #:N-LIST1022)
(LET ((LBL (TRULY-THE (MEMBER C B A) (CAR #:N-LIST1022))))
(SETQ #:N-LIST1022 (CDR #:N-LIST1022))
(TAGBODY (DEFVAR LBL 0)))
(GO #:START1023))))
NIL)
T
Now, we can still see LBL in two places, including in (defvar LBL 0). So why don't things get "matched up"? To see that, we need to remember that the defvar inside the let will also be macroexpanded. To what? This:
CL-USER> (macroexpand '(DEFVAR LBL 0))
(PROGN
(EVAL-WHEN (:COMPILE-TOPLEVEL) (SB-IMPL::%COMPILER-DEFVAR 'LBL))
(SB-IMPL::%DEFVAR 'LBL (UNLESS (BOUNDP 'LBL) 0) 'T NIL 'NIL
(SB-C:SOURCE-LOCATION)))
But now we see that SBCL's internals are getting the symbol named "LBL"; the call (sb-impl::%defvar 'lbl …) is calling the function sb-impl::%defvar with the symbol lbl, and there's no connection between that symbol and the lexical variable that happens to be represented in the source by the same symbol. After all, if you write:
CL-USER> (let ((a 89))
(list 'a a))
(A 89)
You want to be able to get the symbol a and the number 89, right? The macroexpansion of defvar includes a call to a function with the quotation of one of the arguments to macro.
By "artificial", I mean one created from a string using intern or make-symbol.
I have a section of my code that declares up to 49 global variables:
(defparameter *CHAR-COUNT-1-1* (make-hash-table))
...
(defparameter *CHAR-COUNT-1-7* (make-hash-table))
...
(defparameter *CHAR-COUNT-7-7* (make-hash-table))
I thought, instead, I could create a function to do all that:
(loop for n from 1 to 7 do
(loop for i from 1 to 7 do
(defparameter (symbol-value (intern (concatenate 'string "*CHAR-COUNT-" (write-to-string n) "-" (write-to-string i) "*")))
(make-hash-table :test 'equalp))))
But get the error(sbcl):
unhandled SIMPLE-ERROR in thread #<SB-THREAD:THREAD "main thread" RUNNING
{1002978EE3}>:
Can't declare a non-symbol as SPECIAL: (SYMBOL-VALUE
(INTERN
(CONCATENATE 'STRING "*CHAR-COUNT-"
(WRITE-TO-STRING N) "-"
(WRITE-TO-STRING I)
"*")))
What is the correct way to do this?
Defparameter is a macro, not a function. That means that it defines a special syntax. The defparameter form needs to have a symbol as its second argument, but you're providing the list:
(symbol-value (intern (concatenate 'string "*CHAR-COUNT-" (write-to-string n) "-" (write-to-string i) "*")))
What you want is a form like
(progn
(defparameter *foo-1-1* (make-hash-table ...))
...
(defparameter *foo-n-n* (make-hash-table ...)))
You seem familiar enough with loop and creating the symbols to create that list; just change
(loop … do (loop … do (defparameter …)))
to
`(progn
,#(loop … nconcing
(loop … collecting
`(defparameter ,(intern …) …))))
and you can get the form you need. Then it's just a matter of putting it all into a macro
(defmacro … (…)
`(progn
,#(loop … nconcing
(loop … collecting
`(defparameter ,(intern …) …)))))
and calling the macro.
One of "use a macro that returns a PROGN with DEFPARAMETER stanzas" or "use PROCLAIM, it is a function, not a macro".
The correct way is to use a proper data structure instead of encoding dimensions in symbol names. Do you really want to calculate and encode symbol names any time you want to access the correct table?
(defparameter *char-counts* (make-array '(7 7)))
(dotimes (i 49) ; or (reduce #'* (array-dimensions *char-counts*))
(setf (row-major-aref *char-counts* i) (make-hash-table)))
Now you can access the array of tables just with the indices (x and y in this example):
(gethash (aref *char-counts* x y) :foo)