Related
Often when I try to write a macro, I run up against the following difficulty: I need one form that is passed to the macro to be evaluated before being processed by a helper function that is invoked while generating the macro's expansion. In the following example, we are only interested in how we could write a macro to emit the code we want, and not in the uselessness of the macro itself:
Imagine (bear with me) a version of Common Lisp's lambda macro, where only the number of arguments is important, and the names and order of the arguments are not. Let's call it jlambda. It would be used like so:
(jlambda 2
...body)
where 2 is the arity of the function returned. In other words, this produces a binary operator.
Now imagine that, given the arity, jlambda produces a dummy lambda-list which it passes to the actual lambda macro, something like this:
(defun build-lambda-list (arity)
(assert (alexandria:non-negative-integer-p arity))
(loop for x below arity collect (gensym)))
(build-lambda-list 2)
==> (#:G15 #:G16)
The expansion of the above call to jlambda will look like this:
(lambda (#:G15 #:16)
(declare (ignore #:G15 #:16))
…body))
Let's say we need the jlambda macro to be able to receive the arity value as a Lisp form that evaluates to a non-negative integer (as opposed to receiving a non-negative integer directly) eg:
(jlambda (+ 1 1)
...body)
The form (+ 1 1) needs to be evaluated, then the result needs to be passed to build-lambda-list and that needs to be evaluated, and the result of that is inserted into the macro expansion.
(+ 1 1)
=> 2
(build-lambda-list 2)
=> (#:G17 #:18)
(jlambda (+ 1 1) ...body)
=> (lambda (#:G19 #:20)
(declare (ignore #:G19 #:20))
…body))
So here's a version of jlambda that works when the arity is provided as a number directly, but not when it's passed as a form to be evaluated:
(defun jlambda-helper (arity)
(let ((dummy-args (build-lambda-list arity)))
`(lambda ,dummy-args
(declare (ignore ,#dummy-args))
body)))
(defmacro jlambda (arity &body body)
(subst (car body) 'body (jlambda-helper arity)))
(jlambda 2 (print “hello”)) ==> #<anonymous-function>
(funcall *
'ignored-but-required-argument-a
'ignored-but-required-argument-b)
==> “hello”
“hello”
(jlambda (+ 1 1) (print “hello”)) ==> failed assertion in build-lambda-list, since it receives (+ 1 1) not 2
I could evaluate the (+ 1 1) using the sharp-dot read macro, like so:
(jlambda #.(+ 1 1) (print “hello”)) ==> #<anonymous-function>
But then the form cannot contain references to lexical variables, since they are not available when evaluating at read-time:
(let ((x 1))
;; Do other stuff with x, then:
(jlambda #.(+ x 1) (print “hello”))) ==> failure – variable x not bound
I could quote all body code that I pass to jlambda, define it as a function instead, and then eval the code that it returns:
(defun jlambda (arity &rest body)
(let ((dummy-args (build-lambda-list arity)))
`(lambda ,dummy-args
(declare (ignore ,#dummy-args))
,#body)))
(eval (jlambda (+ 1 1) `(print “hello”))) ==> #<anonymous-function>
But I can't use eval because, like sharp-dot, it throws out the lexical environment, which is no good.
So jlambda must be a macro, because I don't want the function body code evaluated until the proper context for it has been established by jlambda's expansion; however it must also be a function, because I want the first form (in this example, the arity form) evaluated before passing it to helper functions that generate the macro expansion. How do I overcome this Catch-22 situation?
EDIT
In response to #Sylwester 's question, here's an explanation of the context:
I'm writing something akin to an “esoteric programming language”, implemented as a DSL in Common Lisp. The idea (admittedly silly but potentially fun) is to force the programmer, as far as possible (I'm not sure how far yet!), to write exclusively in point-free style. To do this, I will do several things:
Use curry-compose-reader-macros to provide most of the functionality required to write in point-free style in CL
Enforce functions' arity – i.e. override CL's default behaviour that allows functions to be variadic
Instead of using a type system to determine when a function has been “fully applied” (like in Haskell), just manually specify a function's arity when defining it.
So I'll need a custom version of lambda for defining a function in this silly language, and – if I can't figure that out - a custom version of funcall and/or apply for invoking those functions. Ideally they'll just be skins over the normal CL versions that change the functionality slightly.
A function in this language will somehow have to keep track of its arity. However, for simplicity, I would like the procedure itself to still be a funcallable CL object, but would really like to avoid using the MetaObject Protocol, since it's even more confusing to me than macros.
A potentially simple solution would be to use a closure. Every function could simply close over the binding of a variable that stores its arity. When invoked, the arity value would determine the exact nature of the function application (i.e. full or partial application). If necessary, the closure could be “pandoric” in order to provide external access to the arity value; that could be achieved using plambda and with-pandoric from Let Over Lambda.
In general, functions in my language will behave like so (potentially buggy pseudocode, purely illustrative):
Let n be the number of arguments provided upon invocation of the function f of arity a.
If a = 0 and n != a, throw a “too many arguments” error;
Else if a != 0 and 0 < n < a, partially apply f to create a function g, whose arity is equal to a – n;
Else if n > a, throw a “too many arguments” error;
Else if n = a, fully apply the function to the arguments (or lack thereof).
The fact that the arity of g is equal to a – n is where the problem with jlambda would arise: g would need to be created like so:
(jlambda (- a n)
...body)
Which means that access to the lexical environment is a necessity.
This is a particularly tricky situation because there's no obvious way to create a function of a particular number of arguments at runtime. If there's no way to do that, then it's probably easiest to write a a function that takes an arity and another function, and wraps the function in a new function that requires that is provided the particular number of arguments:
(defun %jlambda (n function)
"Returns a function that accepts only N argument that calls the
provided FUNCTION with 0 arguments."
(lambda (&rest args)
(unless (eql n (length args))
(error "Wrong number of arguments."))
(funcall function)))
Once you have that, it's easy to write the macro around it that you'd like to be able to:
(defmacro jlambda (n &body body)
"Produces a function that takes exactly N arguments and and evalutes
the BODY."
`(%jlambda ,n (lambda () ,#body)))
And it behaves roughly the way you'd want it to, including letting the arity be something that isn't known at compile time.
CL-USER> (let ((a 10) (n 7))
(funcall (jlambda (- a n)
(print 'hello))
1 2 3))
HELLO
HELLO
CL-USER> (let ((a 10) (n 7))
(funcall (jlambda (- a n)
(print 'hello))
1 2))
; Evaluation aborted on #<SIMPLE-ERROR "Wrong number of arguments." {1004B95E63}>.
Now, you might be able to do something that invokes the compiler at runtime, possibly indirectly, using coerce, but that won't let the body of the function be able to refer to variables in the original lexical scope, though you would get the implementation's wrong number of arguments exception:
(defun %jlambda (n function)
(let ((arglist (loop for i below n collect (make-symbol (format nil "$~a" i)))))
(coerce `(lambda ,arglist
(declare (ignore ,#arglist))
(funcall ,function))
'function)))
(defmacro jlambda (n &body body)
`(%jlambda ,n (lambda () ,#body)))
This works in SBCL:
CL-USER> (let ((a 10) (n 7))
(funcall (jlambda (- a n)
(print 'hello))
1 2 3))
HELLO
CL-USER> (let ((a 10) (n 7))
(funcall (jlambda (- a n)
(print 'hello))
1 2))
; Evaluation aborted on #<SB-INT:SIMPLE-PROGRAM-ERROR "invalid number of arguments: ~S" {1005259923}>.
While this works in SBCL, it's not clear to me whether it's actually guaranteed to work. We're using coerce to compile a function that has a literal function object in it. I'm not sure whether that's portable or not.
NB: In your code you use strange quotes so that (print “hello”) doesn't actually print hello but the whatever the variable “hello” evaluates to, while (print "hello") does what one would expect.
My first question is why? Usually you know how many arguments you are taking compile time or at least you just make it multiple arity. Making an n arity function only gives you errors when passwd with wrong number of arguments as added feature with the drawback of using eval and friends.
It cannot be solved as a macro since you are mixing runtime with macro expansion time. Imagine this use:
(defun test (last-index)
(let ((x (1+ last-index)))
(jlambda x (print "hello"))))
The macro is expanded when this form is evaluated and the content replaced before the function is assigned to test. At this time x doesn't have any value whatsoever and sure enough the macro function only gets the symbols so that the result need to use this value. lambda is a special form so it again gets expanded right after the expansion of jlambda, also before any usage of the function.
There is nothing lexical happening since this happens before the program is running. It could happen before loading the file with compile-file and then if you load it will load all forms with the macros already expanded beforehand.
With compile you can make a function from data. It is probably as evil as eval is so you shouldn't be using it for common tasks, but they exist for a reason:
;; Macro just to prevent evaluation of the body
(defmacro jlambda (nexpr &rest body)
`(let ((dummy-args (build-lambda-list ,nexpr)))
(compile nil (list* 'lambda dummy-args ',body))))
So the expansion of the first example turns into this:
(defun test (last-index)
(let ((x (1+ last-index)))
(let ((dummy-args (build-lambda-list x)))
(compile nil (list* 'lambda dummy-args '((print "hello")))))))
This looks like it could work. Lets test it:
(defparameter *test* (test 10))
(disassemble *test*)
;Disassembly of function nil
;(CONST 0) = "hello"
;11 required arguments <!-- this looks right
;0 optional arguments
;No rest parameter
;No keyword parameters
;4 byte-code instructions:
;0 (const&push 0) ; "hello"
;1 (push-unbound 1)
;3 (calls1 142) ; print
;5 (skip&ret 12)
;nil
Possible variations
I've made a macro that takes a literal number and makes bound variables from a ... that can be used in the function.
If you are not using the arguments why not make a macro that does this:
(defmacro jlambda2 (&rest body)
`(lambda (&rest #:rest) ,#body))
The result takes any number of arguments and just ignores it:
(defparameter *test* (jlambda2 (print "hello")))
(disassemble *test*)
;Disassembly of function :lambda
;(CONST 0) = "hello"
;0 required arguments
;0 optional arguments
;Rest parameter <!-- takes any numer of arguments
;No keyword parameters
;4 byte-code instructions:
;0 (const&push 0) ; "hello"
;1 (push-unbound 1)
;3 (calls1 142) ; print
;5 (skip&ret 2)
;nil
(funcall *test* 1 2 3 4 5 6 7)
; ==> "hello" (prints "hello" as side effect)
EDIT
Now that I know what you are up to I have an answer for you. Your initial function does not need to be runtime dependent so all functions indeed have a fixed arity, so what we need to make is currying or partial application.
;; currying
(defmacro fixlam ((&rest args) &body body)
(let ((args (reverse args)))
(loop :for arg :in args
:for r := `(lambda (,arg) ,#body)
:then `(lambda (,arg) ,r)
:finally (return r))))
(fixlam (a b c) (+ a b c))
; ==> #<function :lambda (a) (lambda (b) (lambda (c) (+ a b c)))>
;; can apply multiple and returns partially applied when not enough
(defmacro fixlam ((&rest args) &body body)
`(let ((lam (lambda ,args ,#body)))
(labels ((chk (args)
(cond ((> (length args) ,(length args)) (error "too many args"))
((= (length args) ,(length args)) (apply lam args))
(t (lambda (&rest extra-args)
(chk (append args extra-args)))))))
(lambda (&rest args)
(chk args)))))
(fixlam () "hello") ; ==> #<function :lambda (&rest args) (chk args)>
;;Same but the zero argument functions are applied right away:
(defmacro fixlam ((&rest args) &body body)
`(let ((lam (lambda ,args ,#body)))
(labels ((chk (args)
(cond ((> (length args) ,(length args)) (error "too many args"))
((= (length args) ,(length args)) (apply lam args))
(t (lambda (&rest extra-args)
(chk (append args extra-args)))))))
(chk '()))))
(fixlam () "hello") ; ==> "hello"
If all you want is lambda functions that can be applied either partially or fully, I don't think you need to pass the amount of parameters explicitly. You could just do something like this (uses Alexandria):
(defmacro jlambda (arglist &body body)
(with-gensyms (rest %jlambda)
`(named-lambda ,%jlambda (&rest ,rest)
(cond ((= (length ,rest) ,(length arglist))
(apply (lambda ,arglist ,#body) ,rest))
((> (length ,rest) ,(length arglist))
(error "Too many arguments"))
(t (apply #'curry #',%jlambda ,rest))))))
CL-USER> (jlambda (x y) (format t "X: ~s, Y: ~s~%" x y))
#<FUNCTION (LABELS #:%JLAMBDA1046) {1003839D6B}>
CL-USER> (funcall * 10) ; Apply partially
#<CLOSURE (LAMBDA (&REST ALEXANDRIA.0.DEV::MORE) :IN CURRY) {10038732DB}>
CL-USER> (funcall * 20) ; Apply fully
X: 10, Y: 20
NIL
CL-USER> (funcall ** 100) ; Apply fully again
X: 10, Y: 100
NIL
CL-USER> (funcall *** 100 200) ; Try giving a total of 3 args
; Debugger entered on #<SIMPLE-ERROR "Too many arguments" {100392D7E3}>
Edit: Here's also a version that lets you specify the arity. Frankly, I don't see how this could possibly be useful though. If the user cannot refer to the arguments, and nothing is done with them automatically, then, well, nothing is done with them. They might as well not exist.
(defmacro jlambda (arity &body body)
(with-gensyms (rest %jlambda n)
`(let ((,n ,arity))
(named-lambda ,%jlambda (&rest ,rest)
(cond ((= (length ,rest) ,n)
,#body)
((> (length ,rest) ,n)
(error "Too many arguments"))
(t (apply #'curry #',%jlambda ,rest)))))))
CL-USER> (jlambda (+ 1 1) (print "hello"))
#<CLOSURE (LABELS #:%JLAMBDA1085) {1003B7913B}>
CL-USER> (funcall * 2)
#<CLOSURE (LAMBDA (&REST ALEXANDRIA.0.DEV::MORE) :IN CURRY) {1003B7F7FB}>
CL-USER> (funcall * 5)
"hello"
"hello"
Edit2: If I understood correctly, you might be looking for something like this (?):
(defvar *stack* (list))
(defun jlambda (arity function)
(lambda ()
(push (apply function (loop repeat arity collect (pop *stack*)))
*stack*)))
CL-USER> (push 1 *stack*)
(1)
CL-USER> (push 2 *stack*)
(2 1)
CL-USER> (push 3 *stack*)
(3 2 1)
CL-USER> (push 4 *stack*)
(4 3 2 1)
CL-USER> (funcall (jlambda 4 #'+)) ; take 4 arguments from the stack
(10) ; and apply #'+ to them
CL-USER> (push 10 *stack*)
(10 10)
CL-USER> (push 20 *stack*)
(20 10 10)
CL-USER> (push 30 *stack*)
(30 20 10 10)
CL-USER> (funcall (jlambda 3 [{reduce #'*} #'list])) ; pop 3 args from
(6000 10) ; stack, make a list
; of them and reduce
; it with #'*
I have a doubt on how parameters passed to the macros are getting evaluated, details below.
This macro is defined
(defmacro test-macro (xlist)
`(* ,#xlist))
and there is this global variable (defvar *test-list* '(1 100 2 200)).
When *test-list* is passed to this macro (test-macro *test-list*) , this error is returned -
value *TEST-LIST* is not of the expected type LIST.
[Condition of type TYPE-ERROR]
But if the function is modified to this, list is returned
(defmacro test-macro (xlist)
`(,#xlist)) ;; removed the * operator
(test-macro *test-list*) will return (1 100 2 200).
So my doubt is why ,#xlist is not getting evaluated in the first case, i.e when the * operator is applied. Any help is highly appreciated.
When debugging macros, The Right Way is to use macroexpand, not evaluate the macro forms. E.g., in your case:
(defmacro test-macro1 (xlist) `(* ,#xlist))
(macroexpand '(test-macro1 foo))
==> (* . FOO)
(defmacro test-macro2 (xlist) `(,#xlist))
(macroexpand '(test-macro2 foo))
==> FOO
neither is probably what you want.
The confusion is that the macro is a pre-processor: it has no built-in mechanism to know of runtime values. So when you use the term:
(test-macro test-list)
all that the macro sees is the identifier test-list: it does not know up-front that the runtime value is a list, only that the source program has used this variable identifier.
A macro is a source-to-source rewriter: it doesn't know about the dynamics of your program. A smarter compiler might be able to see that test-list is a constant and do an inlining, but the macro expander isn't that clever.
What you can do is probably something like this:
(defmacro test-macro (xlist)
(cond
(;; If we see test-macro is being used with a quoted list of things
;; then we can rewrite that statically.
(and (pair? xlist)
(eq? (car xlist) 'quote)
(list? (cadr xlist)))
`(list 'case-1 (* ,#(cadr xlist))))
(;; Also, if we see test-macro is being used with "(list ...)"
;; then we can rewrite that statically.
(and (pair? xlist)
(eq? (car xlist) 'list))
`(list 'case-2 (* ,#(cdr xlist))))
(else
;; Otherwise, do the most generic thing:
`(list 'case-3 (apply * ,xlist)))))
;; This hits the first case:
(test-macro '(3 4 5))
;; ... the second case:
(test-macro (list 5 6 7))
;; ... and the third case:
(defvar test-list '(1 100 2 200))
(test-macro test-list)
With regards to your second version: the macro:
(defmacro test-macro (xlist)
`(,#xlist))
is equivalent to:
(defmacro test-macro (xlist)
xlist)
so that's why you're not getting the error that you received in the first version.
I'm reading/working through Practical Common Lisp. I'm on the chapter about building a test framework in Lisp.
I have the function "test-+" implemented as below, and it works:
(defun test-+ ()
(check
(= (+ 1 2) 3)
(= (+ 5 6) 11)
(= (+ -1 -6) -7)))
Remember, I said, it works, which is why what follows is so baffling....
Here is some code that "test-+" refers to:
(defmacro check (&body forms)
`(combine-results
,#(loop for f in forms collect `(report-result ,f ',f))))
(defmacro combine-results (&body forms)
(with-gensyms (result)
`(let ((,result t))
,#(loop for f in forms collect `(unless ,f (setf ,result nil)))
,result)))
(defmacro with-gensyms ((&rest names) &body body)
`(let ,(loop for n in names collect `(,n (gensym)))
,#body))
(defun report-result (value form)
(format t "~:[FAIL~;pass~] ... ~a~%" value form)
value)
Now, what I've been doing is using Slime to macro-expand these, step by step (using ctrl-c RET, which is mapped to macroexpand-1).
So, the "check" call of "test-+" expands to this:
(COMBINE-RESULTS
(REPORT-RESULT (= (+ 1 2) 3) '(= (+ 1 2) 3))
(REPORT-RESULT (= (+ 5 6) 11) '(= (+ 5 6) 11))
(REPORT-RESULT (= (+ -1 -6) -7) '(= (+ -1 -6) -7)))
And then that macro-expands to this:
(LET ((#:G2867 T))
(UNLESS (REPORT-RESULT (= (+ 1 2) 3) '(= (+ 1 2) 3)) (SETF #:G2867 NIL))
(UNLESS (REPORT-RESULT (= (+ 5 6) 11) '(= (+ 5 6) 11)) (SETF #:G2867 NIL))
(UNLESS (REPORT-RESULT (= (+ -1 -6) -7) '(= (+ -1 -6) -7))
(SETF #:G2867 NIL))
#:G2867)
And it is THAT code, directly above this sentence, which doesn't work. If I paste that into the REPL, I get the following error (I'm using Clozure Common Lisp):
Unbound variable: #:G2867 [Condition of type UNBOUND-VARIABLE]
Now, if I take that same code, replace the gensym with a variable name such as "x", it works just fine.
So, how can we explain the following surprises:
The "test-+" macro, which calls all of this, works fine.
The macro-expansion of the "combine-results" macro does not run.
If I remove the gensym from the macro-expansion of "combine-results", it
does work.
The only thing I can speculate is that you cannot use code the contains literal usages of gensyms. If so, why not, and how does one work around that? And if that is not the explanation, what is?
Thanks.
GENSYM creates uninterned symbols. When the macro runs normally, this isn't a problem, because the same uninterned symbol is being substituted throughout the expression.
But when you copy and paste the expression into the REPL, this doesn't happen. #: tells the reader to return an uninterned symbol. As a result, each occurrence of #:G2867 is a different symbol, and you get the unbound variable warning.
If you do (setq *print-circle* t) before doing the MACROEXPAND it will use #n= and #n# notation to link the identical symbols together.
The code, after being printed and read back, is no longer the same code. In particular, the two instances of #:G2867 in the printed representation would be read back as two separated symbols (albeit sharing the same name), while they should be the same in the original internal representation.
Try setting *PRINT-CIRCLE* to T to preserve the identity in the printed representation of the macro-expanded code.
I am having trouble with Lisp's backquote read macro. Whenever I try to write a macro that seems to require the use of embedded backquotes (e.g., ``(w ,x ,,y) from Paul Graham's ANSI Common Lisp, page 399), I cannot figure out how to write my code in a way that compiles. Typically, my code receives a whole chain of errors preceded with "Comma not inside a backquote." Can someone provide some guidelines for how I can write code that will evaluate properly?
As an example, I currently need a macro which takes a form that describes a rule in the form of '(function-name column-index value) and generates a predicate lambda body to determine whether the element indexed by column-index for a particular row satisfies the rule. If I called this macro with the rule '(< 1 2), I would want a lambda body that looks like the following to be generated:
(lambda (row)
(< (svref row 1) 2))
The best stab I can make at this is as follows:
(defmacro row-satisfies-rule (rule)
(let ((x (gensym)))
`(let ((,x ,rule))
(lambda (row)
(`,(car ,x) (svref row `,(cadr ,x)) `,(caddr ,x))))))
Upon evaluation, SBCL spews the following error report:
; in: ROW-SATISFIES-RULE '(< 1 2)
; ((CAR #:G1121) (SVREF ROW (CADR #:G1121)) (CADDR #:G1121))
;
; caught ERROR:
; illegal function call
; (LAMBDA (ROW) ((CAR #:G1121) (SVREF ROW (CADR #:G1121)) (CADDR #:G1121)))
; ==>
; #'(LAMBDA (ROW) ((CAR #:G1121) (SVREF ROW (CADR #:G1121)) (CADDR #:G1121)))
;
; caught STYLE-WARNING:
; The variable ROW is defined but never used.
; (LET ((#:G1121 '(< 1 2)))
; (LAMBDA (ROW) ((CAR #:G1121) (SVREF ROW (CADR #:G1121)) (CADDR #:G1121))))
;
; caught STYLE-WARNING:
; The variable #:G1121 is defined but never used.
;
; compilation unit finished
; caught 1 ERROR condition
; caught 2 STYLE-WARNING conditions
#<FUNCTION (LAMBDA (ROW)) {2497F245}>
How can I write macros to generate the code I need, and in particular, how do I implement row-satisfies-rule?
Using the ideas from Ivijay and discipulus, I have modified the macro so that it compiles and works, even allowing forms to be passed as the arguments. It runs a bit differently from my originally planned macro since I determined that including row as an argument made for smoother code. However, it is ugly as sin. Does anyone know how to clean it up so it performs the same without the call to eval?
(defmacro row-satisfies-rule-p (row rule)
(let ((x (gensym))
(y (gensym)))
`(let ((,x ,row)
(,y ,rule))
(destructuring-bind (a b c) ,y
(eval `(,a (svref ,,x ,b) ,c))))))
Also, an explanation of clean, Lispy ways to get macros to generate code to properly evaluate the arguments at runtime would be greatly appreciated.
First of all, Lisp macros have "destructuring" argument lists. This is a nice feature that means instead of having an argument list (rule) and then taking it apart with (car rule) (cadr rule) (caddr rule), you can simply make the argument list ((function-name column-index value)). That way the macro expects a list of three elements as an argument, and each element of the list is then bound to the corresponding symbol in the arguemnt list. You can use this or not, but it's usually more convenient.
Next, `, doesn't actually do anything, because the backquote tells Lisp not to evaluate the following expression and the comma tells it to evaluate it after all. I think you meant just ,(car x), which evaluates (car x). This isn't a problem anyway if you use destructuring arguments.
And since you're not introducing any new variables in the macro expansion, I don't think (gensym) is necessary in this case.
So we can rewrite the macro like this:
(defmacro row-satisfies-rule ((function-name column-index value))
`(lambda (row)
(,function-name (svref row ,column-index) ,value)))
Which expands just how you wanted:
(macroexpand-1 '(row-satisfies-rule (< 1 2)))
=> (LAMBDA (ROW) (< (SVREF ROW 1) 2))
Hope this helps!
If you need the argument to be evaluated to get the rule set, then here's a nice way to do it:
(defmacro row-satisfies-rule (rule)
(destructuring-bind (function-name column-index value) (eval rule)
`(lambda (row)
(,function-name (svref row ,column-index) ,value))))
Here's an example:
(let ((rules '((< 1 2) (> 3 4))))
(macroexpand-1 '(row-satisfies-rule (car rules))))
=> (LAMBDA (ROW) (< (SVREF ROW 1) 2))
just like before.
If you want to include row in the macro and have it give you your answer straightaway instead of making a function to do that, try this:
(defmacro row-satisfies-rule-p (row rule)
(destructuring-bind (function-name column-index value) rule
`(,function-name (svref ,row ,column-index) ,value)))
Or if you need to evaluate the rule argument (e.g. passing '(< 1 2) or (car rules) instead of (< 1 2)) then just use (destructuring-bind (function-name column-index value) (eval rule)
Actually, a function seems more appropriate than a macro for what you're trying to do. Simply
(defun row-satisfies-rule-p (row rule)
(destructuring-bind (function-name column-index value) rule
(funcall function-name (svref row column-index) value)))
works the same way as the macro and is much neater, without all the backquoting mess to worry about.
In general, it's bad Lisp style to use macros for things that can be accomplished by functions.
One thing to understand is that the backquote feature is completely unrelated to macros. It can be used for list creation. Since source code usually consists of lists, it may be handy in macros.
CL-USER 4 > `((+ 1 2) ,(+ 2 3))
((+ 1 2) 5)
The backquote introduces a quoted list. The comma does the unquote: the expression after the comma is evaluated and the result inserted. The comma belongs to the backquote: the comma is only valid inside a backquote expression.
Note also that this is strictly a feature of the Lisp reader.
Above is basically similar to:
CL-USER 5 > (list '(+ 1 2) (+ 2 3))
((+ 1 2) 5)
This creates a new list with the first expression (not evaluated, because quoted) and the result of the second expression.
Why does Lisp provide backquote notation?
Because it provides a simple template mechanism when one wants to create lists where most of the elements are not evaluated, but a few are. Additionally the backquoted list looks similar to the result list.
you don't need nested backquotes to solve this problem. Also, when it's a macro, you don't have to quote your arguments. So (row-satisfies-rule (< 1 2)) is lispier than (row-satisfies-rule '(< 1 2)).
(defmacro row-satisfies-rule (rule)
(destructuring-bind (function-name column-index value) rule
`(lambda (row)
(,function-name (svref row ,column-index) ,value))))
will solve the problem for all calls in the first form. Solving the problem when in the second form is left as an exercise.
I am learning common lisp and tried to implement a swap value function to swap two variables' value. Why the following does not work?
(defun swap-value (a b)
(setf tmp 0)
(progn
((setf tmp a)
(setf a b)
(setf b tmp))))
Error info:
in: LAMBDA NIL
; ((SETF TMP A) (SETF A B) (SETF B TMP))
;
; caught ERROR:
; illegal function call
; (SB-INT:NAMED-LAMBDA SWAP-VALUE
; (A B)
You can use the ROTATEF macro to swap the values of two places. More generally, ROTATEF rotates the contents of all the places to the left. The contents of the
leftmost place is put into the rightmost place. It can thus be used with more than two places.
dfan is right, this isn't going to swap the two values.
The reason you are getting that error though is that this:
(progn
((setf tmp a)
(setf a b)
(setf b tmp)))
should be this:
(progn
(setf tmp a)
(setf a b)
(setf b tmp))
The first progn has one s-expression in the body, and it's treated
as an application of the function (setf tmp a). In Common Lisp, I
think that only variables or lambda forms can be in the function
position of an application. I could be wrong about the details here,
but I know there are restrictions in CL that aren't in Scheme. That's
why it's an illegal call.
For instance, this is illegal in CL and results in the same error:
CL-USER> ((if (< 1 2) #'+ #'*) 2 3)
; in: LAMBDA NIL
; ((IF (< 1 2) #'+ #'*) 2 3)
;
; caught ERROR:
; illegal function call
;
; compilation unit finished
; caught 1 ERROR condition
You COULD write a swap as a macro (WARNING: I'm a Lisp noob, this
might be a terrible reason for a macro and a poorly written one!)
(defmacro swap (a b)
(let ((tmp (gensym)))
`(progn
(setf ,tmp ,a)
(setf ,a ,b)
(setf ,b ,tmp))))
Nope! Don't do this. Use rotatef as Terje Norderhaug points out.
A function (rather than macro) swapping two special variables can take the variable symbols as arguments. That is, you quote the symbols in the call. Below is an implementation of such a swap function:
(defvar *a* 1)
(defvar *b* 2)
(defun swap-values (sym1 sym2)
(let ((tmp (symbol-value sym1)))
(values
(set sym1 (symbol-value sym2))
(set sym2 tmp))))
? (swap-values '*a* '*b*)
2
1
? *a*
2
Note the use of defvar to define global/special variables and the per convention use of earmuffs (the stars) in their names. The symbol-value function provides the value of a symbol, while set assigns a value to the symbol resulting from evaluating its first argument. The values is there to make the function return both values from the two set statements.
You can not use setf to build a lexical variable tmp. You can use let, as follow:
(defun swap-value (a b)
(let ((tmp 0))
(setf tmp a)
(setf a b)
(setf b tmp))
(values a b))
which will do you hope.
Complimentary to other answers, the OP's targeted problem - the (multiple) value assignment issue - can be solved by parallel assignment using psetf:
(let ((a 21)
(b 42))
(psetf a b
b a)
(print (list a b)))
;; (42 21)