I often find the following type of incremental definition useful:
(define (foo) (display "bar"))
(foo)
;prints bar
(define foo (let ((bar foo))
(lambda ()
(display "foo")
(bar))))
(foo)
;prints foobar
How do I preform this type of incremental definition with macros?
I could not get let-syntax to provide the same functionality.
Currently I use plt scheme, but would like to see answers in different lisp implementations as well.
Edit:
Naively I would want to do the following:
(define-syntax foo
(syntax-rules ()
((_) (display "bar"))))
(define-syntax foo
(let-syntax ((old-foo (syntax-rules () ((_) (foo)))))
(syntax-rules ()
((_) (begin
(display "foo")
(old-foo))))))
Translation of naive macros to working plt scheme macros:
(require-for-syntax scheme/base)
(define-syntax foo
(syntax-rules ()
[(foo) (display "bar")]))
(define-syntax foo
(let ([old (syntax-local-value #'foo)])
(lambda (stx)
#`(begin #,((syntax-rules ()
[(_) (begin (display "foo"))]) stx)
#,(old #'(_))))))
(foo)
If I am missing a better method let me know.
I do not think that you can do something like this with macros. I also do not see any point in trying.
Please note that macros are not just some functions with extra magic! Macros are something different altogether.
Perhaps you are looking for something like method combinations in Common Lisp?
With macros doing this you are on the way to create the hardest to maintain software on the planet.
Edit: in Common Lisp it is possible. I can't remember that I have ever seen it used in source code.
Adding behavior to functions or macros is often called 'advise' or 'advice' in the Common Lisp community. Some 'advise' tools have also the ability to advise macros.
FWIW (and this is definitely not much, since this is pretty much an exercise in target practicing with your feet), here is how you would do this in PLT Scheme with only hygienic syntax-rules macros:
(define-syntax foo
(syntax-rules ()
[(foo x) (list 'x '= x)]))
(define-syntax foo
(let ([old (syntax-local-value #'foo)])
(compose (syntax-rules ()
[(_ x ...) (list 'begin x ... 'end)])
old)))
(printf ">>> ~s\n" (foo (+ 1 2)))
This will not work inside a module, only on the REPL -- and that is a good thing. It is possible to do something similar in modules too, but if you're going for that, you can just as well use procedural macros (aka syntax-case macros), with an identifier that is bound at the syntax level and `set!'-ing its value to extend it. Still not a great idea, and can still lead to eyes bleeding, but some people like to hurt themselves...
(Oh, and BTW -- even doing this is still completely unrelated to whether the macros in question are hygienic or not.)
I think you could get this by using unhygienic macros, which I believe PLT Scheme supports. Then you would use exactly the same mechanisms as with regular functions, since the macros would be regular functions that happened to operate on S-expressions.
I don't know how to do this with hygienic macros, but I'm quite surprised you can't - I would consider asking on a PLT mailing list.
What about let*?
(define foobar
(let* ((foo (lambda () (display "foo")))
(bar (lambda () (foo) (display "bar"))) )
bar))
Related
I would like to get the name of the file where a particular function or macro definition was last written, for various documentation & testing purposes?
I have posted an answer that works well from me, after the help of PJB on #CommonLisp (IRC.LIBERA.CHAT), but will accept any answers which provide a portable solution that does not depend on SWANK or provides additional detail on how to achieve the same in other IDEs like LispWorks, ACL, etc.
If what you seek is a portable solution – one that is written in portable CL – then the answer to that is to define wrappers for defining forms and then use the wrappers.
(defvar *flocs* (make-hash-table :test #'equal))
(defgeneric function-location (f/name)
(:method ((name t))
(values (gethash name *flocs* nil) t))
(:method ((f function))
(multiple-value-bind (le cp nm) (function-lambda-expression f)
(declare (ignore le cp))
(if nm
(function-location nm)
(values nil nil)))))
(defmacro define-function (f args &body doc/decls/forms)
(when (or *load-pathname* *compile-file-pathname*)
;; Prefer *load-pathname*
(setf (gethash f *flocs*) (or *load-pathname* *compile-file-pathname*)))
`(defun ,f ,args ,#doc/decls/forms))
In real life you'd call define-function defun of course, and similarly with define-variable etc, and then construct a conduit package for CL which exported all the CL symbols while replacing the defining forms with these ones.
If what you seek is a portable solution in the sense that it exports some standard interface but has varying implementation-dependent backends, then probably looking at what SLY or SWANK do is a good start. In the case of LW you would want the backend to use DSPECs which are how it deals with location information:
> (dspec:dspec-definition-locations '(defun foo))
(((defun foo) :listener))
> (dspec:dspec-definition-locations '(defun needs))
(((defmacro needs)
#P"..."))
> (defclass foo () ())
#<standard-class foo 402000B763>
> (dspec:name-definition-locations dspec:*dspec-classes* 'foo)
(((defclass foo) :listener) ((defun foo) :listener))
PJB on #CommonLisp (IRC.LIBERA.CHAT) greatly assisted and provided the below answer (which I have paraphrased so any mistakes are on my end):
Firstly, CL is a lisp-2 (actually, lisp-∞) and thus there may not be a single definition attached to a symbol. For example, which foo is being referred to in the below?
(deftype foo () (member foo))
(defvar foo 'foo)
(defun foo () foo)
In addition, as an example, symbols may be defined in the REPL (no associated filename), so this is not a straightforward question that can be answered without making assumptions on the intent.
Now, when the behaviour of M-. is satisfactory for your requirements, you could look in slime/swank what the implementation specific API is to do it.
This should point you to swank:find-definition-for-emacs, which may be what you are after:
(swank:find-definitions-for-emacs "foo") #| -->
(("#'foo"
(:location (:file "/private/tmp/foo.lisp")
(:position 50)
(:snippet "(defun foo () foo)")))
("(type foo)"
(:location (:file "/private/tmp/foo.lisp")
(:position 1)
(:snippet "(deftype foo () '(member foo))")))
("(variable foo)"
(:location (:file "/private/tmp/foo.lisp")
(:position 32)
(:snippet "(defvar foo 'foo)"))))
Make sure to load swank as a dependency in your .asd files if you want to rely on the above.
EDIT: I also found the below very useful (and swank has a similar file for most implementations, so just go through each to see their equivalents):
https://github.com/slime/slime/blob/68c58c0194ff03cd147fcec99f0ee90ba9178875/swank/sbcl.lisp#L811
The function call (sb-introspect:find-definition-sources-by-name name type) (name is a symbol, type is a keyword, e.g. :function - refer above link) returns the file in which a definition is stored, assuming you are using SBCL. More (SBCL) details also in:
https://github.com/sbcl/sbcl/blob/master/contrib/sb-introspect/introspect.lisp
I am using syntax transformers to define macros in Racket. I want to create some helper functions to help me manipulate the syntax. However, the functions I defined outside the syntax transformer are not available inside the syntax transformer. For example, in the following code
(define (my-function x) (+ x 1))
(define-syntax my-macro
(lambda (stx)
(datum->syntax stx (my-function (cadr (syntax->datum stx))))))
I got the error "my-function: reference to an unbound identifier at phase: 1; the transformer environment".
After some searching, I am able to write the following code so that my-function is available inside the syntax transformer.
(begin-for-syntax
(define (my-function x) (+ x 1)))
(provide (for-syntax my-function))
(define-syntax my-macro
(lambda (stx)
(datum->syntax stx (my-function (cadr (syntax->datum stx))))))
But the problem is, my-function is not available outside the syntax transformer this time. Sometimes I want to check those helper functions in ordinary code, so I need to be able to call it from both inside and outside the syntax transformer, just like the function cadr. How can I achieve that?
I know my question has something to do with Racket's syntax model, in particular the concept of "phase level", but I never really understand it. If you could provide some easy-to-follow tutorials explaining it I would even be more grateful.
A common way is to define your function that you want to share across phases in another (sub)module. Then, require it twice.
#lang racket
(module common racket
(provide my-function)
(define (my-function x) (+ x 1)))
(require 'common
(for-syntax 'common))
(define-syntax my-macro
(lambda (stx)
(datum->syntax stx (my-function (cadr (syntax->datum stx))))))
(my-function 1)
(my-macro 123)
In this post, I ask tangentially why when I declare in SBCL
(defun a (&rest x)
x)
and then check what the function cell holds
(describe 'a)
COMMON-LISP-USER::A
[symbol]
A names a compiled function:
Lambda-list: (&REST X)
Derived type: (FUNCTION * (VALUES LIST &OPTIONAL))
Source form:
(LAMBDA (&REST X) (BLOCK A X))
I see this particular breakdown of the original function. Could someone explain what this output means? I'm especially confused by the last line
Source form:
(LAMBDA (&REST X) (BLOCK A X))
This is mysterious because for some reason not clear to me Lisp has transformed the original function into a lambda expression. It would also be nice to know the details of how a function broken down like this is then called. This example is SBCL. In Elisp
(symbol-function 'a)
gives
(lambda (&rest x) x)
again, bizarre. As I said in the other post, this is easier to understand in Scheme -- but that created confusion in the answers. So once more I ask, Why has Lisp taken a normal function declaration and seemingly stored it as a lambda expression?
I'm still a bit unclear what you are confused about, but here is an attempt to explain it. I will stick to CL (and mostly to ANSI CL), because elisp has a lot of historical oddities which just make things hard to understand (there is an appendix on elisp). Pre-ANSI CL was also a lot less clear on various things.
I'll try to explain things by writing a macro which is a simple version of defun: I'll call this defun/simple, and an example of its use will be
(defun/simple foo (x)
(+ x x))
So what I need to do is to work out what the expansion of this macro should be, so that it does something broadly equivalent (but simpler than) defun.
The function namespace & fdefinition
First of all I assume you are comfortable with the idea that, in CL (and elisp) the namespace of functions is different than the namespace of variable bindings: both languages are lisp-2s. So in a form like (f x), f is looked up in the namespace of function bindings, while x is looked up in the namespace of variable bindings. This means that forms like
(let ((sin 0.0))
(sin sin))
are fine in CL or elisp, while in Scheme they would be an error, as 0.0 is not a function, because Scheme is a lisp-1.
So we need some way of accessing that namespace, and in CL the most general way of doing that is fdefinition: (fdefinition <function name>) gets the function definition of <function name>, where <function name> is something which names a function, which for our purposes will be a symbol.
fdefinition is what CL calls an accessor: this means that the setf macro knows what to do with it, so that we can mutate the function binding of a symbol by (setf (fdefinition ...) ...). (This is not true: what we can access and mutate with fdefinition is the top-level function binding of a symbol, we can't access or mutate lexical function bindings, and CL provides no way to do this, but this does not matter here.)
So this tells us what our macro expansion needs to look like: we want to set the (top-level) definition of the name to some function object. The expansion of the macro should be like this:
(defun/simple foo (x)
x)
should expand to something involving
(setf (fdefinition 'foo) <form which makes a function>)
So we can write this bit of the macro now:
(defmacro defun/simple (name arglist &body forms)
`(progn
(setf (fdefinition ',name)
,(make-function-form name arglist forms))
',name))
This is the complete definition of this macro. It uses progn in its expansion so that the result of expanding it is the name of the function being defined, which is the same as defun: the expansion does all its real work by side-effect.
But defun/simple relies on a helper function, called make-function-form, which I haven't defined yet, so you can't actually use it yet.
Function forms
So now we need to write make-function-form. This function is called at macroexpansion time: it's job is not to make a function: it's to return a bit of source code which will make a function, which I'm calling a 'function form'.
So, what do function forms look like in CL? Well, there's really only one such form in portable CL (this might be wrong, but I think it is true), which is a form constructed using the special operator function. So we're going to need to return some form which looks like (function ...). Well, what can ... be? There are two cases for function.
(function <name>) denotes the function named by <name> in the current lexical environment. So (function car) is the function we call when we say (car x).
(function (lambda ...)) denotes a function specified by (lambda ...): a lambda expression.
The second of these is the only (caveats as above) way we can construct a form which denotes a new function. So make-function-form is going to need to return this second variety of function form.
So we can write an initial version of make-function-form:
(defun make-function-form (name arglist forms)
(declare (ignore name))
`(function (lambda ,arglist ,#forms)))
And this is enough for defun/simple to work:
> (defun/simple plus/2 (a b)
(+ a b))
plus/2
> (plus/2 1 2)
3
But it's not quite right yet: one of the things that functions defined by defun can do is return from themselves: they know their own name and can use return-from to return from it:
> (defun silly (x)
(return-from silly 3)
(explode-the-world x))
silly
> (silly 'yes)
3
defun/simple can't do this, yet. To do this, make-function-form needs to insert a suitable block around the body of the function:
(defun make-function-form (name arglist forms)
`(function (lambda ,arglist
(block ,name
,#forms))))
And now:
> (defun/simple silly (x)
(return-from silly 3)
(explode-the-world x))
silly
> (silly 'yes)
3
And all is well.
This is the final definition of defun/simple and its auxiliary function.
Looking at the expansion of defun/simple
We can do this with macroexpand in the usual way:
> (macroexpand '(defun/simple foo (x) x))
(progn
(setf (fdefinition 'foo)
#'(lambda (x)
(block foo
x)))
'foo)
t
The only thing that's confusing here is that, because (function ...) is common in source code, there's syntactic sugar for it which is #'...: this is the same reason that quote has special syntax.
It's worth looking at the macroexpansion of real defun forms: they usually have a bunch of implementation-specific stuff in them, but you can find the same thing there. Here's an example from LW:
> (macroexpand '(defun foo (x) x))
(compiler-let ((dspec::*location* '(:inside (defun foo) :listener)))
(compiler::top-level-form-name (defun foo)
(dspec:install-defun 'foo
(dspec:location)
#'(lambda (x)
(declare (system::source-level
#<eq Hash Table{0} 42101FCD5B>))
(declare (lambda-name foo))
x))))
t
Well, there's a lot of extra stuff in here, and LW obviously has some trick around this (declare (lambda-name ...)) form which lets return-from work without an explicit block. But you can see that basically the same thing is going on.
Conclusion: how you make functions
In conclusion: a macro like defun, or any other function-defining form, needs to expand to a form which, when evaluated, will construct a function. CL offers exactly one such form: (function (lambda ...)): that's how you make functions in CL. So something like defun necessarily has to expand to something like this. (To be precise: any portable version of defun: implementations are somewhat free to do implementation-magic & may do so. However they are not free to add a new special operator.)
What you are seeing when you call describe is that, after SBCL has compiled your function, it's remembered what the source form was, and the source form was exactly the one you would have got from the defun/simple macro given here.
Notes
lambda as a macro
In ANSI CL, lambda is defined as a macro whose expansion is a suitable (function (lambda ...)) form:
> (macroexpand '(lambda (x) x))
#'(lambda (x) x)
t
> (car (macroexpand '(lambda (x) x)))
function
This means that you don't have to write (function (lambda ...)) yourself: you can rely on the macro definition of lambda doing it for you. Historically, lambda wasn't always a macro in CL: I can't find my copy of CLtL1, but I'm pretty certain it was not defined as one there. I'm reasonably sure that the macro definition of lambda arrived so that it was possible to write ISLisp-compatible programs on top of CL. It has to be in the language because lambda is in the CL package and so users can't portably define macros for it (although quite often they did define such a macro, or at least I did). I have not relied on this macro definition above.
defun/simple does not purport to be a proper clone of defun: its only purpose is to show how such a macro can be written. In particular it doesn't deal with declarations properly, I think: they need to be lifted out of the block & are not.
Elisp
Elisp is much more horrible than CL. In particular, in CL there is a well-defined function type, which is disjoint from lists:
> (typep '(lambda ()) 'function)
nil
> (typep '(lambda ()) 'list)
t
> (typep (function (lambda ())) 'function)
t
> (typep (function (lambda ())) 'list)
nil
(Note in particular that (function (lambda ())) is a function, not a list: function is doing its job of making a function.)
In elisp, however, an interpreted function is just a list whose car is lambda (caveat: if lexical binding is on this is not the case: it's then a list whose car is closure). So in elisp (without lexical binding):
ELISP> (function (lambda (x) x))
(lambda (x)
x)
And
ELISP> (defun foo (x) x)
foo
ELISP> (symbol-function 'foo)
(lambda (x)
x)
The elisp intepreter then just interprets this list, in just the way you could yourself. function in elisp is almost the same thing as quote.
But function isn't quite the same as quote in elisp: the byte-compiler knows that, when it comes across a form like (function (lambda ...)) that this is a function form, and it should byte-compile the body. So, we can look at the expansion of defun in elisp:
ELISP> (macroexpand '(defun foo (x) x))
(defalias 'foo
#'(lambda (x)
x))
(It turns out that defalias is the primitive thing now.)
But if I put this definition in a file, which I byte compile and load, then:
ELISP> (symbol-function 'foo)
#[(x)
"\207"
[x]
1]
And you can explore this a bit further: if you put this in a file:
(fset 'foo '(lambda (x) x))
and then byte compile and load that, then
ELISP> (symbol-function 'foo)
(lambda (x)
x)
So the byte compiler didn't do anything with foo because it didn't get the hint that it should. But foo is still a fine function:
ELISP> (foo 1)
1 (#o1, #x1, ?\C-a)
It just isn't compiled. This is also why, if writing elisp code with anonymous functions in it, you should use function (or equivalently #'). (And finally, of course, (function ...) does the right thing if lexical scoping is on.)
Other ways of making functions in CL
Finally, I've said above that function & specifically (function (lambda ...)) is the only primitive way to make new functions in CL. I'm not completely sure that's true, especially given CLOS (almost any CLOS will have some kind of class instances of which are functions but which can be subclassed). But it does not matter: it is a way and that's sufficient.
DEFUN is a defining macro. Macros transform code.
In Common Lisp:
(defun foo (a)
(+ a 42))
Above is a definition form, but it will be transformed by DEFUN into some other code.
The effect is similar to
(setf (symbol-function 'foo)
(lambda (a)
(block foo
(+ a 42))))
Above sets the function cell of the symbol FOO to a function. The BLOCK construct is added by SBCL, since in Common Lisp named functions defined by DEFUN create a BLOCK with the same name as the function name. This block name can then be used by RETURN-FROM to enable a non-local return from a specific function.
Additionally DEFUN does implementation specific things. Implementations also record development information: the source code, the location of the definition, etc.
Scheme has DEFINE:
(define (foo a)
(+ a 10))
This will set FOO to a function object.
My son and I are learning Racket together and are building a very simple text-based adventure for use directly from the REPL. So, for example, the player can type (go 'north) or (take 'apple).
After getting some basic stuff working, my son thought that quoting the noun was a bit of a pain (strangely, the parens don't bother him!), and so we hacked around with macros for a bit and we did get something working but it required an explicit function and a corresponding macro e.g.
(define (do-take item) ...)
(define-syntax (take stx)
(define item (cadr (syntax->datum stx)))
(datum->syntax stx `(do-take ',item)))
I figured we could do better than this, so I read around a bit more and came up with this:
(require (for-syntax racket/syntax))
(define-syntax (define-verb stx)
(syntax-case stx ()
[(_ (verb noun) body-first body-rest ...)
(with-syntax ([verb-fun (format-id stx "do-~a" #'verb)])
#'(begin
(define-syntax-rule (verb noun) (verb-fun 'noun))
(define (verb-fun noun) body-first body-rest ...)))]))
So now, we can write (define-verb (take item) ...) and the player at the REPL can type (take apple).
My question is whether, given what we want to achieve, this is a reasonable approach or whether there a more simple / idiomatic way to achieve the same thing?
In general, the main thing I would recommend doing is using the syntax/parse library. It has more tools for parsing syntax. You can even use forms like define-syntax-parser to make your macro even more concise. Rewriting your code using syntax/parse (dropping that one line because it doesn't seem to be doing anything), your macro would look like this:
#lang racket
(require syntax/parse/define
(for-syntax syntax/parse racket/syntax))
(define-syntax-parser define-verb
[(_ (verb:id noun) body ...+)
(define/syntax-parse verb-fun (format-id stx "do-~a" #'verb))
#'(begin
(define-simple-macro (verb noun) (verb-fun 'noun))
(define (verb-fun noun) body ...))])
This gives you a few nice things above the example you gave:
the :id ensures that verb is a literal identifier, rather than an expression.
the ...+ means you only need to have one body pattern, rather than two.
Using define/syntax-parse means your code does not get more indented than with-syntax. (Although this one is a matter of preference.)
What's the simplest way to define a capturing macro using define-syntax or define-syntax-rule in Racket?
As a concrete example, here's the trivial aif in a CL-style macro system.
(defmacro aif (test if-true &optional if-false)
`(let ((it ,test))
(if it ,if-true ,if-false)))
The idea is that it will be bound to the result of test in the if-true and if-false clauses. The naive transliteration (minus optional alternative) is
(define-syntax-rule (aif test if-true if-false)
(let ((it test))
(if it if-true if-false)))
which evaluates without complaint, but errors if you try to use it in the clauses:
> (aif "Something" (displayln it) (displayln "Nope")))
reference to undefined identifier: it
The anaphora egg implements aif as
(define-syntax aif
(ir-macro-transformer
(lambda (form inject compare?)
(let ((it (inject 'it)))
(let ((test (cadr form))
(consequent (caddr form))
(alternative (cdddr form)))
(if (null? alternative)
`(let ((,it ,test))
(if ,it ,consequent))
`(let ((,it ,test))
(if ,it ,consequent ,(car alternative)))))))))
but Racket doesn't seem to have ir-macro-transformer defined or documented.
Racket macros are designed to avoid capture by default. When you use define-syntax-rule it will respect lexical scope.
When you want to "break hygiene" intentionally, traditionally in Scheme you have to use syntax-case and (carefully) use datum->syntax.
But in Racket the easiest and safest way to do "anaphoric" macros is with a syntax parameter and the simple define-syntax-rule.
For example:
(require racket/stxparam)
(define-syntax-parameter it
(lambda (stx)
(raise-syntax-error (syntax-e stx) "can only be used inside aif")))
(define-syntax-rule (aif condition true-expr false-expr)
(let ([tmp condition])
(if tmp
(syntax-parameterize ([it (make-rename-transformer #'tmp)])
true-expr)
false-expr)))
I wrote about syntax parameters here and also you should read Eli Barzilay's Dirty Looking Hygiene blog post and Keeping it Clean with Syntax Parameters paper (PDF).
See Greg Hendershott's macro tutorial. This section uses anaphoric if as example:
http://www.greghendershott.com/fear-of-macros/Syntax_parameters.html
Although the answer above is the accepted way to implement aif in the Racket community, it has severe flaws. Specifically, you can shadow it by defining a local variable named it.
(let ((it 'gets-in-the-way))
(aif 'what-i-intended
(display it)))
The above would display gets-in-the-way instead of what-i-intended, even though aif is defining its own variable named it. The outer let form renders aif's inner let definition invisible. This is what the Scheme community wants to happen. In fact, they want you to write code that behaves like this so badly, that they voted to have my original answer deleted when I wouldn't concede that their way was better.
There is no bug-free way to write capturing macros in Scheme. The closest you can come is to walk down the syntax tree that may contain variables you want to capture and explicitly strip the scoping information that they contain, replacing it with new scoping information that forces them to refer to your local versions of those variables. I wrote three "for-syntax" functions and a macro to help with this:
(begin-for-syntax
(define (contains? atom stx-list)
(syntax-case stx-list ()
(() #f)
((var . rest-vars)
(if (eq? (syntax->datum #'var)
(syntax->datum atom))
#t
(contains? atom #'rest-vars)))))
(define (strip stx vars hd)
(if (contains? hd vars)
(datum->syntax stx
(syntax->datum hd))
hd))
(define (capture stx vars body)
(syntax-case body ()
(() #'())
(((subform . tl) . rest)
#`(#,(capture stx vars #'(subform . tl)) . #,(capture stx vars #'rest)))
((hd . tl)
#`(#,(strip stx vars #'hd) . #,(capture stx vars #'tl)))
(tl (strip stx vars #'tl)))))
(define-syntax capture-vars
(λ (stx)
(syntax-case stx ()
((_ (vars ...) . body)
#`(begin . #,(capture #'(vars ...) #'(vars ...) #'body))))))
That gives you the capture-vars macro, which allows you to explicitly name the variables from the body you'd like to capture. aif can then be written like this:
(define-syntax aif
(syntax-rules ()
((_ something true false)
(capture-vars (it)
(let ((it something))
(if it true false))))
((_ something true)
(aif something true (void)))))
Note that the aif I have defined works like regular Scheme's if in that the else-clause is optional.
Unlike the answer above, it is truly captured. It's not merely a global variable:
(let ((it 'gets-in-the-way))
(aif 'what-i-intended
(display it)))
The inadequacy of just using a single call to datum->syntax
Some people think that all you have to do to create a capturing macro is use datum->syntax on one of the top forms passed to your macro, like this:
(define-syntax aif
(λ (stx)
(syntax-case stx ()
((_ expr true-expr false-expr)
(with-syntax
((it (datum->syntax #'expr 'it)))
#'(let ((it expr))
(if it true-expr false-expr))))
((_ expr true-expr)
#'(aif expr true-expr (void))))))
Just using datum->syntax is only a 90% solution to writing capturing macros. It will work in most cases, but break in some cases, specifically if you incorporate a capturing macro written this way in another macro. The above macro will only capture it if the expr comes from the same scope as the true-expr. If they come from different scopes (this can happen merely by wrapping the user's expr in a form generated by your macro), then the it in true-expr will not be captured and you'll be left asking yourself "WTF won't it capture?"
You may be tempted to quick-fix this by using (datum->syntax #'true-expr 'it) instead of (datum->syntax #'expr 'it). In fact this makes the problem worse, since now you won't be able to use aif to define acond:
(define-syntax acond
(syntax-rules (else)
((_) (void))
((_ (condition . body) (else . else-body))
(aif condition (begin . body) (begin . else-body)))
((_ (condition . body) . rest)
(aif condition (begin . body) (acond . rest)))))
If aif is defined using the capture-vars macro, the above will work as expected. But if it's defined by using datum->syntax on the true-expr, the the addition of begin to the bodies will result in it being visible in the scope of acond's macro definition instead of the code that invoked acond.
The impossibility of really writing a capturing macro in Racket
This example was brought to my attention, and demonstrates why you just can't write a real capturing macro in Scheme:
(define-syntax alias-it
(syntax-rules ()
((_ new-it . body)
(let ((it new-it)) . body))))
(aif (+ 1 2) (alias-it foo ...))
capture-vars cannot capture the it in alias-it's macroexpansion, because it won't be on the AST until after aif is finished expanding.
It is not possible at all to fix this problem, because the macro definition of alias-it is most probably not visible from the scope of aif's macro definition. So when you attempt to expand it within aif, perhaps by using expand, alias-it will be treated as a function. Testing shows that the lexical information attached to alias-it does not cause it to be recognized as a macro for a macro definition written out of scope from alias-it.
Some would argue that this shows why the syntax-parameter solution is the superior solution, but perhaps what it really shows is why writing your code in Common Lisp is the superior solution.