Can someone explain the difference between using drracket with swindle (any of the versions) and firing up the interpreter as: racket -I swindle
Many expressions evaluate the same in both environments, but some don't. For example:
(let-syntax ((let* (syntax-rules () ((let* a b) "You used let*")))) (let* 3 4))
... evaluates to "You used let*" in every scheme-based language I tried in drracket. However, it fails in the interpreter brought up by "racket -I swindle" on the command line with the error
; readline-input:1:20: syntax-rules: unbound identifier;
; also, no #%app syntax transformer is bound in the transformer phase
; at: syntax-rules
; in: (syntax-rules () ((let* a b) "You used let*"))
; [,bt for context]
Your example uses syntax-rules at phase 1 (aka the "for-syntax phase", aka the "transformer phase"), but it appears that the swindle module (unlike racket/base and racket) does not provide any bindings for-syntax. (It looks like swindle uses an old technique to provide phase-1 bindings that works for modules written in the swindle language but not for a REPL started with racket -l swindle.)
One workaround is to evaluate
(require (for-syntax racket/base))
before evaluating your example, and then your example should work as you expect.
Related
I would like to get the results of a function which calls a shell command and returns a string.
I am using racket, and this is my first attempt:
(define (run-function)
(let*
([stdout (some-function)]
[output (process-string stdout)])
;; many more lines...
output))
It seems to work well enough, but suppose that I would like to write functions similar to run-function for many other shell commands.
To avoid code duplication, I could just define a more general function like this:
(define (shell cmd)
(let*
([stdout (cmd)]
[output (process-string stdout)])
;; many more lines...
output))
and then call for example (shell ls) or (shell pwd).
An alternative would be using a simple macro:
(define-syntax shell
(syntax-rules ()
[(shell cmd)
(let*
([stdout (cmd)]
[output (process-string stdout)])
;; many more lines...
output)]))
This would also have the advantage of allowing a more general syntax, for example I could easily change the macro so that it takes as many parameters (commands) as I want, but I am sure the same behaviour could be replicated by writing the higher order function more sensibly.
Q. What are the pros/cons of writing a higher order function vs a macro? Is there a clear winner between the two?
I agree with what #MLavrentyev said, "use a function if you can".
And the Racket style guide also says:
Define functions when possible, Or, do not introduce macros when functions will do.
But why? One reason is that if you write shell as a function, you can pass shell to other functions. You can do the same with macros via the identifier macro feature, but it's much more difficult to do so (and you will effectively end up creating a function anyway).
Another reason is that using macros will make compiled code larger than using functions. This is because macros are expanded at compile-time, and the expanded code is then compiled (into bytecode in Racket BC, machine code in Racket CS). So it's as if you write (let* ...) over and over again. If you distribute the compiled or executable Racket program, you wouldn't want the size to be large.
In fact, a good practice when writing a macro is to try to stuff code into functions as much as possible. Instead of writing:
(define-syntax-value (debug code)
(let ([val code])
(printf "~a evaluates to ~a\n" (quote code) val)
val))
it would be better to write it as:
(define (debug-core expr val)
(printf "~a evaluates to ~a\n" expr val)
val)
(define-syntax-value (debug code)
(debug-core (quote code) code))
I have a program that uses the at-exp meta-language:
#lang at-exp racket
(define v '#f{str})
(displayln v)
I want to manually type it line-by-line into the Racket REPL. Unfortunately, #lang at-exp racket is not valid in the REPL:
$ racket
Welcome to Racket v6.11.
> #lang at-exp racket
; readline-input:1:0: read: #lang not enabled in the current context [,bt for
; context]
; at-exp: undefined;
; cannot reference undefined identifier
; [,bt for context]
; racket: undefined;
; cannot reference undefined identifier
; [,bt for context]
>
How do I set the meta-language in the Racket REPL?
For the beginners who have ended up here, the short answer is no. The #lang shorthand should only be used in files. If you want to run your example program in the REPL, you could load it using this command.
racket --repl --eval '(enter! (file "your-file.rkt"))'
The more detailed answer is that the #lang shorthand gets re-written behind the scenes into module form.
So for example something like this...
#lang racket
(define v "Hello!")
(displayln v))
Is re-written to what you see below and you could copy and paste this into the repl and then require the module, e.g. in the repl type...
> (module the-hello-module racket
(define v "Hello!")
(displayln v))
> (require 'the-hello-module)
Hello!
But from what I can tell, that re-write is dependent on the languages you're using, so the rewrite is probably not something you want to do manually and I don't think the at-exp metalanguage is something you would be add manually to module form.
I don't know why they're different, but the REPL inside DrRacket can use the #lang shorthand:
Unlike the command-line REPL, the DrRacket REPL adopts the language specified by the #lang line at the top of the definitions panel. DrRacket also uses the REPL itself to print output from the program.
https://beautifulracket.com/explainer/repl.html#a_Rv5YS
So if you have #lang at-exp racket/base in the definitions panel and click "Run" the bottom REPL panel will show Language: at-exp racket/base, etc., and this will work:
> #string-append{this is a test}
"this is a test"
That page also says:
More intricate REPL hacking is possible. For instance, the REPL can be configured to use a custom reader (other than Racket’s default S-expression reader).
https://beautifulracket.com/basic-3/the-repl.html
However, the steps described there are much more involved than I care to get into. I don't know what DrRacket is doing in the background to make #lang at-exp work.
I have sample code like this:
#!/usr/bin/guile -s
!#
(define (process body)
(list 'list (map (lambda (lst)
(list 'quote (car lst)))
body)))
(defmacro macro (body)
(list 'quote (process body)))
(display (macro ((foo bar) (bar baz))))
(newline)
it run but I've got error from compiler
ERROR: Unbound variable: process
functions inside macros should be allowed, why I got this error?
Functions inside macros are allowed in Guile and in most other Scheme dialects.
However, the crucial question is: Which functions are available for a macro to call out during the expansion process?
Think of it this way: When the compiler is processing your code, it is first focused on turning your source code into something that can be run at some point in the future. But the compiler might not necessarily be able to execute those same functions right now while it is compiling them, at the same time that your macro is running and expanding the source code in general.
Why wouldn't such a function be available? Well, one example would be: What if the function's body used the macro you are in the midst of defining? Then you would have a little chicken/egg problem. The function would need to run the macro to be compiled (since the macro use in the body needs to be expanded, at compile-time) ... But the macro would need the compiled function available in order to even run!
(Furthermore, there might be some functions that you only want to be available at compile-time, as a helper for your macros, but that you do not want to be available at run-time, so that it will not be included in your program executable when you deploy it, as that would waste space in the deployed binary.)
One of my favorite papers describing this problem, and the particular solution adopted by MzScheme (now known as Racket), is the "You Want It When" paper by Matthew Flatt.
So, this is a problem that any Scheme dialect with a procedural macro system has to deal with in some way, and Guile is no exception.
In Guile's case, one fix that is directly documented in the Guile manual is to use the eval-when special form, which allows you to specify at what phases a particular definition is meant to be available.
(The "You Want It When" paper referenced above describes some problems with eval-when, but since it is what the Guile manual documents, I'm going to stick with it for now. I do recommend that after you understand eval-when, that you then look into Racket's solution, and see if Guile offers anything similar.)
So in your case, since you want the process function to be available at compile-time (for use in the macro definition), you could write:
#!/usr/bin/guile -s
!#
(eval-when (expand)
(define (process body)
(list 'list (map (lambda (lst)
(list 'quote (car lst)))
body))))
(defmacro macro (body)
(list 'quote (process body)))
(display (macro ((foo bar) (bar baz))))
(newline)
Given this situation:
(defmacro mac1 ...)
(defun func1 ()
(mac1 ...))
Redefinition of mac1 in the live environment will not affect func1 until it is itself recompiled.
Is there a way to get emacs or lisp to automatically recompile func1 whenever mac1 is recompiled?
Something like this in the source would be acceptable:
(watch
(defmacro mac1 ...))
(on-signal (mac1)
(defun func1 ...))
Not that hard to implement, but I would rather avoid wheel reinvention.
Macros are not necessarily compiled. If you are using SBCL (see manual), there is a variable named sb-ext:*evaluator-mode* which can be set to :interpret so that macro are expanded during evaluation. Other implementations might provide something similar. That allows you to change the definitions of macros without recompiling call sites like you already do with functions.
Alternatively, slime defines a function named slime-who-macroexpands. You'd have to dig a little bit to see how it works and maybe exploit it, either in the Common Lisp environment (swank) or the emacs side.
For example in LispWorks you can do the following. Probably SBCL has a similar facility.
Let's say we have this:
(defmacro foo ()
`(list 1 2 3))
(defun bar () (first (foo)))
(defun baz () (second (foo)))
Now you can ask who calls foo:
CL-USER 11 > (who-calls 'foo)
(BAZ BAR)
This makes it easy to re-compile both functions:
CL-USER 12 > (mapcar 'compile (who-calls 'foo))
;;;*** Warning in BAZ: The definition of BAZ is already compiled.
;;;*** Warning in BAR: The definition of BAR is already compiled.
(BAZ BAR)
Since LispWorks keeps a who-calls database, one can recompile all functions which directly use and depend on other functions/macros.
The editor has the command Edit Callers and Continue Tags Search to find the callers and then to recompile them manually. It should be easy/possible to write an editor command which recompiles all callers.
I'm trying to define a new language in racket, let's call it wibble. Wibble will allow modules to be loaded so it has to translate it's forms to Racket require forms. But I'm having trouble getting require to work when used in a language extension. I eventually tracked down my problems to the following strange behaviour.
Here's my reader which redefines read and read-syntax
=== wibble/lang/reader.rkt ===
#lang racket/base
(provide (rename-out (wibble-read read) (wibble-read-syntax read-syntax)))
(define (wibble-read in)
(wibble-read-syntax #f in))
(define (wibble-read-syntax src in)
#`(module #,(module-name src) wibble/lang
#,#(read-all src in)))
(define (module-name src)
(if (path? src)
(let-values (((base name dir?) (split-path src)))
(string->symbol (path->string (path-replace-suffix name #""))))
'anonymous-module))
(define (read-all src in)
(let loop ((all '()))
(let ((obj (read-syntax src in)))
(if (eof-object? obj)
(reverse all)
(loop (cons obj all))))))
and here's my much simplified language module, this introduces (require racket/base) into each wibble module
=== wibble/lang.rkt ===
#lang racket/base
(require (for-syntax racket/base))
(provide (rename-out (wibble-module-begin #%module-begin)) #%app #%datum #%top)
(define-syntax wibble-module-begin
(lambda (stx)
(syntax-case stx ()
((_ x ...) #`(#%module-begin (require #,(datum->syntax stx 'racket/base)) x ...)))))
With the above code then this wibble code 'works', i.e. there are no errors
#lang wibble
(cons 1 2)
(cons 3 4)
but the following
#lang wibble
(cons 1 2)
gives error message cons: unbound identifier in module in: cons
Really I'm just looking for an explanation as to what going on. I'm sure the difference is related to this from the racket docs (Racket Reference 3.1)
If a single form is provided, then it is partially expanded in a
module-begin context. If the expansion leads to #%plain-module-begin,
then the body of the #%plain-module-begin is the body of the module.
If partial expansion leads to any other primitive form, then the form
is wrapped with #%module-begin using the lexical context of the module
body; this identifier must be bound by the initial module-path import,
and its expansion must produce a #%plain-module-begin to supply the
module body. Finally, if multiple forms are provided, they are wrapped
with #%module-begin, as in the case where a single form does not
expand to #%plain-module-begin.
but even with that I don't understand why having a single form makes any difference, it's seems to be somthing to do with the timing of partial expansion but I'm not really sure. Nor do I understand why Racket treats a single form as a special case.
Incidentally I can fix the problem with a slight modification to my reader
(define (wibble-read-syntax src in)
#`(module #,(module-name src) wibble/lang
#,#(read-all src in) (void)))
Hard-coding a (void) form means I always have more than one form and eveything works.
Sorry for the long post, I'm just looking for some understanding of how this stuff works.
Alright, I think that I've figured it out.
Your intuition is correct in that the problem lies within the timing of the partial expansion of the single-form module body. Inside of your reader.rkt file, you produce a (module ...) form. As the quoted excerpt from your question states, the forms ... portion of this is then treated specially, since there is only one. Let's take a look at an excerpt from the documentation on partial expansion:
As a special case, when expansion would otherwise add an #%app, #%datum, or #%top identifier to an expression, and when the binding turns out to be the primitive #%app, #%datum, or #%top form, then expansion stops without adding the identifier.
I am almost certain that the partial expansion which occurs at this point does something to the cons identifier. This is the one part that I remain unsure of... my gut tells me that what's happening is that the partial expansion is attempting to find the binding for the cons identifier (since it is the first part of the parentheses, the identifier could be bound to a macro which should be expanded, so that needs to be checked) but is unable to, so it throws a tantrum. Note that even if cons has no phase 1 (syntax-expansion time) binding, the macro expander still expects there to be a phase 0 (runtime) binding for the identifier (among other things, this helps the expander remain hygienic). Because all of this partial expansion happens to the body of your (module ...) form (which is done before your (#%module-begin ...) form where you inject the (#%require ...) form), cons has no binding during the expansion, so the expansion, I believe, fails.
Nevertheless, a naive fix for your problem is to rewrite wibble-read-syntax as follows:
(define (wibble-read-syntax src in)
(let* ((read-in (read-all src in))
(in-stx (and (pair? read-in) (car read-in))))
#`(module #,(module-name src) wibble/lang
(require #,(datum->syntax in-stx 'racket/base))
#,#read-in))
You can then remove the (#%require ...) form from your (#%module-begin ...) macro.
That's not, in my opinion, the best way to fix the issue, however. As a matter of cleanliness, hard-coding in a require form like you've done in wibble/lang.rkt would make Eli Barzilay and co. cry. A much simpler way to do what you are trying to do is by updating your lang.rkt file to something like so:
=== wibble/lang.rkt ===
#lang racket/base
(require (for-syntax racket/base))
(provide (rename-out (wibble-module-begin #%module-begin))
(except-out (all-from-out racket/base) #%module-begin #%app #%datum #%top)
#%app #%datum #%top)
(define-syntax wibble-module-begin
(lambda (stx)
(syntax-case stx ()
((_ x ...) #`(#%module-begin x ...)))))
Writing in this convention removes the need for any hard-coded (require ...) forms and prevents subtle bugs like the one you've unearthed from occuring. If you are confused why this works, remember that you've already provided the #%module-begin identifier using this file, which is subsequently bound in all #lang wibble files. In principle, there is no limit on what identifiers you can bind in this fashion. If you would like some further reading, here's a shameless self-advertisement for a blog post I wrote a little while back on the subject.
I hope I've helped.
The problem is with the require (though I'm not sure I 100% understand all the behavior).
(require X) imports bindings from X with the lexical context of #'X. #'X here has the context of stx, which is the entire #'(module-begin x ...), which is not the context you want. You want the context of one of the cons expressions, i.e., one of the #'xs.
Something like this should work:
(define-syntax wibble-module-begin
(lambda (stx)
(syntax-case stx ()
[(_) #'(#%module-begin)]
[(m x y ...)
#`(#%module-begin
(require #,(datum->syntax #'x 'racket/base))
x y ...)])))
Though, as #belph warned, there's probably a more idiomatic way to accomplish what you want.
The behavior of your original program, and as you intuited, likely has to do with module's different treatment of single and multi sub-forms, but I think the "working" case might be an accident and could be a bug in the racket compiler.