I'm not very familiar with Clojure/Lisp macros. I would like to write apply-recur macro which would have same meaning as (apply recur ...)
I guess there is no real need for such macro but I think it's a good exercise. So I'm asking for your solution.
Well, there really is no need for that, if only because recur cannot take varargs (a recur to the top of the function takes a single final seqable argument grouping all arguments pass the last required argument). This doesn't affect the validity of the exercise, of course.
However, there is a problem in that a "proper" apply-recur should presumably handle argument seqs returned by arbitrary expressions and not only literals:
;; this should work...
(apply-recur [1 2 3])
;; ...and this should have the same effect...
(apply-recur (vector 1 2 3))
;; ...as should this, if (foo) returns [1 2 3]
(apply-recur (foo))
However, the value of an arbitrary expression such as (foo) is simply not available, in general, at macro expansion time. (Perhaps (vector 1 2 3) might be assumed to always yield the same value, but foo might mean different things at different times (one reason eval wouldn't work), be a let-bound local rather than a Var (another reason eval wouldn't work) etc.)
Thus to write a fully general apply-recur, we would need to be able to determine how many arguments a regular recur form would expect and have (apply-recur some-expression) expand to something like
(let [seval# some-expression]
(recur (nth seval# 0)
(nth seval# 1)
...
(nth seval# n-1))) ; n-1 being the number of the final parameter
(The final nth might need to be nthnext if we're dealing with varargs, which presents a problem similar to what is described in the next paragraph. Also, it would be a good idea to add an assertion to check the length of the seqable returned by some-expression.)
I am not aware of any method to determine the proper arity of a recur at a particular spot in the code at macro-expansion time. That does not mean one isn't available -- that's something the compiler needs to know anyway, so perhaps there is a way to extract that information from its internals. Even so, any method for doing that would almost certainly need to rely on implementation details which might change in the future.
Thus the conclusion is this: even if it is at all possible to write such a macro (which might not even be the case), it is likely that any implementation would be very fragile.
As a final remark, writing an apply-recur which would only be capable of dealing with literals (actually the general structure of the arg seq would need to be given as a literal; the arguments themselves -- not necessarily, so this could work: (apply-recur [foo bar baz]) => (recur foo bar baz)) would be fairly simple. I'm not spoiling the exercise by giving away the solution, but, as a hint, consider using ~#.
apply is a function that takes another function as an argument. recur is a special form, not a function, so it cannot be passed to apply.
Related
I'd like to compare the code contents of two syntax objects and ignore things like contexts. Is converting them to datum the only way to do so? Like:
(equal? (syntax->datum #'(x+1)) (syntax->datum #'(x+1)))
If you want to compare both objects without deconstructing them at all, then yes.
HOWEVER, the problem with this method is that it only compares the datum attached to two syntax objects, and won't actually compare their binding information.
The analogy that I've heard (from Ryan Culpepper), is this is kind of like taking two paintings, draining of them of their color, and seeing if they are identical. While they might be similar in some ways, you will miss a lot of differences from the different colors.
A better approach (although it does require some work), is to use syntax-e to destruct the syntax object into more primitive lists of syntax objects, and do this until you get identifiers (basically a syntax object whose datum is a symbol), from there, you can generally use free-identifier=? (and sometimes bound-identifier=? to see if each identifier can bind each other, and identifier-binding to compare module level identifiers.
The reason why there isn't a single simple predicate to compare two arbitrary syntax objects is because, generally, there isn't really one good definition for what makes two pieces of code equal, even if you only care about syntactic equality. For example, using the functions referenced above doesn't track internal bindings in a syntax object, so you will still get a very strict definition of what it means to be 'equal'. that is, both syntax objects have the same structure with identifiers that are either bound to the same module, or are free-identifier=?.
As such, before you use this answer, I highly recommend you take a step back and make sure this is really what you want to do. Once in a blue moon it is, but most of the time you actually are trying to solve a similar, yet simpler, problem.
Here's a concrete example of one possible way you could do the "better approach" Leif Andersen mentioned.
I have used this in multiple places for testing purposes, though if anyone wanted to use it in non-test code, they would probably want to re-visit some of the design decisions.
However, things like the equal?/recur pattern used here should be helpful no matter how you decide to define what equality means.
Some of the decisions you might want to make different choices on:
On identifiers, do you want to check that the scopes are exactly the same (bound-identifier=?), or would you want to assume that they would be bound outside of the syntax object and check that they are bound to the same thing, even if they have different scopes (free-identifier=?)? Note that if you choose the first one, then checking the results of macro expansion will sometimes return #false because of scope differences, but if you choose the second one, then if any identifier is not bound outside of the syntax object, then it would be as if you only care about symbol=? equality on names, so it will return #true in some places where it shouldn't. I chose the first one bound-identifier=? here because for testing, a "false positive" where the test fails is better than a "false negative" where the tests succeeds in cases it shouldn't.
On source locations, do you want to check that they are equal, or do you want to ignore them? This code ignores them because it's only for testing purposes, but if you want equality only for things which have the same source location, you might want to check that using functions like build-source-location-list.
On syntax properties, do you want to check that they are equal, or do you want to ignore them? This code ignores them because it's only for testing purposes, but if you want to check that you might want to use functions like syntax-property-symbol-keys.
Finally here is the code. It may not be exactly what you want depending on how you answered the questions above. However, its structure and how it uses equal?/recur might be helpful to you.
(require rackunit)
;; Works on fully wrapped, non-wrapped, and partially
;; wrapped values, and it checks that the inputs
;; are wrapped in all the same places. It checks scopes,
;; but it does not check source location.
(define-binary-check (check-stx=? stx=? actual expected))
;; Stx Stx -> Bool
(define (stx=? a b)
(cond
[(and (identifier? a) (identifier? b))
(bound-identifier=? a b)]
[(and (syntax? a) (syntax? b))
(and (bound-identifier=? (datum->syntax a '||) (datum->syntax b '||))
(stx=? (syntax-e a) (syntax-e b)))]
[else
(equal?/recur a b stx=?)]))
Is the values function in Common Lisp just syntactic sugar for packaging multiple values into a list that gets destructured by the caller?. I am asking because I thought Common Lisp supports "true" multiple value return rather than returning a tuple or a list as in other languages, such as python. Someone just told me that it's just syntactic sugar, so I would like someone to kindly explain it. To try to understand the type that is returned by the values function, I typed (type-of (values 1 2 3)), and the output was BIT. I searched in the Common Lisp reference for that and I couldn't find it mentioned in the datatypes section. Also, can anyone share some resources that suggest how the values function is implemented in Common Lisp?. Thank you.
Multiple Values in CL
The language Common lisp
is described in the ANSI standard INCITS 226-1994 (R2004) and has many
implementations.
Each can implement multiple values
as it sees fit, and they are allowed, of course, to cons up a list for them
(in fact, the Emacs Lisp compatibility layer for CL does just
that -
but it is, emphatically and intentionally, not a Common Lisp implementation).
Purpose
However, the intent of this facility is to enable passing (at least
some) multiple values without consing (i.e., without allocating
heap memory) and all CL
implementations I know of do that.
In this sense the multiple values facility is an optimization.
Of course, the implementation of this feature can be very different for
different platforms and scenarios. E.g., the first few (say, 20 -
required by the standard) are
stored in a static of thread-local vector, the next several (1000?) are
allocated on the stack, and the rest (if needed) are allocated on the
heap as a vector or list.
Usage
E.g., the function floor returns two values.
If you write
(setq a (floor 10 3))
you capture only the first one and discard the second one, you need to
write
(setf (values q r) (floor 10 3))
to capture both values. This is similar to what other
languages might express as
q,r = floor(10,3)
using tuples, except that CL does
not allocate memory to pass (just a few) multiple values, and the
other languages often do.
IOW, one can think of multiple values as an ephemeral struct.
Note that CL can convert multiple values to lists:
(destructuring-bind (q r) (multiple-value-list (floor 10 3))
; use q & r here
...)
instead of the more efficient and concise
(multiple-value-bind (q r) (floor 10 3)
; use q & r here
...)
MV & type
CL does not have a special type for the "multiple value object"
exactly because it does not allocate a separate object to pass
around multiple values. In this sense one can, indeed, claim that
values is syntactic sugar.
However, in CL one can declare a
function type returning
multiple values:
(declaim (ftype (real &optional real) (values real real)) floor)
This means that floor returns two values, both
reals (as opposed to returning
a value of type (values real real)), i.e., in this case one might
claim abuse of notation.
Your case
In your specific case, type-of
is an ordinary function (i.e., not a macro or special operator).
You pass it a single object, 1, because, unless you are using
multiple-value-bind and
friends, only the first value is used, so
(type-of (values 1 2 3))
is identical to
(type-of 1)
and type of 1 is bit.
PS: Control return values
One use of values is to
control the return values of a function.
Normally a CL function's return values are those of the last form.
Sometimes it is not desirable, e.g., the last form return multiple
values and you want your function to return one value (or none,
like void in C):
(defun 2values (x y)
(floor y x))
(defun 1value (x y)
(values (floor y x)))
(defun no-values (x)
(print x)
(values))
The values function isn't just syntactic sugar for making a list for the caller to destructure.
For one, if the caller expects only a single value, it will get only one value (the first), not a list, from a form that returns multiple values. Since type-of takes only one value as an argument, it is giving you the type of the first value, 1. 1 is of type BIT.
Each Common Lisp implementation is free to pursue its own strategy for implementing multiple values. I learned a lot from what Frode Fjeld wrote about how his implementation, Movitz, handles it in The Movitz development platform, section 2.5.
If you make a CL implementation you could implement it with lists as long as it coheres to the spec. You need to handle one value specific and you need some way to tag zero, 2..n values and the other functions need to understand that format and print can be made to display it the same way as in other makes.
Most likely values and its sister functions is an optimization where the implementations use the stack instead of consing the values to a list structure just to get it destructured in the next level. In the olden times where RAM and CPU was not to be wasted it was very important, but I doubt you'll notice real trouble should you use destructuring-bind instead of multiple-value-bind today.
Common Lisp differs from Scheme a great deal in the positive direction that you can make a function, eg. floor where in it's calculations end up with the remainder in addition to the quotient answer, return all values at the same time but you are allowed to use it as if it only returned the very first value. I really miss that sometimes when writing Scheme since it demands you have a call-with-values that is similar to multiple-value-call or syntactic sugar like let-values to handle all the returned values that again makes you end up with making three versions in case you only need just one of the values.
I have been trying to use Clojure tagged literals, and noticed that the reader does not evaluate the arguments, very much like a macro. This makes sense, but what is the appropriate solution for doing this? Explicit eval?
Example: given this function
(defn my-data
([[arg]]
(prn (symbol? arg))
:ok))
and this definition data_readers.clj
{myns/my-data my.ns/my-data}
The following behave differently:
> (let [x 1] (my.ns/my-data [x]))
false
:ok
So the x passed in is evaluated before being passed into my-data. On the other hand:
> (let [x 1] #myns/my-data [x])
true
:ok
So if I want to use the value of x inside my-data, the my-data function needs to do something about it, namely check that x is a symbol, and if so, use (eval x). That seems ugly. Is there a better approach?
Summary
There is no way to get at the value of the local x in your example, primarily because locals only get assigned values at runtime, whereas tagged literals are handled at read time. (There's also compile time in between; it is impossible to get at locals' values at compile time; therefore macros cannot get at locals' values either.)1
The better approach is to use a regular function at runtime, since after all you want to construct a value based on the runtime values of some parameters. Tagged literals really are literals and should be used as such.
Extended discussion
To illustrate the issue described above:
(binding [*data-readers* {'bar (fn [_] (java.util.Date.))}]
(eval (read-string "(defn foo [] #bar x)")))
foo will always return the same value, because the reader has only one opportunity to return a value for #bar x which is then baked into foo's bytecode.
Notice also that instead of passing it to eval directly, we could store the data structure returned by the call to read-string and compile it at an arbitrary point in the future; the value returned by foo would remain the same. Clearly there's no way for such a literal value to depend on the future values of any locals; in fact, during the reader's operation, it is not even clear which symbols will come to name locals -- that's for the compiler to determine and the result of that determination may be non-obvious in cases where macros are involved.
Of course the reader is free to return a form which looks like a function call, the name of a local etc. To show one example:
(binding [*data-readers* {'bar (fn [sym] (list sym 1 2 3 4 5))}]
(eval (read-string "#bar *")))
;= 120
;; substituting + for * in the string yields a value of 15
Here #bar f becomes equivalent to (f 1 2 3 4 5). Needless to say, this is an abuse of notation and doesn't really do what you asked for.
1 It's worth pointing out that eval has no access to locals (it always operates in the global scope), but the issue with locals not having values assigned before runtime is more fundamental.
Is there a use in being able to quote multiple times?
Like 'a is different from ''a and (quote a) is different from (quote (quote a))
I mean, why not have it so that ''a is the same as 'a or even ''''''''a. quote would quote if the argument it's given has not been quoted already and that way ''''a would evaluate to 'a.
The way it is feels wrong to me because I think the question should be: is it quoted? and not how many times has it been quoted? none? once? a 100 times?
Nesting quotes is largely useless. It might happen in the output of a compiler for the backquote syntax, but I can't think of any reason why you would do that, even in nested macros. If you find yourself cascading quotes, that probably means you've inserted the expression in a backquote template at the wrong unquote level.
Concretely, suppose you have written something like this:
````(,,,,'''''a) ;; goal is to be left with ('a) at the end of four eval rounds
But here, the commas and quotes basically cancel and you can do it like this:
````('a)
There is no need to assign the form ''''a to the innermost backquote, and have it work backwards toward 'a across multiple evaluations. Just assign 'a to the outermost level and you're done.
Another example would be a slight modification of the above. We expect four evaluation rounds to happen, and in the very last round a is substituted for the binding which it has in the environment of that evaluation:
````(,,,,''''a) ;; only four quotes this time
So now, after three evals of the result of the above backquote, we are left with (a) and if we eval that, it wants to call the function a. But, again, we could achieve the same thing like this:
````(a)
Exactly like before, three evals produce (a) and one more wants to call a.
Now what if the symbol a is not literally a, but computed? Say a is to be replaced by b:
(let ((a 'b))
````(,,,''',a))
The substitution has to happen in the first evaluation of the backquote, inside the let, and so the b needs four commas on the left, so that it connects to the outermost backquote. The resulting object is equivalent to
```(,,,'''a)
In this example, we cannot just cancel the three commas and quotes. However, the customary way is to have the two interspersed:
(let ((a 'b))
````(,',',',a)) ;; instead of ,,,''',a
Each evaluation round strips away a quote but then adds one back. The first substitution replaces ,a with b, but then protects it with the quote. Then in the next round, the comma before that quote causes ,'b to be replaced with b, but another quote before that protects it once again, and so on.
That is to say, the two situations in which you might see directly compounded quotes doing something useful would be: triply (or more) nested backquote (rare to begin with!) in which someone used ,,'',expr instead of ,',',expr; and superfluous use of multiple quotes used to combat the multiple evaluation of some form that was needlessly submerged into multiple evaluation rounds.
Of course there is. When the expression is evaluated more than once and you need to pass the expression through more than one evaluation as is. Remember, when writing macros, the expression is usually evaluated twice (once during expansion and once as the macro result) and in some rare cases may need to be evaluated even more times.
#Kaz is no doubt right about usual practice, but I would agree with #JanHudec about what is appropriate for CL. 'a means the same thing as (quote a), and ''a means the same thing as (quote (quote a)). What you're suggesting, user1076329, is that evaluating (quote (quote a)) should have the same result as evaluating (quote a). It feels contrary to the spirit of CL to make it difficult for a function to return 'a. (Suppose, for example, that you were writing a program that would generate Lisp source code to be written to a file and evaluated later? Or writing a program that generates Lisp source code for a program that writes Lisp programs? Lisp is designed to make it easy to do this sort of thing.) You can and should use macros in many cases, but in essence a macro is just way of constructing lists and then evaluating them. You should be able to construct the same lists without macros or backquote, even if they make it easier.
I'm working my way through Graham's book "On Lisp" and can't understand the following example at page 37:
If we define exclaim so that its return value
incorporates a quoted list,
(defun exclaim (expression)
(append expression ’(oh my)))
> (exclaim ’(lions and tigers and bears))
(LIONS AND TIGERS AND BEARS OH MY)
> (nconc * ’(goodness))
(LIONS AND TIGERS AND BEARS OH MY GOODNESS)
could alter the list within the function:
> (exclaim ’(fixnums and bignums and floats))
(FIXNUMS AND BIGNUMS AND FLOATS OH MY GOODNESS)
To make exclaim proof against such problems, it should be written:
(defun exclaim (expression)
(append expression (list ’oh ’my)))
Does anyone understand what's going on here? This is seriously screwing with my mental model of what quoting does.
nconc is a destructive operation that alters its first argument by changing its tail. In this case, it means that the constant list '(oh my) gets a new tail.
To hopefully make this clearer. It's a bit like this:
; Hidden variable inside exclaim
oh_my = oh → my → nil
(exclaim '(lions and tigers and bears)) =
lions → and → tigers → and → bears → oh_my
(nconc * '(goodness)) destructively appends goodness to the last result:
lions → and → tigers → and → bears → oh → my → goodness → nil
so now, oh_my = oh → my → goodness → nil
Replacing '(oh my) with (list 'oh 'my) fixes this because there is no longer a constant being shared by all and sundry. Each call to exclaim generates a new list (the list function's purpose in life is to create brand new lists).
The observation that your mental model of quoting may be flawed is an excellent one—although it may or may not apply depending on what that mental model is.
First, remember that there are various stages to program execution. A Lisp environment must first read the program text into data structures (lists, symbols, and various literal data such as strings and numbers). Next, it may or may not compile those data structures into machine code or some sort of intermediary format. Finally, the resulting code is evaluated (in the case of machine code, of course, this may simply mean jumping to the appropriate address).
Let's put the issue of compilation aside for now and focus on the reading and evaluation stages, assuming (for simplicity) that the evaluator's input is the list of data structures read by the reader.
Consider a form (QUOTE x) where x is some textual representation of an object. This may be symbol literal as in (QUOTE ABC), a list literal as in (QUOTE (A B C)), a string literal as in (QUOTE "abc"), or any other kind of literal. In the reading stage, the reader will read the form as a list (call it form1) whose first element is the symbol QUOTE and whose second element is the object x' whose textual representation is x. Note that I'm specifically saying that the object x' is stored within the list that represents the expression, i.e. in a sense, it's stored as a part of the code itself.
Now it's the evaluator's turn. The evaluator's input is form1, which is a list. So it looks at the first element of form1, and, having determined that it is the symbol QUOTE, it returns as the result of the evaluation the second element of the list. This is the crucial point. The evaluator returns the second element of the list to be evaluated, which is what the reader read in in the first execution stage (prior to compilation!). That's all it does. There's no magic to it, it's very simple, and significantly, no new objects are created and no existing ones are copied.
Therefore, whenever you modify a “quoted list”, you're modifying the code itself. Self-modifying code is a very confusing thing, and in this case, the behaviour is actually undefined (because ANSI Common Lisp permits implementations to put code in read-only memory).
Of course, the above is merely a mental model. Implementations are free to implement the model in various ways, and in fact, I know of no implementation of Common Lisp that, like my explanation, does no compilation at all. Still, this is the basic idea.
In Common Lisp.
Remember:
'(1 2 3 4)
Above is a literal list. Constant data.
(list 1 2 3 4)
LIST is a function that when call returns a fresh new list with its arguments as list elements.
Avoid modifying literal lists. The effects are not standardized. Imagine a Lisp that compiles all constant data into a read only memory area. Imagine a Lisp that takes constant lists and shares them across functions.
(defun a () '(1 2 3)
(defun b () '(1 2 3))
A Lisp compiler may create one list that is shared by both functions.
If you modify the list returned by function a
it might not be changed
it might be changed
it might be an error
it might also change the list returned by function b
Implementations have the freedom to do what they like. This leaves room for optimizations.