Is there a way in Chicken Scheme to determine at run-time if a variable is currently defined?
(let ((var 1))
(print (is-defined? var)) ; #t
(print (is-defined? var)) ; #f
EDIT: XY problem.
I'm writing a macro that generates code. This generated code must call the macro in mutual recursion - having the macro simply call itself won't work. When the macro is recursively called, I need it to behave differently than when it is called initially. I would use a nested function, but uh....it's a macro.
Rough example:
(defmacro m (nested)
(if nested
BACKQUOTE(print "is nested")
BACKQUOTE(m #t)
(yes, I know scheme doesn't use defmacro, but I'm coming from Common Lisp. Also I can't seem to put backquotes in here without it all going to hell.)
I don't want the INITIAL call of the macro to take an extra argument that only has meaning when called recursively. I want it to know by some other means.
Can I get the generated code to call a macro that is nested within the first macro and doesn't exist at the call site, maybe? For example, generating code that calls (,other-macro) instead of (macro)?
But that shouldn't work, because a macro isn't a first-class object like a function is...
When you write recursive macros I get the impression that you have an macro expansion (m a b ...) that turns into a (m-helper a (b ...)) that might turn into (let (a ...) (m b ...)). That is not directly recursive since you are turning code into code that just happens to contain a macro.
With destructuring-bind you really only need to keep track of two variables. One for car and one for cdr and with an implicit renaming macro the stuff not coming from the form is renamed and thus hygenic:
(define-syntax destructuring-bind
(ir-macro-transformer
(lambda (form inject compare?)
(define (parse-structure structure expression optional? body)
;;actual magic happens here. Returns list structure with a mix of parts from structure as well as introduced variables and globals
)
(match form
[(structure expression) . body ]
`(let ((tmp ,expression))
,(parse-structure structure 'tmp #f body))))))
To check if something from input is the same symbol you use the supplied compare? procedure. eg. (compare? expression '&optional).
There's no way to do that in general, because Scheme is lexically scoped. It doesn't make much sense to ask if a variable is defined if an referencing an undefined variable is an error.
For toplevel/global variables, you can use the symbol-utils egg but it is probably not going to work as you expect, considering that global variables inside modules are also rewritten to be something else.
Perhaps if you can say what you're really trying to do, I can help you with an alternate solution.
Related
How can we get variable value with a string in scheme language as we can achieve this in Common Lisp:
> (defvar s 3)
> S
> (symbol-value (intern "S"))
> 3
I am accessing a parameter of parent function from the closure.
EDIT: I have found this solution, but I can't use eval because it evaluates at top level. Searching for alternatives.
(eval (string->symbol "s"))
EDIT 2: I have found that Common lisp code also try to find symbol in global space. So this question is basically for both Lisps(Common Lisp, Scheme).
Don't do that!
Variables are for when you know the variable at compile time. In that case it is never a string. You can still reason about strings in compile time but your code also needs to have a relation with the name for it to be interesting. When you use eval or other forms that evaluate structure and compile/run data in runtime you are probably not doing it right (but not always. I've in my 20 year career used eval intentionally in production code twice)
If you want to store values you use a data structure. An assoc would mimic a dynamic environment. You can also use a hash with a level indicator if the size is harmless.
You can't do what you want to do and in fact it is a confused thing to want to do.
Here's why what you are trying to do is confused. Consider how a Lisp system for which it was possible to do what you wanted would work. In particular consider something like this:
(define (foo a name)
(let ([b 10])
(display (get-value name))
(* a b)))
Where get-value is meant to be how you get the binding of whatever something is.
So, if I call (foo 10 "b") it should print 10 and return 100.
But wait: b is a compile-time constant in this code. Any compiler worth its salt is going to immediately turn this into
(define (foo a name)
(display (get-value name))
(* a 10))
And now there is no binding of b.
So there are two options here: what you want to work works and it is impossible to ever write a reasonable compiler for Scheme, or what you want to work doesn't work, and it is.
I am reading an article which uses Scheme for describing an implementation. I know a bit of Common Lisp but no Scheme.
I am hoping you will be so kind as to explain two Scheme code samples and show me how they correspond to Common Lisp.
First, what does this Scheme mean:
(define (content cell)
(cell ’content))
I believe it means this: define a function named content which has one argument named cell. In Common Lisp it is written as:
(defun content (cell)
(...))
Am I right so far?
I am uncertain what the function's body is doing. Is the argument (cell) actually a function and the body is invoking the function, passing it a symbol, which happens to be the name of the current function? Is this the corresponding Common Lisp:
(defun content (cell)
(funcall cell ’content))
Here is the second Scheme code sample:
(define nothing #(*the-nothing*))
I believe it is creating a global variable and initializing it to #(*the-number*)). So the corresponding Common Lisp is:
(defvar nothing #(*the-nothing*))
Is that right? Does the pound symbol (#) have a special meaning? I'm guessing that *the-nothing* is referring to a global variable, yes?
Broadly speaking: yes to both, with one major caveat. More specifically, the first one is accepting an argument called cell and calling it with the symbol 'content. (BTW, your unicode quotation mark is freaking me out a bit. Is that just a copy-paste issue?)
In the second case, the hash is a shortcut for defining a vector. So, for instance:
(vector? #(abc)) ;; evaluates to #t
However, the hash also has quoting behavior. Just as the first element of
'(a b c)
is the symbol 'a (and not the value of the variable named a), the first value in the vector
#(*the-nothing*)
is the symbol '*the-nothing*, rather than the value of a global variable.
How does one call a method object as a function?
Closer-mop and clos packages both provide method-function for turning a method object into a function. However, is there a way to do it without including another package? And if not, which package? (Using SBCL), but if a package is needed then how does the discrimination function do it?
Here is an example of using find-method to get a method object. The question is then how to call method-to-be-called.
(defclass a () ((x :accessor x :initform 0)))
(defgeneric inc (i))
(defmethod inc ((i a)) (incf (x i)))
(defvar r (make-instance 'a))
;; ... in a land far far away:
(defvar method-to-be-called (find-method #'inc '() '(a)))
(funcall method-to-be-called r);; crashes and burns
As a secondary question, the docs say that the discrimination function first tries to compute-applicable-methods-by-class to find a method object, and if that fails, it uses compute-applicable-methods. Why do this two layer approach? Is it correct to assume the find-method is doing this two layer approach, so it is better to use find-method ?
-- Appendix --
In the comments below Rainer Joswig pointed out that this find-method form is implementation dependent:
(find-method #'inc '() '(a))) ; works on sbcl 1.3.1
He says the specifier list should be classes and suggests instead:
(find-method #'inc '() (list (find-class 'a))))
So I thought to just put my class in there:
(find-method #'inc '() (list a)) ; crashes and burns
Apparently (defclass a ... ) does not set a to a class. In fact it doesn't set it to anything!
* (defclass a () ((x :accessor x :initform 0)))
#<STANDARD-CLASS COMMON-LISP-USER::A>
* a
...
The variable A is unbound.
However, this works:
* (defvar ca (defclass a () ((x :accessor x :initform 0))))
CA
* (defmethod inc ((i a)) (incf (x i)))
WARNING: Implicitly creating new generic function COMMON-LISP-USER::INC.
#<STANDARD-METHOD COMMON-LISP-USER::INC (A) {1005EE8263}>
enter code here
* (find-method #'inc '() (list ca))
#<STANDARD-METHOD COMMON-LISP-USER::INC (A) {1005EE8263}>
*
So a class is the return value from the defclass, not the value of the symbol that is provided to defclass.
(find-method #'inc '() '(a))
Above does not work. We need a list of classes, not a list of symbols.
(funcall (method-function (find-method #'inc
'()
(list (find-class 'a))))
r)
Since the function method-function belongs to the MOP, many implementations provide it and it is in some implementation specific package. CLOSER-MOP makes it available, too.
But usually, if you are already trying extracting method functions, then you are probably using CLOS the wrong way or you are really knowing what you are doing...
How does one call a method object as a function?
Honest question: why do you want to do that? Did you specify how the method's function is built in the first place, or not?
Even with closer-mop, I believe that the function returned by closer-mop:method-function is, at most, consistent with closer-mop:make-method-lambda in terms of its lambda-list, so perhaps you can use a package to know what you can count on portably.
A method's function does not have to be a function with the same lambda-list as the generic function, and usually it isn't due to next-method-p and call-next-method. Some implementations might use dynamic bindings for the next method list, so these might have a method lambda-list congruent with the generic function. Just don't count on it, generically.
I believe SBCL is not one of these implementations, the next method list is passed to the method's function to support next-method-p and call-next-method.
Why do this two layer approach?
Because it allows memoizing (or caching) based on the list of classes, when possible. If the generic function is called again with arguments of the same classes, and the generic function has not been updated (see "Dependent Maintenance Protocol" in the MOP), it can reuse the last result without further processing, for instance, by keeping the results in a hash table which keys are lists of classes.
However, if compute-applicable-methods-using-classes returns a false second value, then compute-applicable-methods is used. The reason is that no method could be found using classes alone, and this means some method has a non-class specializer.
This is different than saying there are no applicable methods, for instance, if all methods are specialized on classes and there are no applicable methods, compute-applicable-methods-using-classes should return an empty list and a true second value. There's no point in calling compute-applicable-methods, it won't (or rather, it shouldn't, if well implemented) find anything further.
It's still possible to perform memoization when compute-applicable-methods is used, but the memoization is no longer as trivial as, for instance, using a list of classes as a key in a hash table. Perhaps you could use a tree structure where you'd try to look up a method for each specializer (instance, then class) sequentially for each argument, until a tree node matched the whole specializable parameter list.
With non-standard specializers, you'd have to change the search order for each node. Unless such specializer's priority is not strictly before, between or after eql and a class, then you're in uncharted areas.
Actually, you'll have to change compute-applicable-methods-using-classes to recognize the non-standard specializers and return false early, and you'll have to change compute-applicable-methods to process these specializers, anyway, so perhaps you'll have a good knowledge on, if possible, how to memoize with the results of compute-applicable-methods anyway.
Is it correct to assume the find-method is doing this two layer approach, so it is better to use find-method ?
No, the purpose of find-method is to find a specific method, not an applicable method. It does not use compute-applicable-methods-using-classes or compute-applicable-methods at all. In fact, it couldn't use the latter ever, as it takes actual arguments instead of specializers.
For the particular case of method-function, closer-mop for SBCL simply reexport the existing symbol from sb-pcl, as seen in closer-mop-packages.lisp. The whole file make use of read-time conditionals (see 1.5.2.1 Use of Implementation-Defined Language Features).
That means that if you are working with SBCL, you might call sb-pcl:method-function (PCL means Portable Common Loops).
The generic function compute-applicable-methods-by-class allows you to know which methods are applicable given classes. This is useful if you don't have actual instances on which you can operate.
It seems also that compute-applicable-methods-using-classes allows the implementation to memoize the applicable methods when the second return value is true. This generic method does not allow you to find applicable methods specialized with eql specializers.
I am speculating here, but it makes sense to fall back on compute-applicable-methods to allow for example eql-specializers or because it is slightly easier to define a method for compute-applicable-methods.
Note the paragraph about consistency:
The following consistency relationship between compute-applicable-methods-using-classes and compute-applicable-methods must be maintained: for any given generic function and set of arguments, if compute-applicable-methods-using-classes returns a second value of true, the first value must be equal to the value that would be returned by a corresponding call to compute-applicable-methods. The results are undefined if a portable method on either of these generic functions causes this consistency to be violated.
I don't think there is a find-method-using-classes generic function specified anywhere.
I am reading DrRacket document http://docs.racket-lang.org/guide/binding.html
There is a function
(define f
(lambda (append)
(define cons (append "ugly" "confusing"))
(let ([append 'this-was])
(list append cons))))
> (f list)
'(this-was ("ugly" "confusing"))
I see that we define function f, inside we define lambda that takes (append), why ?
Procedure (body) for lambda is another function called cons, that appends two strings.
I don't understand this function at all.
Thanks !
The section that you're referring to demonstrates lexical scope in Racket. As in other Scheme implementations, the main point is that you can "shadow" every binding in the language. Unlike most "mainstream" languages, there are no real keywords that are "sacred" in the sense that they can never be shadowed by a local binding.
Note that a really good tool to visualize what is bound where is DrRacket's "check syntax" button: click it, and you'll see your code with highlights that shows which parts are bindings, which are special forms -- and if you hover the mouse over a specific name, you'll see an arrow that tells you where it came from.
Scheme takes some getting used to :)
f is assigned the function returned by the lambda.
lambda defines the function that takes a parameter (called append).
(define cons (append "ugly" "confusing")) is not a function per se, but calls append with the two strings as parameter and assigns the result to cons.
inside the let block, append is re-assigned a different value, the symbol this-was.
the let block creates a list of append (which now contains 'this-was) and cons (which contains '("ugly" "confusing") from 3 above
since 5 is the last statement that value is returned by the whole function which is called f
f is called with the parameter list (the list function). which gets passed as the parameter append. And this is why 3 above creates a list '("ugly" "confusing") which gets assigned to cons.
Hope that cleared up things a bit.
Cheers!
How do I declare that a symbol will always stand for a particular value and cannot be changed throughout the execution of the program?
As far as I know, this isn't possible in Scheme. And, for all intents and purposes, it's not strictly necessary. Just define the value at the toplevel like a regular variable and then don't change it. To help you remember, you can adopt a convention for naming these kinds of constants - I've seen books where toplevel variables are defined with *stars* around their name.
In other languages, there is a danger that some library will override the definition you've created. However, Scheme's lexical scoping coupled with PLT's module system ensure this will never happen.
In PLT Scheme, you write your definitions in your own module -- and if your own code is not using `set!', then the binding can never change. In fact, the compiler uses this to perform various optimizations, so this is not just a convention.
You could define a macro that evaluates to the constant, which would protect you against simple uses of set!
(define-syntax constant
(syntax-rules ()
((_) 25)))
Then you just use (constant) everywhere, which is no more typing than *constant *
A really hackish answer that I thought of was to define a reader macro that returns your constant:
#lang racket
(current-readtable
(make-readtable (current-readtable)
#\k 'dispatch-macro (lambda (a b c d e f) 5)))
#k ;; <-- is read as 5
It would then be impossible to redefine this (without changing your reader macro):
(set! #k 6) ;; <-- error, 5 is not an identifier