I have the following dumb test code:
#lang racket
(define vars '('g1 'g2 'g3 'g1))
(define addrs '(123 456 789 012))
(define immhs (make-immutable-hasheq empty))
(define immhs* (for/fold ([imhs immhs]) ([var (in-list vars)] [addr (in-list addrs) ]) (hash-set imhs var addr )))
immhs*
(hash-ref immhs* 'g1)
The output is:
'#hasheq(('g1 . 123) ('g2 . 456) ('g3 . 789) ('g1 . 12))
hash-ref: no value found for key: 'g1
Why can't the hash-ref reference to the 'g1? (it will also fail on 'g2, etc)
Then I uses (hash-keys immhs*), it returns '('g1 'g2 'g3 'g1), where there is 'g1;
and I use further (car (hash-keys immhs*)), it returns ''g1 ; then the question is that why there are two quote ' before?
The problem is exactly the two quotes that you see: x evaluates to whatever x is bound to, 'x evaluates to a symbol, and ''x evaluates to a quoted form -- 'x. Try this:
(define vars '(g1 g2 g3 g1))
and it will work.
Related
I would like to dynamically create a list of chars to use with read-char-choice. From this answer of similar question, how to construct the list programmatically with unquoted values as choices, ?1 with a function. The function that I ended up is
(defun prompt-list (name-list)
"docstring"
(let ((names name-list)
(name-num 1)
(choice-list (list)))
(dolist (x names)
(add-to-list 'choice-list
`(,name-num ;; in that part how to create the ?1, ?2 but dynamically
,x (lambda () (setq project-headers x))))
(setq name-num (+ 1 name-num)))
choice-list))
when I try to run it it returns error : Format specifier doesn’t match argument type.
In my understanding it need a char type, so my question is how to produce char types programmatically?
?1 is the read syntax for the character '1', which has the integer value 49. You can use the read syntax to initialize your name-num variable, rather than using the integer 1:
(defun prompt-list (name-list)
"docstring"
(let ((names name-list)
(name-num ?1)
(choice-list (list)))
(dolist (x names)
(add-to-list 'choice-list `(,name-num
,x (lambda () (setq project-headers ',x)))
t)
(setq name-num (+ 1 name-num)))
choice-list))
A couple of things to note:
You want the use ,x when setting project-headers, not just x, as the latter won't be valid by the time the lambda is invoked.
You want to append to choice-list as you build it, which is what the final t argument to add-to-list above does, otherwise your read-char-choice prompt will be in reverse order.
I am writing a simple interpreter that should
output: +inf or -inf for the following computations:
(/ 0)
(/ 1 0)
(/ -0)
I notice that replaced 0 by 0.0 gives me the behavior I want.
But I haven't figured out to convert -0 to -0.0.
exact->inexact loses the negative sign.
(exact->inexact -0) gives 0.0.
To do this, with the literal text -0 you would have to change the Reader so that -0 reads equal to -0.0, an inexact version. To be consistent you might want to read every number as an inexact number, whether it has a decimal point or not.
It's worth noting that you can do this by prefixing a number #i, for example #i-0 reads equal to -0.0. However, it sounds like you might want to change the reader so that every number reads the same as if it had #i prefixed onto it including -0.
One of the easier ways to extend the reader is with a readtable. You can make a function that extends a readtable like this:
;; Readtable -> Readtable
(define (extend-readtable orig-rt)
;; Char InputPort Any Nat Nat Nat -> Any
(define (rt-proc char in src ln col pos)
....)
...
(make-readtable orig-rt
#f 'non-terminating-macro rt-proc
...))
To use this to define a #lang language, you need to put the reader implementation in your-language/lang/reader.rkt. Here that's inexact-number/lang/reader.rkt, where the inexact-number directory is installed as a single-collection package (raco pkg install path/to/inexact-number).
inexact-number/lang/reader.rkt
#lang racket
(provide (rename-out [-read read]
[-read-syntax read-syntax]
[-get-info get-info]))
(require syntax/readerr
syntax/module-reader)
;; Readtable -> Readtable
(define (extend-readtable orig-rt)
;; Char InputPort Any Nat Nat Nat -> Any
(define (rt-proc char in src ln col pos)
....)
...
(make-readtable orig-rt
#f 'non-terminating-macro rt-proc))
;; [X ... -> Y] -> [X ... -> Y]
(define ((wrap-reader rd) . args)
(parameterize ([current-readtable (extend-readtable (current-readtable))])
(apply rd args)))
(define-values [-read -read-syntax -get-info]
(make-meta-reader 'inexact-number
"language path"
lang-reader-module-paths
wrap-reader
wrap-reader
identity))
The main work goes into filling in the .... holes in the extend-readtable function. Within the rt-proc, you can have it "peek" into the input port in to see if its a number, and if it is, call the Racket reader on a port that has #i appended to the front of in. You might be able to do this with something like input-port-append:
(input-port-append #f (open-input-string "#i") in)
In the context of extend-readtable, here's some code that does the peeking and re-reading:
;; Readtable -> Readtable
(define (extend-readtable orig-rt)
;; Char InputPort Any Nat Nat Nat -> Any
(define (rt-proc char in src ln col pos)
(define try-in (peeking-input-port in))
(define try (read/recursive try-in char orig-rt))
(cond
[(number? try)
;; read it as if it had #i on the front
(define prefix (string-append "#i" (string char)))
(define inexact-in
(input-port-append #f (open-input-string prefix) in))
(read-syntax/recursive src inexact-in #f orig-rt)]
[else
;; read normally
(read-syntax/recursive src in char orig-rt)]))
(make-readtable orig-rt
#f 'non-terminating-macro rt-proc))
Once you have this done, you should be able to use it like this:
#lang inexact-number racket
-0
; => -0.0
(/ -0)
; => -inf.0
P.S. I have now made #lang inexact-number available on the Racket package server, as the package inexact-number-lang. By the way, there's also #lang exact-decimal, which has the opposite affect of making numbers like 5.2 into exact rational numbers.
This is because -0 in Racket is the same literal datum that can also be written as 0, +0, 0000000, #e0, #e-0/1, -00000/1, or in various other ways. All of these syntaxes produce the very same value from the reader, which is an exact integer. Unlike floating-point numbers, exact numbers don't have signed zeros. To illustrate:
> (exact-integer? 0)
#t
> (exact-integer? +0)
#t
> (exact-integer? -0)
#t
> (eq? +0 -0)
#t
> (eq? -0 0)
#t
To get the behavior you want, you need to adjust the reader layer so that 0 and -0 don't produce the same value. I don't think there is a built-in parameter for doing what you want (there are for many other reader customizations), but you can do it by creating a custom readtable.
I would need more context to give you further guidance (for example, you say you're writing an "interpreter," which isn't the usual way of making a DSL in Racket), but my first inclination would be that, rather than re-implementing number parsing, you might map - to a "non-terminating-macro" and handle the rest in the expander (or interpreter) layer.
-0 doesn't exist! If you enter something silly like (define x -0) and press the Macro stepper #'> before any transformations you'll see it has read it in as (define x 0). The sign is truncated by the reader since integers doesn't have positive and negative zero like IEEE754 does. Thus unless you create your own language with your own reader that can discriminate on signed zero mapped to a data structure that supports it you cannot solve this.
IEEE754 floating points has the sign as it's own bit and thus every number, including 0, can be both positive and negative.
As for Scheme standard: Before R6RS there was no requirement for a full numeric tower and the report even mentions that a floating point only Scheme might be useful. Thus I believe some implementations of R5RS actually might read -0 as -0.0.
I'm trying to create a function that will take a string and display it.
(defun closing (s)
(format t "~{~a~}" ("Sincerely," "\n" s)))
What I hope to get is
Sincerely,
Frank
if "Frank" is the string I passed in. It complains of the variable S is defined but never used. What am I doing wrong?
Trying to use format alone: If I declare urname as a defparameter to be "Frank", the following doesn't print Frank, rather just the variable name. (Without quote it complains of urname not being a function.)
(format t "~{~a~}" '(urname urname urname))
How can I feed variables to format?
There are three issues here: (1) The code you posted doesn't just have the problem of not using s; it's also trying to call the string "Sincerely" as a function; (2) quoting a list means you'll get exactly what's quoted (e.g., a list of symbols, not a list of values of variables); (3) calling format with lists.
(something other-stuff...) is a function call
When I put the code you posted into SBCL, I get some very specific and helpful output:
CL-USER> (defun closing (s)
(format t "~{~a~}" ("Sincerely," "\n" s)))
; in: DEFUN CLOSING
; ("Sincerely," "n" S)
;
; caught ERROR:
; illegal function call
; (SB-INT:NAMED-LAMBDA CLOSING
; (S)
; (BLOCK CLOSING (FORMAT T "~{~a~}" ("Sincerely," "n" S))))
;
; caught STYLE-WARNING:
; The variable S is defined but never used.
;
; compilation unit finished
; caught 1 ERROR condition
; caught 1 STYLE-WARNING condition
("Sincerely," "\n" s) is an illegal function call, since a string, like "Sincerely", can't have a function binding. Since SBCL sees the problem in that, it recognizes that the one thing that s might have been used for (i.e., an argument to a function call), can't happen. That's why you'll get the error, and then the associated style warning.
Creating lists of values
The second is probably answered in other questions already, but the short answer is that you want (list x y z), not '(x y z). The former calls the function list with the values of the variables x, y, and z, while the latter denotes a literal list of the symbols x, y, and z.
CL-USER> (let ((a 42)
(b 89))
(print '(a b)) ; a list of two symbols
(print (list a b))) ; a list of two numbers
(A B)
(42 89)
Format, iteration, &c.
The third is probably more interesting, since format has so many possibilities. The ~{ and ~} in your example are used for iterating over values in a list. First, let's look at a simple example: you can just use the format directive ~a and call format with the arguments you want to splice in:
CL-USER> (let ((closing "Sincerely")
(name "Frank"))
(format t "~a,~%~a" closing name))
Sincerely,
Frank
Now, if you need to print multiple values, you can use ~{ and ~} to have format iterate over a list of values:
CL-USER> (let ((closing "Sincerely")
(names '("Frank" "John")))
(format t "~a,~{~%~a~}" closing names))
Sincerely,
Frank
John
If the names are the values of variables, then you can either create a list containing those values:
CL-USER> (let ((closing "Sincerely")
(name1 "Frank")
(name2 "John"))
(format t "~a,~{~%~a~}" closing (list name1 name2)))
Sincerely,
Frank
John
or you can change ~{ to ~#{ and have format read the remaining arguments as the list:
CL-USER> (let ((closing "Sincerely")
(name1 "Frank")
(name2 "John"))
(format t "~a,~#{~%~a~}" closing name1 name2))
Sincerely,
Frank
John
You should read a tutorial about format, from here for example
For an easy explanation
(format
(destination-stream usually t for standard output nil to form a string)
(here comes the string)
(&rest variables that should write in the string where is an ~A and by position like String.format in java or c))
in your case, you need the symbol ~% or use the character for return in common lisp
CL-USER> (defun closing (s) (format t "~A~%~%~A" "Sincerely," s))
CLOSING
CL-USER> (closing "paco")
Sincerely,
paco
NIL
The nil say that the functions returns null, and the other is the standard output if you want to return a string, put nil instead of t
I defined a generic function taking 2 arguments:
(defgeneric interact (a b))
The order of the arguments should not be important, so (interact x y) and (interact y x) should be the same, but I don't want to define two methods that do the same for every combination of different objects.
A Method-Combination of this type should help:
(defmethod interact :around (a b)
(if (some-function a b)
;;some-function has to be true if (eq (class-of a) (class-of b))
;;else (some-function a b) is (not (some-function b a))
;;similar #'<=
(call-next method)
(interact b a))
But I would have to know #'some-function and be able to know the type of the arguments I have to define.
Edit: both proposed approaches have a few limitations discussed in the comments below. Please read them before using this answer!
Can I suggest two options - a working but hacky option for when you only have two arguments, and a vaguely sketched out generic approach which I think should work but I haven't written:
Option 1:
(defparameter *in-interact-generic-call* nil)
(defgeneric interact (x y))
(defmethod interact ((x T) (y T))
; this can be called on pretty much anything
(if *in-interact-generic-call*
(cause-some-kind-of-error) ; Replace this with a more sensible error call
(let ((*in-interact-generic-call* T))
(interact y x))))
(defmethod interact ((x integer) (y string))
; example
(print x )(prin1 y))
(interact 5 "hello") ; should print 5 "hello"
(interact "hello" 5) ; should print 5 "hello"
;(interact "hello" "hello") ; should cause an error
Essentially the idea is to define a generic function which always matches anything, use it to try to swap the arguments (to see if that matches anything better) and if it's already swapped the arguments then to raise some kind of error (I've not really done that right here).
Option 2
Define the generic function as something like interact-impl. Actually call the standard function (defined by defun) interact.
In interact, define a loop over all permutations of the order of your arguments. For each permutation try calling interact-impl (e.g. using (apply #'interact-impl current-permutation).)
At least in sbcl, no matching arguments gives me a simple-error. You probably would want to do a more detailed check that it's actually the right error. Thus the code in interact looks something like
; completely untested!
(do (all-permutations all-permutations (cdr all-permutations))
(...) ; some code to detect when all permutations are exhausted and raise an error
(let (current-permutation (first all-permutations))
(handler-case
(return (apply #'interact-impl current-permutation))
(simple-error () nil)) ; ignore and try the next option
)
)
So what you are looking for is an arbitrary linear order on the class objects.
How about string order on class names?
(defun class-less-p (a b)
"Lexicographic order on printable representation of class names."
(let* ((class-a (class-of a))
(name-a (symbol-name class-a))
(pack-a (package-name (symbol-package name-a)))
(class-b (class-of b))
(name-b (symbol-name class-b))
(pack-b (package-name (symbol-package name-b))))
(or (string< pack-a pack-b)
(and (string= pack-a pack-b)
(string<= name-a name-b)))))
Let's say I have a program like this:
(define (foo x)
(local
((define y (- x 1)))
(* x y)))
(foo 3)
I want to be able to open a REPL between lines 3 and 4, such that I can explore (and possibly modify) the values of x and y by executing arbitrary statements.
To do this in Ruby, I would take the equivalent program:
def foo(x)
lambda {
y = x - 1
x * y
}.call
end
puts (foo 3)
And modify it by adding a call to pry to give me a nicely-scoped repl where I want it:
require 'pry'
def foo(x)
lambda {
y = x - 1
binding.pry
x * y
}.call
end
puts (foo 3)
To do it in js, I would run this program under Firebug and just put a breakpoint on line 4:
foo = function(x) {
return (function(){
var y = x - 1;
return x * y;
})();
};
console.log(foo(3));
And then I could explore stuff in the evaluation window.
Is there anything I can do to get this in Racket? The closest I've found is DrScheme's debugger, but that just presents all the values of the current scope, it doesn't let you explore them in a REPL as far as I can see.
This isn't answering your original question, it's in response to your comment about making your own. I thought that was a really interesting idea so I explored it. What I was able to figure out:
Let's say you want this to work:
(define top-x 10)
(define (f)
(for ([i 10])
(displayln i)
(when (= i 5)
(pry)))) ; <= drop into a REPL here, resume after exiting REPL
A first attempt at pry:
(define (pry)
(let loop ()
(display "PRY> ")
(define x (read))
(unless (or (eof-object? x) (equal? x '(unquote exit)))
(pretty-print (eval x))
(loop))))
This seems to work:
> (f)
0
1
2
PRY> (+ 10 10)
20
PRY> ,exit
3
4
>
But although it lets you access Racket functions like +, you can't access even your top-level variables like top-x:
> (f)
0
1
2
PRY> top-x
; top-x: undefined;
; cannot reference undefined identifier
You can get the top-level stuff by giving eval access to the current namespace, as explained here. So pry needs a namespace argument:
(define (pry ns)
(let loop ()
(display "PRY> ")
(define x (read))
(unless (or (eof-object? x) (equal? x '(unquote exit)))
(pretty-print (eval x ns)) ; <---
(loop))))
And to get that argument you need this incantation to your debugee file:
(define-namespace-anchor a) ; <---
(define ns (namespace-anchor->namespace a)) ; <---
(define top-x 10)
(define (f)
(for ([i 5])
(displayln i)
(when (= i 2)
(pry ns)))) ; <---
Now the REPL can see and change top-x:
> (f)
0
1
2
PRY> top-x
10
PRY> (set! top-x 20)
#<void>
PRY> top-x
20
PRY> ,exit
3
4
>
Cool! But it can't change the local variable, i:
> (f)
0
1
2
PRY> i
; i: undefined;
; cannot reference an identifier before its definition
Shoot. The reason why is explained here.
You might imagine that even though eval cannot see the local bindings in broken-eval-formula, there must actually be a data structure mapping x to 2 and y to 3, and you would like a way to get that data structure. In fact, no such data structure exists; the compiler is free to replace every use of x with 2 at compile time, so that the local binding of x does not exist in any concrete sense at run-time. Even when variables cannot be eliminated by constant-folding, normally the names of the variables can be eliminated, and the data structures that hold local values do not resemble a mapping from names to values.
You might say, OK, but in that case...
How does DrRacket provide a debugger?
From what I was able to figure out, DrRacket does this by annotating the syntax before evaluating the program. From drracket/gui-debugger/annotator.rkt:
;; annotate-stx inserts annotations around each expression that introduces a
;; new scope: let, lambda, and function calls. These annotations reify the
;; call stack, and allows to list the current variable in scope, look up
;; their value, as well as change their value. The reified stack is accessed
;; via the CURRENT-CONTINUATION-MARKS using the key DEBUG-KEY
So I think that would be the jumping-off point if you wanted to tackle this.
In the DrRacked IDE you have a DEBUG Q >| button. You can step through your program or you can do as you said in other languages, press right mouse button at the expression you want to investigate and either choose continue to this point for once only or pause at this point for a breakpoint, then press GO > to run the program.
To inspect or change x, put you mouse pointer over it and use right mouse button. To change you choose (set! x ...) in the menu.
As for the in language repl, You could make your own (pry) to start a repl in there and in Common Lisp you could have just signaled an error to get to the nice debugger.