Just started learning and coding lisp,
I'm trying to create a program that will continuously accept a number and stops only if and only if the last input number is twice the previous number.
Here's my code
----------
(let((a 0)
(b 0)
(count 0))
(loop
(= a b))
(princ"Enter Number: ")
(defvar a(read))
(format t "~% a = ~d" a)
(setq count (1+ count))
(while
(!= b(* a 2) || <= count 1)
(princ "Program Terminated Normally")
)
Thank you
a bit feedback
(let ((a 0)
(b 0)
(count 0))
(loop
(= a b)) ; here the LOOP is already over.
; You have a closing parenthesis
; -> you need better formatting
(princ"Enter Number: ")
(defvar a(read))
(format t "~% a = ~d" a)
(setq count (1+ count))
(while
(!= b(* a 2) || <= count 1)
(princ "Program Terminated Normally")
)
some improved formatting:
(let ((a 0)
(b 0)
(count 0))
(loop
(= a b)) ; LOOP ends here, that's not a good idea
(princ "Enter Number: ")
(defvar a(read)) ; DEFVAR is the wrong construct,
; you want to SETQ an already defined variable
(format t "~% a = ~d" a)
(setq count (1+ count))
(while ; WHILE does not exist as an operator
(!= b(* a 2) || <= count 1) ; This expression is not valid Lisp
(princ "Program Terminated Normally")
)
You may need to learn a bit more Lisp operators, before you really can write such loops. You also may want to use Lisp interactively and try out things, instead of trying to write code into an editor and never get feedback from a Lisp...
Here's an answer which definitely is not how you would do this in real life, but if you understand what it does you will understand one of the two big important things about Lisps.
(If you understand why the equivalent program would not work reliably in Scheme you'll also understand one of the important things about writing safe programs! Fortunately this is Common Lisp, not Scheme, so it's OK here.)
First of all let's write a little helper function to read integers. This is just fiddly detail: it's not important.
(defun read-integer (&key (prompt "Integer: ")
(stream *query-io*))
;; Read an integer. This is just fiddly details
(format stream "~&~A" prompt)
(values (parse-integer (read-line stream))))
OK, now here's a slightly odd function called mu (which stands for 'mutant U'):
(defun mu (f &rest args)
(apply f f args))
And now here is our program:
(defun read-integers-until-double-last ()
(mu (lambda (c getter current next factor)
(if (= next (* current factor))
(values current next)
(mu c getter next (funcall getter) factor)))
#'read-integer
(read-integer)
(read-integer)
2))
And here it is working:
> (read-integers-until-double-last)
Integer: 0
Integer: 4
Integer: 3
Integer: -2
Integer: -4
-2
-4
For extra mysteriosity you can essentially expand out the calls to mu here, which makes it either more clear or less clear: I'm not quite sure which:
(defun read-integers-until-double-last ()
((lambda (c)
(funcall c c
#'read-integer
(read-integer)
(read-integer)
2))
(lambda (c getter current next factor)
(if (= next (* current factor))
(values current next)
(funcall c c getter next (funcall getter) factor)))))
Again, this is not how you do it in real life, but if you understand what this does and how it does it you will understand quite an important thing about Lisps and their theoretical underpinnings. This is not all (not even most) of the interesting things about them, but it is a thing worth understanding, I think.
Related
During the execution of my code I get the following errors in the different Scheme implementations:
Racket:
application: not a procedure;
expected a procedure that can be applied to arguments
given: '(1 2 3)
arguments...:
Ikarus:
Unhandled exception
Condition components:
1. &assertion
2. &who: apply
3. &message: "not a procedure"
4. &irritants: ((1 2 3))
Chicken:
Error: call of non-procedure: (1 2 3)
Gambit:
*** ERROR IN (console)#2.1 -- Operator is not a PROCEDURE
((1 2 3) 4)
MIT Scheme:
;The object (1 2 3) is not applicable.
;To continue, call RESTART with an option number:
; (RESTART 2) => Specify a procedure to use in its place.
; (RESTART 1) => Return to read-eval-print level 1.
Chez Scheme:
Exception: attempt to apply non-procedure (1 2 3)
Type (debug) to enter the debugger.
Guile:
ERROR: In procedure (1 2 3):
ERROR: Wrong type to apply: (1 2 3)
Chibi:
ERROR in final-resumer: non procedure application: (1 2 3)
Why is it happening
Scheme procedure/function calls look like this:
(operator operand ...)
Both operator and operands can be variables like test, and + that evaluates to different values. For a procedure call to work it has to be a procedure. From the error message it seems likely that test is not a procedure but the list (1 2 3).
All parts of a form can also be expressions so something like ((proc1 4) 5) is valid syntax and it is expected that the call (proc1 4) returns a procedure that is then called with 5 as it's sole argument.
Common mistakes that produces these errors.
Trying to group expressions or create a block
(if (< a b)
((proc1)
(proc2))
#f)
When the predicate/test is true Scheme assumes will try to evaluate both (proc1) and (proc2) then it will call the result of (proc1) because of the parentheses. To create a block in Scheme you use begin:
(if (< a b)
(begin
(proc1)
(proc2))
#f)
In this (proc1) is called just for effect and the result of teh form will be the result of the last expression (proc2).
Shadowing procedures
(define (test list)
(list (cdr list) (car list)))
Here the parameter is called list which makes the procedure list unavailable for the duration of the call. One variable can only be either a procedure or a different value in Scheme and the closest binding is the one that you get in both operator and operand position. This would be a typical mistake made by common-lispers since in CL they can use list as an argument without messing with the function list.
wrapping variables in cond
(define test #t) ; this might be result of a procedure
(cond
((< 5 4) result1)
((test) result2)
(else result3))
While besides the predicate expression (< 5 4) (test) looks correct since it is a value that is checked for thurthness it has more in common with the else term and whould be written like this:
(cond
((< 5 4) result1)
(test result2)
(else result3))
A procedure that should return a procedure doesn't always
Since Scheme doesn't enforce return type your procedure can return a procedure in one situation and a non procedure value in another.
(define (test v)
(if (> v 4)
(lambda (g) (* v g))
'(1 2 3)))
((test 5) 10) ; ==> 50
((test 4) 10) ; ERROR! application: not a procedure
Undefined values like #<void>, #!void, #<undef>, and #<unspecified>
These are usually values returned by mutating forms like set!, set-car!, set-cdr!, define.
(define (test x)
((set! f x) 5))
(test (lambda (x) (* x x)))
The result of this code is undetermined since set! can return any value and I know some scheme implementations like MIT Scheme actually return the bound value or the original value and the result would be 25 or 10, but in many implementations you get a constant value like #<void> and since it is not a procedure you get the same error. Relying on one implementations method of using under specification makes gives you non portable code.
Passing arguments in wrong order
Imagine you have a fucntion like this:
(define (double v f)
(f (f v)))
(double 10 (lambda (v) (* v v))) ; ==> 10000
If you by error swapped the arguments:
(double (lambda (v) (* v v)) 10) ; ERROR: 10 is not a procedure
In higher order functions such as fold and map not passing the arguments in the correct order will produce a similar error.
Trying to apply as in Algol derived languages
In algol languages, like JavaScript and C++, when trying to apply fun with argument arg it looks like:
fun(arg)
This gets interpreted as two separate expressions in Scheme:
fun ; ==> valuates to a procedure object
(arg) ; ==> call arg with no arguments
The correct way to apply fun with arg as argument is:
(fun arg)
Superfluous parentheses
This is the general "catch all" other errors. Code like ((+ 4 5)) will not work in Scheme since each set of parentheses in this expression is a procedure call. You simply cannot add as many as you like and thus you need to keep it (+ 4 5).
Why allow these errors to happen?
Expressions in operator position and allow to call variables as library functions gives expressive powers to the language. These are features you will love having when you have become used to it.
Here is an example of abs:
(define (abs x)
((if (< x 0) - values) x))
This switched between doing (- x) and (values x) (identity that returns its argument) and as you can see it calls the result of an expression. Here is an example of copy-list using cps:
(define (copy-list lst)
(define (helper lst k)
(if (null? lst)
(k '())
(helper (cdr lst)
(lambda (res) (k (cons (car lst) res))))))
(helper lst values))
Notice that k is a variable that we pass a function and that it is called as a function. If we passed anything else than a fucntion there you would get the same error.
Is this unique to Scheme?
Not at all. All languages with one namespace that can pass functions as arguments will have similar challenges. Below is some JavaScript code with similar issues:
function double (f, v) {
return f(f(v));
}
double(v => v * v, 10); // ==> 10000
double(10, v => v * v);
; TypeError: f is not a function
; at double (repl:2:10)
// similar to having extra parentheses
function test (v) {
return v;
}
test(5)(6); // == TypeError: test(...) is not a function
// But it works if it's designed to return a function:
function test2 (v) {
return v2 => v2 + v;
}
test2(5)(6); // ==> 11
I'm very new to LISP programming and I'm having a real hard time with the syntax. The following code is from my notes and I know what it does but I'd really appreciate a line by line breakdown to better understand what's happening here. The "when" loop seemed pretty simple to understand but specifically I'm having a hard time trying to understand the first 3 lines in the "do" loop. Also I'm not sure why (:= acc (1+ acc) was used in the last line of the when loop.
(defun count-lower-case-vowels (str)
(do ((i 0 (1+ i))
(acc 0)
(len (length str)))
((= i len) acc)
(when (or (equal (aref str i) #\a) (equal (aref str i) #\e)
(equal (aref str i) #\i) (equal (aref str i) #\o)
(equal (aref str i) #\u))
(:= acc (1+ acc)))))
I'm a big proponent of lots and lots of extra white space, to achieve visual code alignment (in 2D, yes, as if on a piece of paper) to improve readability:
(defun count-lower-case-vowels (str)
(do ( (i 0 (1+ i) ) ; loop var `i`, its init, step exprs
(acc 0 ) ; loop var `acc`, its init expr
(len (length str) ) ) ; loop var `len`, its init expr
((= i len) ; loop stop condition
acc) ; return value when loop stops
(if ; loop body: if
(find (aref str i) "aeiou") ; test
(setf acc (1+ acc))))) ; consequent
Is this better?
It is definitely not the accepted standard of LISP code formatting. But whatever makes it more readable, I think is for the best.
The i's step expression's meaning is that on each step after the loop didn't stop and its body was evaluated, (setf i (1+ i)) is called. acc and len have no step expressions, so for them nothing is called on each step.
As to the "when loop" you mention, it is not a loop at all, and is not a part at all of the do loop's looping mechanism. A when form is just like an if without the alternative, which also allows for multiple statements in the consequent, as if with an implicit progn:
(when test a1 a2 ...)
===
(if test (progn a1 a2 ...))
It just so happens that this loop's body consists of one form which is a when form. I have re-written it with an equivalent if.
do is a macro expecting 3 parameters:
(do ((i 0 (1+ i))
(acc 0)
(len (length str))) ;; first argument
((= i len) acc) ;; Second one
(when ...) ;; third
)
The first argument is itself a list, each element of this element being of the following form:
<var-name> <var-initial-value> <var-next-value>
In your case, the form (i 0 (1+ i)) means that in the body of the do macro (= in the third argument), you introduce a new, local variable called i. It starts with the value 0, and at each step of the loop, it gets updated to the value (1+ i) (i.e. it gets incremented by 1).
You see that the second element of this list is acc 0 with no <var-next-value> in it. It means that acc won't get updated automatically by the macro, and its value will change only according to what is done in the body.
The second argument is a list of one or (optionally) two elements <condition> <return-val> The first one <condition> is stating when to stop the iteration: once it evaluates to true, the macro stops. It gets evaluated before each iteration. The second, optional part, is a form stating what the do form returns. By default, it returns nil, but if you specify a form there, it will be evaluated before exiting the loop and return-val is returned instead.
The third argument is simply a list of forms that will get executed at each step, provided the condition is false.
Note that the code you have posted is older style.
Nowadays it can be written much shorter with loop and find:
(defun count-lower-case-vowels (string)
(loop for c across string
count (find c "aeiou")))
I have an immutable datastructure that is a functional hashmap (see fash.scm) that is shared among several threads.
Imagine a thread wants to change the global hashmap to a new version. Do I need a lock to change the value? If that's is the case, I assume I also need to lock the value to read it, isn't it?
It seems to me it boils down to whether setting a value in Scheme is an atomic operation or not. According to this answer for C language, you must
acquire a lock for both read and write access of pointer.
If it matters I am using guile 2.2.3 and bigloo 4.3.
It all depends on what you want to do. In general, if the value can be guaranteed to be read (e.g. always a number), then it's okay not to lock when the value is read. For example,
(import (rnrs) (srfi :18))
(define count 0)
(define t
(thread-start!
(make-thread
(lambda ()
(let loop ()
(display count) (newline)
(thread-sleep! 0.1)
(loop))))))
(do ((i 0 (+ i 1))) ((= i 10))
(set! count (+ count 1))
(thread-sleep! 0.1))
This is pretty much safe to read. However if the value is, say a vector of length 1, then you may want to lock if the other threads may change the value to #f or a vector of length 0. For example:
(import (rnrs) (srfi :18))
(define count (vector 1))
(define t
(thread-start!
(make-thread
(lambda ()
(let loop ()
(display (vector-ref count 0)) (newline)
(thread-sleep! 0.1)
(loop))))))
(do ((i 0 (+ i 1))) ((= i 10))
(vector-set! count 0 (+ (vector-ref count 0) i))
(thread-sleep! 0.1))
(set! count #f) ;; if this happens, the you may want to lock on reader thread
I haven't check how fash is implemented, but as long as entries are not updated to unexpected values, I would think it's okay not to lock.
I want to delete some characters at the end of a string.
I made this function :
(defun del-delimiter-at-end (string)
(cond
((eq (delimiterp (char string (- (length string) 1))) nil)
string )
(t
(del-delimiterp-at-end (subseq string 0 (- (length string) 1))) ) ) )
with this :
(defun delimiterp (c) (position c " ,.;!?/"))
But I don't understand why it doesn't work. I have the following error :
Index must be positive and not -1
Note that I want to split a string in list of strings, I already looked here :
Lisp - Splitting Input into Separate Strings
but it doesn't work if the end of the string is a delimiter, that's why I'm trying to do that.
What am I doing wrong?
Thanks in advance.
The Easy Way
Just use string-right-trim:
(string-right-trim " ,.;!?/" s)
Your Error
If you pass an empty string to you del-delimiter-at-end, you will be passing -1 as the second argument to char.
Your Code
There is no reason to do (eq (delimiterp ...) nil); just use (delimiterp ...) instead (and switch the clauses!)
It is mode idiomatic to use if and not cond when you have just two clauses and each has just one form.
You call subseq recursively, which means that you not only allocate memory for no reason, your algorithm is also quadratic in string length.
There are really two questions here. One is more specific, and is described in the body of the question. The other is more general, and is what the title asks about (how to split a sequence). I'll handle the immediate question that's in the body, of how to trim some elements from the end of a sequence. Then I'll handle the more general question of how to split a sequence in general, and how to split a list in the special case, since people who find this question based on its title may be interested in that.
Right-trimming a sequence
sds answered this perfectly if you're only concerned with strings. The language already includes string-right-trim, so that's probably the best way to solve this problem, if you're only concerned with strings.
A solution for sequences
That said, if you want a subseq based approach that works with arbitrary sequences, it makes sense to use the other sequence manipulation functions that the language provides. Many functions take a :from-end argument and have -if-not variants that can help. In this case, you can use position-if-not to find the rightmost non-delimiter in your sequence, and then use subseq:
(defun delimiterp (c)
(position c " ,.;!?/"))
(defun right-trim-if (sequence test)
(let ((pos (position-if-not test sequence :from-end t)))
(subseq sequence 0 (if (null pos) 0 (1+ pos)))))
(right-trim-if "hello!" 'delimiterp) ; some delimiters to trim
;=> "hello"
(right-trim-if "hi_there" 'delimiterp) ; nothing to trim, with other stuff
;=> "hi_there"
(right-trim-if "?" 'delimiterp) ; only delimiters
;=> ""
(right-trim-if "" 'delimiterp) ; nothing at all
;=> ""
Using complement and position
Some people may point out that position-if-not is deprecated. If you don't want to use it, you can use complement and position-if to achieve the same effect. (I haven't noticed an actual aversion to the -if-not functions though.) The HyperSpec entry on complement says:
In Common Lisp, functions with names like xxx-if-not are related
to functions with names like xxx-if in that
(xxx-if-not f . arguments) == (xxx-if (complement f) . arguments)
For example,
(find-if-not #'zerop '(0 0 3)) ==
(find-if (complement #'zerop) '(0 0 3)) => 3
Note that since the xxx-if-not functions and the :test-not
arguments have been deprecated, uses of xxx-if functions or :test
arguments with complement are preferred.
That said, position and position-if-not take function designators, which means that you can pass the symbol delimiterp to them, as we did in
(right-trim-if "hello!" 'delimiterp) ; some delimiters to trim
;=> "hello"
complement, though, doesn't want a function designator (i.e., a symbol or function), it actually wants a function object. So you can define right-trim-if as
(defun right-trim-if (sequence test)
(let ((pos (position-if (complement test) sequence :from-end t)))
(subseq sequence 0 (if (null pos) 0 (1+ pos)))))
but you'll have to call it with the function object, not the symbol:
(right-trim-if "hello!" #'delimiterp)
;=> "hello"
(right-trim-if "hello!" 'delimiterp)
; Error
Splitting a sequence
If you're not just trying to right-trim the sequence, then you can implement a split function without too much trouble. The idea is to increment a "start" pointer into the sequence. It first points to the beginning of the sequence. Then you find the first delimiter and grab the subsequence between them. Then find the the next non-delimiter after that, and treat that as the new start point.
(defun split (sequence test)
(do ((start 0)
(results '()))
((null start) (nreverse results))
(let ((p (position-if test sequence :start start)))
(push (subseq sequence start p) results)
(setf start (if (null p)
nil
(position-if-not test sequence :start p))))))
This works on multiple kinds of sequences, and you don't end up with non delimiters in your subsequences:
CL-USER> (split '(1 2 4 5 7) 'evenp)
((1) (5 7))
CL-USER> (split '(1 2 4 5 7) 'oddp)
(NIL (2 4))
CL-USER> (split "abc123def456" 'alpha-char-p)
("" "123" "456")
CL-USER> (split #(1 2 3 foo 4 5 6 let 7 8 list) 'symbolp)
(#(1 2 3) #(4 5 6) #(7 8))
Although this works for sequences of all types, it's not very efficient for lists, since subseq, position, etc., all have to traverse the list up to the start position. For lists, it's better to use a list specific implementation:
(defun split-list (list test)
(do ((results '()))
((endp list)
(nreverse results))
(let* ((tail (member-if test list))
(head (ldiff list tail)))
(push head results)
(setf list (member-if-not test tail)))))
CL-USER> (split-list '(1 2 4 5 7) 'oddp)
(NIL (2 4))
CL-USER> (split-list '(1 2 4 5 7) 'evenp)
((1) (5 7))
Instead of member-if and ldiff, you could also us cut from this answer to Idiomatic way to group a sorted list of integers?.
I've written a small Scheme interpreter in C#, and realised that the way I had implemented it, it was very easy to add support for proper continuations.
So I added them... but want to "prove" that they way that I've added them is correct.
My Scheme interpreter however has no support for "mutating" state - everything is immutable.
So it was pretty easy to write a unit test to expose "upwards" continuations:
AssertEqual(Eval("(call/cc (lambda (k) (+ 56 (k 3))))"), 3);
However, I also want to write a unit test that demonstrates that if the continuation "escapes" then that still works too:
AssertEqual(Eval("(call/cc (lambda (k) k))", <some continuation>);
But of course, the above would just test that "I got a continuation"... not that it's actually a valid continuation.
All of the examples I can find, however, always end up using "set!" to demonstrate the escaped continuation.
What's the simplest Scheme example that demonstrates proper support for backwards continuations without relying on mutation?
Are backwards continuations any use without mutation? I am beginning to suspect that they are not, because you could only use it to execute the exact same calculation again... which is meaningless if there are no side-effects. Is this why Haskell does not have continuations?
I don't know if this is the simplest, but here's an example of using backwards continuations without any call to set! or similar:
(apply
(lambda (k i) (if (> i 5) i (k (list k (* 2 i)))))
(call/cc (lambda (k) (list k 1))))
This should evaluate to 8.
Slightly more interesting is:
(apply
(lambda (k i n) (if (= i 0) n (k (list k (- i 1) (* i n)))))
(call/cc (lambda (k) (list k 6 1))))
which computes 6! (that is, it should evaluate to 720).
You can even do the same thing with let*:
(let* ((ka (call/cc (lambda (k) `(,k 1)))) (k (car ka)) (a (cadr ka)))
(if (< a 5) (k `(,k ,(* 2 a))) a))
(Man, stackoverflow's syntax highlighting fails massively on scheme.)
I think you're right -- without mutation, backwards continuations do nothing that forward continuations can't.
Here's the best I've come up with:
AssertEqual(Eval("((call/cc (lambda (k) k)) (lambda (x) 5))", 5);
Not amazing, but it is a backwards continuation which I then "call" with the actual function I wish to invoke, a function that returns the number 5.
Ah and I've also come up with this as a good unit test case:
AssertEqual(Eval("((call/cc call/cc) (lambda (x) 5))", 5);
I agree with Jacob B - I don't think it's that useful without mutable state... but would be still be interested in a counter-example.
Functional Threads:
You can use a recursive loop to update state without mutation. including the state of the next continuation to be called. Now this is more complicated than the other examples given, but all you really need is the thread-1 and main loop. The other thread and "update" function are there to show that continuations can be used for more than a trivial example. Additionally, for this example to work you need an implementation with the named let. This can be translated into an equivalent form made with define statements.
Example:
(let* ((update (lambda (data) data)) ;is identity to keep simple for example
(thread-1
(lambda (cc) ;cc is the calling continuation
(let loop ((cc cc)(state 0))
(printf "--doing stuff state:~A~N" state)
(loop (call/cc cc)(+ state 1))))) ;this is where the exit hapens
(thread-2
(lambda (data) ;returns the procedure to be used as
(lambda (cc) ;thread with data bound
(let loop ((cc cc)(data data)(state 0))
(printf "--doing other stuff state:~A~N" state)
(loop (call/cc cc)(update data)(+ state 1)))))))
(let main ((cur thread-1)(idle (thread-2 '()))(state 0))
(printf "doing main stuff state:~A~N" state)
(if (< state 6)
(main (call/cc idle) cur (+ state 1)))))
Which outputs
doing main stuff state:0
--doing other stuff state:0
doing main stuff state:1
--doing stuff state:0
doing main stuff state:2
--doing other stuff state:1
doing main stuff state:3
--doing stuff state:1
doing main stuff state:4
--doing other stuff state:2
doing main stuff state:5
--doing stuff state:2
doing main stuff state:6