Consider the class account :
(defclass account ()
((name :initarg :name :reader name)
(balance :initarg :balance :initform 0.00 :accessor balance)
(interest-rate :allocation :class :initform 0.06
:reader interest-rate)))
For this class, we define a method withdraw :
(defmethod withdraw ((acct account) amt)
(if (< amt (balance acct))
(decf (balance acct) amt)
'insufficient-funds))
And, another class password-account , that is a subclass of account:
(defclass password-account (account)
((password :initarg :password :reader password )))
And, the method withdraw, for this class:
(defmethod withdraw ((acct password-account) amt pass)
(if (equal (password acct) pass)
(call-next-method acct amt )
'wrong-password))
But this gives an error :
The generic function
#<STANDARD-GENERIC-FUNCTION COMMON-LISP-USER::WITHDRAW (1)>
takes 2 required arguments; was asked to find a method with
specializers
(#<STANDARD-CLASS COMMON-LISP-USER::PASSWORD-ACCOUNT>
#1=#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T> #1#)
[Condition of type SB-PCL::FIND-METHOD-LENGTH-MISMATCH]
See also:
Common Lisp Hyperspec, FIND-METHOD [:function]
Restarts:
0: [RETRY] Retry SLIME REPL evaluation request.
1: [*ABORT] Return to SLIME's top level.
2: [ABORT] abort thread (#<THREAD "repl-thread" RUNNING {1005308033}>)
Why is this happening? And what does
was asked to find a method with specializers
mean here?
Here, the primary withdraw function had two arguments acct and amt, so in order to call it from a more specific method, which uses 3 arguments instead of 2, we can provide call-next-method with the arguments of the less specific withdraw method. But this still isn't working.
Any help appreciated.
Congruent lambda lists for generic functions
Methods of a generic function need to have congruent lambda lists. The language standard describes what that means: Congruent Lambda-lists for all Methods of a Generic Function.
As you can see the first rule says:
Each lambda list must have the same number of required parameters.
Required parameters tell us which arguments always have to be provided. Generic functions additionally allow optional, keyword and rest arguments. But there is no dispatch over these. The dispatch only works over the required arguments and all of those.
Having the same number of required parameters makes dispatch easier and allows the compiler to check for function calls with the wrong number of arguments.
Optional parameters need to be congruent, too
Note also that all methods of a generic function need to have the same number of optional parameters. See the second rule in the standard.
Wording
a parameter is something like a named variable in a lambda list
an argument is provided in a call to a function
Examples:
(defun foo (a b) (list a b))
a and b are parameters for the function foo.
(foo (+ 2 3) (* 4 5))
5 and 20 are the two arguments for the call of the function foo.
Related
Say I have a functions as follows:
(defun my/test-a (n)
(interactive)
(message n))
(defun my/test-b ()
(interactive)
(sleep-for .5)
(message "Message - B.")
(sleep-for .5))
I then advice my/test-a with mytest-b like so: (advice-add 'my/test-a :after #'my/test-b).
However when I call (my/test-a "Message - A.") I get a "Wrong number of arguments" error. My understanding is that add-advice is feeding the argument into my/test-b, which is not expecting any arguments.
How to advice-add a function with no arguments to a function that takes arguments?
I could change my/test-b so it takes an argument and doesn't use it, but that feels very messy.
Related followup - how could I advise find-file with a function with no arguments (like my/test-b)? I understand find-file is an unusual case, because it doesn't need an argument if called interactively. But if I run (advice-add 'find-file :after #'my/test-b) and then (call-interactively 'find-file) I get a "Wrong Number Of Arguments" error again.
TIA.
You can't do that.
Your advice function has to accept the arguments for the original function.
C-hig (elisp)Advice Combinators says:
:after
Call FUNCTION after the old function. Both functions receive the
same arguments, and the return value of the composition is the
return value of the old function. More specifically, the
composition of the two functions behaves like:
(lambda (&rest r) (prog1 (apply OLDFUN r) (apply FUNCTION r)))
A way to take arbitrary arguments and ignore them is:
(defun foo (&rest _args) ...)
The underscore tells the byte-compiler that the arguments are unused in the function body on purpose.
TL;DR
What I'm looking for is a combination of the functions send/apply and dynamic-send. So that it finds a method of an object based on a symbol and unpacks a list of arguments.
Background and more info
For a project I am sending some "commands" trough the network with Racket's tcp-connect. At the receivers end this command should execute a method from a class and pass along its parameters.
Consider the following received 'message':
(define message (list 'set-switch! '3 'on))
(define method-name (car msg)) ;'set-switch!
(define parameters (cdr msg)) ;(list '3 'on)
And the following class:
(define light%
(class object%
(super-new)
...
(define/public (set-switch! id mode)
(vector-set! switches id mode))))
The problem now is that when executing this statement
(dynamic-send light-class method-name parameters)
it perfectly finds the method set-switch! but it calls it with only one parameter (list '3 'on).
The Racket docs mention those three functions for classes:
(send obj-expr method-id arg) which just executes a method of an object
(send/apply obj-expr method-id arg-list-expr) which executes a method AND unpacks the argument list
(dynamic-send obj method-name v) which finds a method-name based on a symbol
What I think I need is something like (dynamic-send/apply obj method-name arg-list-expr) which combines the last two mentioned.
Note: I know that I could just simply accept lists as parameters and use car and cdr in the functions itself to get the right values. But that's not what I want.
dynamic-send is a function (also known as procedure; e.g., car, vector-set!, +), so you can use apply:
(apply dynamic-send light-class method-name parameters)
Or even simply:
(apply dynamic-send light-class message)
The reason why send has the send/apply variant is that send is a form (also known as syntax; e.g., let, define, if), so apply doesn't work and hence send/apply is separately provided.
I created a function which gets two string parameters. The function simply adds each string length. below is a code.
(defun add_twostring_length (mystr1 mystr2)
(+ (length mystr1) (length mystr2))
)
When I call add_twostring_length function like this,
(add_twostring_length "cpp" "lisp")
output is correct. 7
But, when I call the same function in the manner of using comma,
(add_twostring_length "cpp", "lisp")
I got an error message.
Error: Comma not inside a backquote.
[condition type: READER-ERROR]
I want to call function in the manner of (add_twostring_length "cpp", "lisp").
What is the wrong with the code?
picture showing error message
You might as well ask "why can't I call the function without parentheses?" In lisp, you call functions as an sexpr with the function in the car and the arguments in the cdr. There are no commas involved -- that's the syntax of lisp.
What you want is possible, but I will strongly advice against using it:
(set-macro-character #\,
#'(lambda (stream char)
(read stream t nil t)))
The above code creates the so called "read macro". At read-time common lisp will find all occurrences of , and ignore them. This makes possible calling functions like so:
(+ 1, 2, 3) ; => 6
However this will break escaping in templates:
`(1 2 ,(+ 3 4)) ; => (1 2 (+ 3 4))
Perhaps it is possible to make the read macro more intelligent, but I don't want to delve deeper into this, because I don't like the idea. Sorry.
Can anyone briefly explain to me how message passing is implemented in scheme? I think I am little off on the whole concept of message passing.
Take a look at SICP.
http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-17.html#%_sec_2.4.1
http://www.michaelharrison.ws/weblog/?p=50
Message passing in the context of closures
The following example defines a closure implementing a simple calculator. The function make-calculator is similar to what object oriented languages call the constructor. The difference is: make-calculator returns a function, while constructors return object values. In object oriented languages object values are first class values. Scheme does not have such values. An object first class values provides the functionality to access object member variables and object methods. In Scheme this functionality has to be emulated by the definition of a dispatch function. make-calculator returns such a function. The body of make-calculator defines
two variables a and b (member variables)
two mutation functions set-a! and set-b! (accessors)
four evaluation functions addition, subtraction, multiplication and division (methods)
The above definitions are local to the closure make-calculator. In an object oriented language they are called private. The dispatch function makes the functions public and keeps the variables private. This works, because the dispatch function has access to the local scope of the make-calculator closure. The dispatch function accepts a message and returns the matching function. This exposes the local functions to the caller of the dispatch function.
(define (make-calculator)
(define a)
(define b)
(define (set-a! value)
(set! a value))
(define (set-b! value)
(set! b value))
(define (addition)
(+ a b))
(define (subtraction)
(- a b))
(define (multiplication)
(* a b))
(define (division)
(/ a b))
(lambda (message)
(case message
((set-a!) set-a!)
((set-b!) set-b!)
((addition) addition)
((subtraction) subtraction)
((multiplication) multiplication)
((division) division))))
First the constructor has to be called to create an "object". calc is the dispatch function, which accepts different messages, which are just symbols.
(define calc (make-calculator))
Sending a message means calling the dispatch function with a symbol argument. The following sends the message set-a! to calc, which returns the value of the local function set-a!. The name of the message and the name of the local function are the same in this case. This helps to avoid confusion, but it is not required.
(calc 'set-a!) ;; => #<procedure set-a!>
Because calc returns a function, an additional application is necessary to call the accessor. The following sets a to 3 and b to 5.
((calc 'set-a!) 3)
((calc 'set-b!) 5)
Now we can calculate:
((calc 'addition)) ;; => 8
((calc 'subtraction)) ;; => -2
((calc 'multiplication)) ;; => 15
((calc 'division)) ;; => 3/15
The code works this way in Chez Scheme.
I've been looking at the if-let and when-let macros, Im having trouble determining what exactly it is that they "do". In particular, the documentation sais :
clojure.core/when-let
([bindings & body])
Macro
bindings => binding-form test
When test is true, evaluates body with binding-form bound to the value of test
I thus am somewhat confused about the way macros are documented.
1) What does the "=>" symbol mean ?
2) What does "test" refer to ?
Direct answer to your questions:
=> means "expands to", as in BNF notation. In this case it means that you need two forms: binding-form and the test.
"test" means anything that can be evaluated as bool.
By the way, I think that the docs are unclear or even maybe erroneous here. It is hard (or impossible) to deduce that the two forms constituting the bindings need to be enclosed in a vector. IMHO it should be either when-let ([[bindings] & body]) (the vector shown in the args) or bindings => [binding-form test] (the vector shown in the BNF-like expansion.)
It's often helpful when dealing with a macro to call macroexpand and see what the generated code is.
(macroexpand
'(if-let [x (myfunc)]
(foo x)
(bar))
; expands to
(let* [temp__3695__auto__ (myfunc)]
(if temp__3695__auto__ (clojure.core/let [x temp__3695__auto__]
(foo x))
(bar)))
; the generated symbols are necessary to avoid symbol
; capture but can confuse. The above is equivalent to:
(let* [ t (myfunc)]
(if t
(let [x t]
(foo x))
(bar))
So you can see that if-let is a shorthand for "bind local variable to the result of a function call and if that variable is truthy call the first form, else call the other form. The value returned by your function is only available in the 'truthy' branch."
wrt documentation convention
bindings => binding-form test
=> reads something like 'is equivalent to'
test is some form that returns a value
For most of these functions, clojuredocs is your friend, example usage often clarify things. If the clojuredocs example doesn't cut it for you, you can add your own
Consider the following code:
(if-let [x (a-function)]
(do-something-with x) ;; (a-function) returned a truthy result
(do-something-else) ;; (a-function) returned nil or false
This is like let, in that x will be bound to the return value of (a-function). This function could return nil or false. In that case, the implicit test fails and (do-something-else) will be evaluated. If x is not nil and not false, (do-something-with x) will be evaluated.
A scenario where this could be useful:
(if-let [user (find-logged-in-user)]
(do something with logged in user) ;; a user was found
(redirect to login page) ;; no user was found
I sometimes use something like the following, to conditionally add keys to a map of options:
(apply merge {:username "joe"
:email "joe#example.com"}
(when-let [name (find-full-name)] {:name name})
(when-let [dob (find-date-of-birth)] {:dob dob}))
This results in a map with :username and :email keys, and a :name key if the users' full name was found, plus a :dob key if a date of birth was found.
I hope that makes the use of if-let and when-let clearer.