I would like to create a nil function that takes any number of symbols and sets them all to nil.
(defun clean (as many args as given by user)
(setq each-arg nil)
)
(clean x y z)
How to do this 'cleanly'?
Since you're not quoting the arguments, it has to be a macro:
(defmacro clean (&rest symbols)
`(progn
,#(mapcar (lambda (sym) (list 'setq sym 'nil))
symbols)))
Similar idea as Dmitry, but generates slightly less code:
(defmacro clean (&rest variables)
`(setq ,#(loop for var in variables nconc (list var nil))))
(macroexpand '(clean a b c d))
;; (setq a nil b nil c nil d nil)
Regarding your other questions:
simple, but time consuming way to know is to move the point to the function that you don't know and C-h f or M-xdescribe-function this will put the function name in the prompt (if it is indeed an Emacs Lisp function) and show the description and, if availably the location in the source code.
I'll try to explain, but I'm no language reference :)
defmacro - is similar to function, but it doesn't evaluate arguments. Macros are executed when your code is read and compiled into the bytecode. Their primary goal is to generate other code.
lambda - is a macro that creates an anonymous function and returns it.
mapcar - is a high-order function that applies a function to all elements of the list in succession and collects the result into a list in the order it applied the function.
&rest - is a special keyword in the function's lambda-list (i.e. the definition of parameters) which means literally that the identifier following this symbol is a list of all arguments on the right of it).
,# is a special operator used in macros, inside back-quote macros. It instructs the reader that the expression following it must be evaluated, treated as list, and all of its conses must be appended to the form that is being currently parsed.
Related
I'm studying Clojure, and I've read that in Clojure a function definition is just data, i.e. parameters vector is just an ordinary vector. If that's the case, why can I do this
(def add (fn [a b]
(+ a b)))
but not this
(def vector-of-symbols [a b])
?
I know I normally would have to escape symbols like this:
(def vector-of-symbols [`a `b])
but why don't I have to do it in fn/defn? I assume this is due to fn/defn being macros. I tried examining their source, but they are too advanced for me so far. My attempts to recreate defn also fail, and I'm not sure why (I took example from a tutorial):
(defmacro defn2 [name param & body]
`(def ~name (fn ~param ~#body)))
(defn2 add [a b] (+ a b)) ;;I get "Use of undeclared Var app.core/defn2"
Can someone please explain, how exactly does Clojure turn data structures, especially symbols, into code? And what am I missing about the macro example?
Update Apparently, macro does not work because my project is actually in Clojurescript (in Clojure it does work). I did not think it matters, but as I progress - I discover more and more things that somehow don't work for me in with Clojurescript.
Update 2 This helps: https://www.clojurescript.org/about/differences
A function is a first-class citizen as other data in Clojure.
To define a vector you use (vector ...) or reader has syntaxic sugar [...], for a list it's (list ...) or '(...) the quote not to evaluate the list as a function call, for a set (set ...) or #{...}.
So the factory function for a function is fn (in fact fn*, that comes from Java core of Clojure, fn is a series of macros to manage to destructure and all).
(fn args body)
is a function call that returns a function, where args is a vector of argument(s) event. empty and body is a series of Clojure expressions to be evaluated with args bind to the environment. If nothing is to be evaluated it returns nil. There is also a syntactic sugar #(...) with %x as argument x and % as argument 1.
(fn ...) return a value that is a function. So
(def my-super-function (fn [a b c d] (println "coucou") (+ a b c d)))
binds the symbol my-super-function with the anonymous function returned by (fn [a b c d] (println "coucou") (+ a b c d)).
(def my_vector [1 2 3])
binds the symbol my_vector with the vector [1 2 3]
List of learning resources: https://github.com/io-tupelo/clj-template#documentation
As #jas said, your defn2 macro looks fine.
The main point is that macros are an advanced feature that one almost never needs. A macro is equivalent to a compiler extension, and that is almost never the best solution to a problem. Also keep in mind that functions can do some things macros can't.
Another point: the syntax-quote (aka backquote) ` is very different from a single quote '. In your example you want the single quote for ['a 'b]. Even better would be to quote the entire vector form '[a b].
As to your primary question, it is poorly explained how source-file text is converted into code. This is a 2-step process. The Clojure Reader consumes text string data (from a file or a literal string) and produces data structures like lists, vectors, strings, numbers, symbols. The Clojure compiler takes these data structures as input and produces java byte code that can be executed.
It is confusing because, when printed, one can't tell the difference between the text representation of a vector [1 2 3] and the text string that is input to the reader [1 2 3]. Ideally it would be color-coded or something. This problem doesn't exist in Java, etc since they don't have macros and hence there is no confusion between the source code (text) and the data structures used by a macro (not text).
For a more detailed answer on creating macros in Clojure, please see this answer.
I'm unsure of the best way to work with functions that are not defined at the top level.
I've created a list of workhorse lambda expressions, call it *lambdas*, that are derived from a user's input specifications. There is also an *args* list that provides the appropriate arguments for each lambda. I would like to turn each lambda into a function, compile it, apply it to its respective arguments, and collect the results.
Currently, I am doing (setf (symbol-value *dummy-fn-name*) (compile nil lambda)) as a temporary place to hold each function, followed by (apply (symbol-value *dummy-fn-name*) args) to get each result, but this probably qualifies as a hack. Is there a more direct way of working with the lambdas? The functions do not need to hang around after they are apply-ed.
You don't need to put it into a global variable if it's just temporary:
(let ((func (compile nil lambda)))
(apply func args))
or simply
(apply (compile nil lambda) args)
I am trying to determine whether a given argument within a macro is a function, something like
(defmacro call-special? [a b]
(if (ifn? a)
`(~a ~b)
`(-> ~b ~a)))
So that the following two calls would both generate "Hello World"
(call-special #(println % " World") "Hello")
(call-special (println " World") "Hello")
However, I can't figure out how to convert "a" into something that ifn? can understand. Any help is appreciated.
You might want to ask yourself why you want to define call-special? in this way. It doesn't seem particularly useful and doesn't even save you any typing - do you really need a macro to do this?
Having said that, if you are determined to make it work then one option would be to look inside a and see if it is a function definition:
(defmacro call-special? [a b]
(if (#{'fn 'fn*} (first a))
`(~a ~b)
`(-> ~b ~a)))
This works because #() function literals are expanded into a form as follows:
(macroexpand `#(println % " World"))
=> (fn* [p1__2609__2610__auto__]
(clojure.core/println p1__2609__2610__auto__ " World"))
I still think this solution is rather ugly and prone to failure once you start doing more complicated things (e.g. using nested macros to generate your functions)
First, a couple of points:
Macros are simply functions that receive as input [literals, symbols, or collections of literals and symbols], and output [literals, symbols, or collections of literals and symbols]. Arguments are never functions, so you could never directly check the function the symbol maps to.
(call-special #(println % " World") "Hello") contains reader macro code. Since reader macros are executed before regular macros, you should expand this before doing any more analysis. Do this by applying (read-string "(call-special #(println % \" World\") \"Hello\")") which becomes (call-special (fn* [p1__417#] (println p1__417# "world")) "Hello").
While generally speaking, it's not obvious when you would want to use something when you should probably use alternative methods, here's how I would approach it.
You'll need to call macroexpand-all on a. If the code eventually becomes a (fn*) form, then it is guaranteed to be a function. Then you can safely emit (~a ~b). If it macroexpands to eventually be a symbol, you can also emit (~a ~b). If the symbol wasn't a function, then an error would throw at runtime. Lastly, if it macroexpands into a list (a function call or special form call), like (println ...), then you can emit code that uses the thread macro ->.
You can also cover the cases such as when the form macroexpands into a data structure, but you haven't specified the desired behavior.
a in your macro is just a clojure list data structure (it is not a function yet). So basically you need to check whether the data structure a will result is a function or not when it is evaluated, which can be done like show below:
(defmacro call-special? [a b]
(if (or (= (first a) 'fn) (= (first a) 'fn*))
`(~a ~b)
`(-> ~b ~a)))
By checking whether the first element of the a is symbol fn* or fn
which is used to create functions.
This macro will only work for 2 cases: either you pass it a anonymous function or an expression.
I am writing a simple member function that returns the value associated with that member. The first line of the conditional (to test if the first value is the one we're looking for) works fine, but the recursive part (to test parts of the list that are further along) returns an error every time.
Here is my function:
(defun mem (prop L)
(cond
((eq (caar L) prop) (print cadar L)))
(t (mem (prop (cdr L)))))) // error is on this line!
So, if I enter
(mem `i `((i 1) (j 2)))
it correctly returns 1. However, if I enter
(mem `j `((i 1) (j 2)))
it returns an error that "function prop is undefined."
How do I let the program know that prop isn't a function, but is just an input parameter?
This is my first lisp program, so I'm assuming the answer is incredibly simple, but I have tried many things and have yet to succeed.
The problem in particular is the snippet: (prop (cdr L)). This tries to call prop, passing it (cdr L). To pass it as an argument to mem, simply leave out the extra parentheses: (mem prop (cdr L)).
If you're having a problem figuring out where to put parentheses in general, note that Lisp syntax is very similar to mathematical syntax for functions, except the function goes on the inside of the parentheses (and you use spaces instead of commas, but that isn't a problem here). For example, written in mathematical notation, you have: mem(prop(cdr(L))), instead of mem(prop, cdr(L)).
Other Issues
It looks like you have an extra parentheses after the first branch of the cond, which ends it early. Use an editor/IDE with parentheses matching to catch this sort of error. Emacs does it, as does DrRacket. The former can be used with Common LISP via SLIME (search the web for set-up instructions for your platform), while the latter readily supports Scheme.
The semicolon (";") is the comment character used in most LISPs.
print isn't necessary for this function, and (depending on its behavior) may be incorrect. In some LISPs (e.g. Common LISP), it returns its argument, but in others it may return nil or the instance of some void type.
Quasiquote (aka backquote, backtick) is also unnecessary. Quasiquote lets you quote some symbols, while others (those prefixed with a comma) are replaced with their values. Since you don't need substitution in the quoted expressions, a plain quote will work: (mem 'j '((i 1) (j 2))).
For readability, insert more whitespace between closing and opening parentheses.
Is it possible to write an interactive defun with code "r" that has an additional optional argument (so that it does things within the selected region, but with another argument)? I would like something like the following:
(defun my-function (start end &optional arg)
"Do something with selected region"
(interactive "r")
(if arg
(setq val arg)
(setq val 2))
(do things...))
Looking at the documentation it says
'r': Point and the mark, as two numeric
arguments, smallest first. This is the
only code letter that specifies two
successive arguments rather than one.
No I/O.
I'm not sure if the 'No I/O' and 'two successive arguments' means that it takes 2 and only 2 arguments (i.e., limited to the region's start and end point as args). Although it allows me to evaluate and run the defun with an additional argument, Emacs appears to be ignoring it.
Thank you.
To make interactive ask for multiple parameters, separate them with a newline character. For instance, if you want your third parameter be bound to the value of the prefix argument, define your function like this:
(defun my-function (start end &optional arg)
"Do something with selected region"
(interactive "r\np")
(if arg
(setq val arg)
(setq val 2))
(do things...))
M-x describe-function interactive gives you further information.
A function can be called in two ways:
Interactively: This is what happens when a user calls the command, e.g. when it has been bound to a key.
From lisp: When the function is called from another lisp function. e.g. (r 100 200 t).
In your case, you have to make sure that the arguments match the interactive specification, in this case it must accept two arguments. The third will not be used when called interactively (so then it will get the value nil).
NO I/O means that it will not prompt the user for input (like it does when it asks for a file name).
If you want your function to act differently depending in when the region is active, you could ask the function (use-region-p).