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.
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.
(defn recurse
[temp total] ;total is: (and true true(and false))
(map (fn [i]
(cond
(seq? i) (println "");If total is not a single parenthesis (single sequence), recur until it is
(= i 'and) (System/exit 0) ;I want this to be called only when the **second** "and" is called
:else (println "This should never print I think")
))
idealreturn)
)
I want (System/exit 0) to be called only when the second "and" is detected in total and not before. How would I go about doing this?
You are on the right track with mapping a function over the data to transform it. There are a couple of ways to get what you are looking for:
Don't use map, and use reduce instead. Reduce is for building up state over time. So you could reduce it into an expression, and each time you encounter an and, you look to see if there is already an and in the result you are building up, and if that and is already there, call the exit.
Have the function you are mapping over the input do only one thing, convert single items into more meaningful things. Then once it is done, pass that result to a second function that checks if it's time to exit.
Giving each thing one responsibility makes for code that's much easier to write, and composing them afterwords is efficient and easy. It's also much easier on you later when you come back to work on the code later.
I'm attempting to use a string saved in a variable to call a function like so:
(defn update-product-list [] "Test")
(defn handle-state-change [action]
((resolve (symbol action))))
(handle-state-change "update-product-list")
However, this gives me the following error: Assert failed: Argument to resolve must be a quoted symbol
I've also tried changing the above line to:
((resolve (quote (symbol action))))
But this still gives an error. I also tried changing it just to:
((resolve 'action))
But this gives a different error I don't quite understand: js/action is shadowed by a local. I don't want to override the function just call it. Not sure where I'm going wrong. I've looked at a few examples, but can't see to pin it down.
ClojureScript supports :advanced optimization, in which Google Closure Compiler will rename, inline, or eliminate (unused) functions in order to implement minification. In short, the name of the function you want to look up will, in general, simply no longer exist under :advanced.
Because of this, ClojureScript's resolve is a compile-time facility (a macro requiring a literal quoted symbol).
If you are using :simple or self-hosted ClojureScript, more options are available to you because the support needed persists into runtime. For example Planck has a planck.core/resolve that behave's like Clojure's resolve. A similar approach is possible in Lumo, and similar facilities can be fashioned if using :simple.
In general though, given :advanced, if you need to map strings to a set of functions, you need to somehow arrange to have a static mapping constructed at compile time to support this (the set of functions must be known a priori, at compile time).
If you have a namespace (the name of which is statically known at compile time) which defines functions that need to be dynamically called via strings, you could consider making use of ns-publics:
cljs.user=> (ns foo.core)
foo.core=> (defn square [x] (* x x))
#'foo.core/square
foo.core=> (in-ns 'cljs.user)
nil
cljs.user=> (when-some [fn-var ((ns-publics 'foo.core) (symbol "square"))]
(fn-var 3))
9
This will work under :advanced. The mapping constructed by ns-publics is static; built at compile-time. If you have multiple namespaces that need such treatment, you could merge several calls to ns-publics to build a larger map.
The advantage of this approach is that the code involved is pretty short and requires little maintenance. The disadvantage is that it dumps all of the public vars of the namespace (foo.core in this example) into your generated code (and the generated code for vars is somewhat verbose). Another disadvantage is that you need to statically know the namespace(s) involved at compile time.
If you need to further minimize generated code size, you could just build / maintain a simple static map from string to function value as in
(def fns {"square" foo.core/square})
and use it appropriately, keeping it up to date as your codebase evolves.
Another option would be to mark the functions that you need to access using ^:export meta, and then to call those functions using JavaScript interop. For example if you define the function this way
(defn ^:export square [x] (* x x))
then you can use strings / interop to lookup the function and call it at runtime. Here's an example:
((goog.object/getValueByKeys js/window #js ["foo" "core" "square"]) 3)
The use of ^:export and :advanced is covered here. If you know that you are using :simple or less, then you can simply use JavaScript interop to call the functions of interest, without needn't to use ^:export.
Note that there is no general solution that would let you look up a function by name at runtime under :advanced without somehow putting some aspect of that function into your code at compile time. (In fact, if a function is not referenced in a way that Google Closure Compiler can statically, see, the function implementation will be completely eliminated as dead code.) In the above, ns-publics grabs all the vars for a namespace at compile time, rolling your own lookup map sets up static code to refer to the function value, and using ^:export statically arranges to make the name of the function persist into runtime.
You need to use it like this:
((resolve 'inc) 5)) => 6
or, deconstructed a bit:
(let [the-fn (resolve 'inc)]
(the-fn 7))
=> 8
If you have the function name as a string, use the symbol function to convert from string => symbol (from clojuredocs.org):
user=> ((-> "first" symbol resolve) [1 2 3])
1
And, never forget the Clojure CheatSheet!
There's something I don't understand about anonymous functions using the short notation #(..)
The following works:
REPL> ((fn [s] s) "Eh")
"Eh"
But this doesn't:
REPL> (#(%) "Eh")
This works:
REPL> (#(str %) "Eh")
"Eh"
What I don't understand is why (#(%) "Eh") doesn't work and at the same time I don't need to use str in ((fn [s] s) "Eh")
They're both anonymous functions and they both take, here, one parameter. Why does the shorthand notation need a function while the other notation doesn't?
#(...)
is shorthand for
(fn [arg1 arg2 ...] (...))
(where the number of argN depends on how many %N you have in the body). So when you write:
#(%)
it's translated to:
(fn [arg1] (arg1))
Notice that this is different from your first anonymous function, which is like:
(fn [arg1] arg1)
Your version returns arg1 as a value, the version that comes from expanding the shorthand tries to call it as a function. You get an error because a string is not a valid function.
Since the shorthand supplies a set of parentheses around the body, it can only be used to execute a single function call or special form.
As the other answers have already very nicely pointed out, the #(%) you posted actually expands to something like (fn [arg1] (arg1)), which is not at all the same as (fn [arg1] arg1).
#John Flatness pointed out that you can just use identity, but if you're looking for a way to write identity using the #(...) dispatch macro, you can do it like this:
#(-> %)
By combining the #(...) dispatch macro with the -> threading macro it gets expanded to something like (fn [arg1] (-> arg1)), which expands again to (fn [arg1] arg1), which is just want you wanted. I also find the -> and #(...) macro combo helpful for writing simple functions that return vectors, e.g.:
#(-> [%2 %1])
When you use #(...), you can imagine you're instead writing (fn [args] (...)), including the parentheses you started right after the pound.
So, your non-working example converts to:
((fn [s] (s)) "Eh")
which obviously doesn't work because the you're trying to call the string "Eh". Your example with str works because now your function is (str s) instead of (s). (identity s) would be the closer analogue to your first example, since it won't coerce to str.
It makes sense if you think about it, since other than this totally minimal example, every anonymous function is going to call something, so it'd be a little foolish to require another nested set of parens to actually make a call.
If you're in doubt what your anonymous function gets converted to, you can use the macroexpand procedure to get the representation. Remember to quote your expression before passing it to macroexpand. In this case we could do:
(macroexpand '#(%))
# => (fn* [p1__281#] (p1__281#))
This might print different names for p1__281# which are representations of the variables %.
You can also macroexpand the full invocation.
(macroexpand '(#(%) "Eh"))
# => ((fn* [p1__331#] (p1__331#)) "Eh")
Converted to more human readable by replacing the cryptic variable names by short names. We get what the accepted answers have reported.
# => ((fn* [s] (s)) "Eh")
Resources:
https://clojure.org/guides/weird_characters#_n_anonymous_function_arguments
https://clojuredocs.org/clojure.core/macroexpand
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).