Often I need to undefine a function in clojure. If I define something with defn how can I undefine it?
There is no one-argument version, because the same Var can be mapped in more than one namespace. If you are working from the REPL, you often want to unbind from the user namespace, e.g.
(ns-unmap 'user 'symbol-to-unbind)
The first argument to ns-unmap can be a symbol or a namespace, and the second argument should be a symbol.
I think, that you can use ns-unmap to do this.
P.S. Couldn't add this code into comment, so i put it here. To unmap function in current namespace, you need to use following code:
(ns-unmap *ns* 'method)
If you have:
(def x 42)
It might be useful to unbind the var:
(.unbindRoot #'x)
Now, if you try this
x
You get:
#<Unbound Unbound: #'user/x>
Related
Given the following in one file:
(ns demo.first)
(defmacro ^:private private-macro [a] a)
And the following in another file:
(ns demo.second
(:require [demo.first :as first]))
(first/private-macro 10)
The call to the private macro in demo.second will throw: var: #'demo.first/private-macro is not public, as I expect.
Now, is there a way to have this call succeed, without making the macro public?
For functions, I can do:
(#'first/private-macro 10)
But with a macro, it throws: Wrong number of args (1) passed to: first/private-macro.
I'm looking to unit test this private macro, and personally prefer using private meta over an impl namespace. Which is why I'm hoping there's a solution to this.
Thank You.
UPDATE:
I found out that since defmacro is itself a macro, it first expands into a form which creates the symbol and Var for the macro and add its metadata to it.
Thus:
(defmacro ^:private private-macro [a] a)
First is processed by the defmacro macro, and expanded into:
(do
(clojure.core/defn ^{:private true} private-macro
([&form &env a] a))
(. (var ^{:private true} private-macro)
^{:line 487, :column 49}
(setMacro))
(var ^{:private true} private-macro))
As you can see, what then happens is that:
A private-macro fn is declared with defn, and set to private.
This function takes 3 arguments [&form &env a]. This is why we get the wrong number of argument (1) exception when using #' to call the macro.
The private-macro var is set as a macro by calling its setMacro method.
The private-macro var is returned.
In essence, what is happening is that if you call the function pointed to by the private-macro var, such as is the case when using the (#'private-macro) syntax, you're actually calling the function you see above, which takes 3 arguments. If your macro itself took more than one argument, that function would take 2 + the number of args of your macro.
So I still don't know how to call a private macro:
At first I thought stubbing out &form and &env with nils would work:
(#'first/private-macro nil nil 10)
And for my simple macro above it does, and return 10. But on more complicated macros, which need to be expanded further, it doesn't, and instead I get the macro-expansion returned to me ?!?
Then I thought I could use alter-meta! to remove the private meta from the macro temporarily before calling it. As such:
(alter-meta! #'first/private-macro
(fn [meta] (dissoc meta :private)))
(first/private-macro 10)
(alter-meta! #'first/private-macro
(fn [meta] (assoc meta :private true)))
But this only works at the REPL. Try to compile your code afterwards, and it seems the Compiler itself will throw the var: #'demo.first/private-macro is not public error, even before the alter-meta! has a chance to run, thus failing compilation.
I don't really know why #' doesn't work the same as a normal call to the macro, and why passing nil to the &form and &env doesn't work for all macros. And how to make alter-meta! work at compile time. So if someone does know, please answer away!
And for my simple macro above it does, and return 10. But on more complicated macros, which need to be expanded further, it doesn't, and instead I get the macro-expansion returned to me ?!?
Yes. As you discovered, when you write (defmacro m [x] (list x x)), you:
Define a function m that consumes forms as input and produces forms as output
Tell the compiler to look for calls like (m a) and replace them with the result of calling your m function at compile time
By calling #'m instead, you bypass step 2: there is no call to the macro m, and so the compiler does not call it at compile time or replace the calling code with the result. Since #'m is just a regular function which takes code as input and produces code, when you bypass the special compiler behavior and call it at runtime, you of course get code as a result (which you can't do much with because it's runtime already).
Good news, though: there's rarely a compelling reason to make a macro private anyway, since it can do no harm to let other namespace call it. All the private macro does is expand into code the client could have written by hand anyway. So, if you control this macro, you should probably just make it public. If you don't, then you can just write whatever code the macro would have written for you.
If you absolutely insist on calling someone else's private macro, then you can split the parts (1) and (2) up, in a way: define your own macro whose implementation delegates to the function backing the private var in the other namespace:
(defmacro cheat [& args]
(apply #'m &form &env args))
Because cheat is your own macro, you can call it in the usual way, engaging the compiler's "call this at compile time" mechanism. Then you delegate to the function that generates the code you want, passing &form and &env explicitly.
I need a function that can tell whether a symbol is pointing to a special form or a macro.
I found the function? function in the clojure.test namespace, so could use that, but I'm hesitant to use it because it seems to be intended only for testing purposes. Is it okay to use it for normal code? If not, how can I accomplish my goal.
As noted the comments, fn? does not work because it only works on functions themselves, not the symbols that point to them.
If it does what you want, use it. It's in clojure.test because it wasn't expected to be useful for non-test code, but if it's the best function for your particular use case, there's no reason to hamper yourself just because of where the function is located. In other words, clojure.test is separate from clojure.core for organization, not because it should only ever be used for tests.
you can find all special form, :-)
(defn special-symbol?
[s]
(contains? (. clojure.lang.Compiler specials) s))
I have this macro:
(defmacro widget [msg-type value & app-key]
`(defrecord ~msg-type [~value]
Message
(~'process-message [msg# app#]
(let [state# (~#app-key app#)]
(dissoc
(->>
(merge state# msg#)
(assoc app# ~#app-key))
:errors)))))
Message is a protocol defined in a clojurescript dependency, with a process-message function.
When I try to use widget like so
(ns my.cljs.ns
(:require-macros [my.ns.macros :as macro])
(:require [petrol.core :refer [Message]]))
(macro/widget A-Record a-field :a-key)
I get this error:
Bad method signature in protocol implementation,
my.ns.macros/Message does not declare method called
process-message ...
How can I get Message to refer to petrol/Message instead of my.ns.macros/Message?
You need the power of the mystical ~' operator :)
I see you already invoked it for process-message, so perhaps you are already acquainted with why; but for the purposes of the answer, stuff in the backtick gets fully namespace qualified, where as evaluate quote puts the literal symbol in place.
(macroexpand-1 '(widget :a :b))
And the error message indicate that you need to ~'Message if you want to avoid it having the current ns attached to it.
However fully qualifying Message with the petrol namespace would be a good move IMO
petrol.core/Message
That way you don't need to rely on it being referred in the ns declaration. Note you don't need to ~' it either.
Also I would be wary of (~#app-key app#) because app-key are optional... you could get nothing passed in which would call whatever #app is, which doesn't sound like something you want to happen. Similarly passing more than one seems wierd to. Maybe it should be a required param?
In clojure, What is the best possible way to declare a global variable, the value of which needs to be changed in a function and this global variable need to be accessed in other namespaces
Ex:
(ns ..main
.....)
(def global_variable nil)
..
(defn main ...
Change the value of global variable here
)
Another name space
(ns accessvariable ...)
(println (main/global_variable))
What is the best way to do this?
Just store an atom in your global var:
(def global-var (atom nil))
Note: this is in response to the third comment on the question. dAni's answer applies to the question as asked.
Idiomatic Clojure favors functions whose outputs are determined only by their input parameters. This is in contrast to a function which relies on its parameters and global state, too. It is easy to convert a function which uses global data into one that does not - you just need to add a parameter that will contain the data. For example: instead of (defn print-global [] (println #global-atom-defined-elsewhere)), you use (defn print-my-val [val] (println val)).
If you use the command line argument extremely frequently, you might still find it more convenient to put the data in a global atom, instead of having everything use it as a parameter. This is up to you.
According to spec, def should intern the var in the current ns (i.e. *ns*). However, the following code does not look anything like it:
(ns namespace-b)
(defn def_something []
(ns namespace-a)
(println *ns*) ;prints namespace-a as it should
(def something 1)
)
(def_something)
(println namespace-b/something) ; prints 1
(println namespace-a/something) ; throws
What am I missing?
Notes:
defn is used just for clarity. Defining and running anonymous function works just as well.
I know that using def inside function is probably not very idiomatic. However, this is just extracted essence of a bigger problem I ran into.
The parser already interns the var to the current namespace at compile time, although it won't be bound immediately:
(defn dd [] (def x 0))
x ;; => #<Unbound Unbound: #'user/x>
The relevant piece of code can be found here, with the second parameter to lookupVar triggering the aforementioned interning for non-existing vars here.
The parses then generates an expression that references the previously created var, so the expression logic never leaves the current namespace.
TL;DR: def is something that the compiler handles in a special kind of way.
The key thing to understand about def is that it is a macro. This means that it does not resolve the namespace or create the binding at runtime, but beforehand, while the code is being compiled.
If you call a function that calls def, that call to def was already resolved to use the namespace in which the function was defined. Similarly, if you call functions inside a function body, the functions to call are resolved at compile time within the namespace where that function was defined.
If you want to generally bind values to namespaces at runtime, you should use the function intern, which lets you explicitly set the namespace to mutate.
All this said, namespace mutation is just that, it's procedural and is not thread safe and does not have nice declarative semantics like other options Clojure makes available. I would strongly suggest finding a way to express your solution that does not involve unsafe runtime mutation.