As a minimal example of what I want to do:
(defn mkfn [func]
(fn func [a] (print "I am a function")))
(mkfn 'x) ; => #function[user/mkfn/func--10871]
(type x)
(x)
The last two both result in:
Syntax error compiling at (conjure-log-12628.cljc:1:1).
Unable to resolve symbol: x in this context
I'm not sure why this doesn't work since fn takes symbols as input and 'x is a symbol. I'm also not sure how to accomplish this task.
For that matter:
user=> (def (eval 'y) 3)
Syntax error compiling def at (conjure-log-12628.cljc:1:1).
user=> (def 'y 3)
Syntax error compiling def at (conjure-log-12628.cljc:1:1).
First argument to def must be a Symbol
First argument to def must be a Symbol
user=> (type 'y)
clojure.lang.Symbol
Other things that don't work:
(defn mkfn [func]
(fn (sympol func) [a] (print "i am a function")))
(symbol "y") ; => y ; a symbol
(def (symbol "y") 3) ; => an err
You will probably need a macro. It seems that you want to call that function by the provided name, so you also have to replace fn with defn.
And you have to be careful about a number of arguments, because function x with argument vector [a] must be called with one argument, and not like (x).
(defmacro mkfn [func]
`(defn ~func [~'a]
(print "I am a function")))
(mkfn x)
=> #'user/x
(x 1)
I am a function=> nil
There is also other way, using intern, so you can completely avoid writing macros:
(intern *ns* 'x (fn [a] (print "I am a function")))
=> #object...
(x 1)
I am a function=> nil
Example with intern:
(defn mkfn [func]
(intern *ns* func (fn [a] (print "I am a function"))))
=> #'user/mkfn
(mkfn 'y)
=> #'user/y
(y 1)
I am a function=> nil
As for your errors, def is a special form, so it has different evaluation rules. It doesn't evaluate the first argument, which has to be a symbol- and (unevaluated) (eval 'y), 'y or (symbol "y") aren't symbols, while y is.
You gonna need a macro for that since you need code writing code.
(defmacro mkfn [func]
`(fn ~func [~'a] ...))
There 2 ways of doing it, either function plus eval or macro. If you really want to programatically create a new function with your chosen name, the macro solution is the way to go.
The function + eval solution is instructive, but you'll have to either quote the function name when you call it (via a 2nd eval) or save the created function in another variable when you create it.
If you are interested in writing macros, please see this other question first: How do I write a Clojure threading macro?
For the function + eval, we can start with my favorite template project and add the following:
(ns tst.demo.core
(:use demo.core tupelo.core tupelo.test))
(defn maker-eval
[fn-sym]
(let [ll (list 'fn 'anon-fn [] (str "I am " fn-sym))]
(spyx :eval ll)
(eval ll)))
(verify
(let [anon-fn-1 (maker-eval 'aaa)]
(is= "I am aaa" (anon-fn-1))) ; need the temp local variable
(let [anon-fn-2 (maker-eval 'bbb)]
(is= "I am bbb" (anon-fn-2))) ; need the temp local variable
)
and we can see the creation and use of the function, along with printed output:
:eval ll => (fn anon-fn [] "I am aaa")
:eval ll => (fn anon-fn [] "I am bbb")
For the macro version, we type
(defn maker-macro-impl
[fn-sym]
(let [ll `(defn ~fn-sym [] (str "I am " (str (quote ~fn-sym))))]
(spyx :macro ll)
ll))
(defmacro maker-macro
[fn-sym] (maker-macro-impl fn-sym))
(verify
(let [anon-fn-3 (maker-macro-impl 'ccc)]
(is= anon-fn-3 (quote
(clojure.core/defn ccc [] (clojure.core/str "I am " (clojure.core/str (quote ccc)))))))
(maker-macro ddd)
(is= (ddd) "I am ddd"))
and see printed:
:macro ll => (clojure.core/defn ccc [] (clojure.core/str "I am " (clojure.core/str (quote ccc))))
Note that the local variable anon-fn-3 was only used to test the maker-macro-impl function, but was not needed to call the newly-created function ddd
at the end of the unit test.
Related
Which form is better?
(defn my-func [arg arg2]
(fn [] [:div (str arg arg2)]))
Or this one?
(defn my-func []
(fn [arg arg2] [:div (str arg arg2)]))
I'm writting from my phone so forgive me indentation...
There's other difference between these functions aside from arity.
The first function will close over arg, and arg2 on first render, and use these arguments for each subsequent rerender. E.g.
(def a 1)
(def b 2)
[my-func a b]
will always result in [:div '12'] whether or not a or b were changed
The second function will take arg, and arg2 on each render and will update the result when arguments change.
Look for Rookie mistake in reagent docs
I believe your question should be which one is correct(The first one is correct). The second one has got an arity exception. Based on your use case, I would prefer to define and call the anonymous function so as to get the results. Consider this number one edit:
(defn my-func [arg arg2]
((fn [] [:div (str arg arg2)])))
I want to write a macro, sym-def, which has the same behavior as the special form def but uses (symbol "c"), say, as the first argument.
My first step was
(def (symbol "c") 4)
but this returned the error First argument to def must be a Symbol.
My second step was
(eval `(def ~(symbol "c") 4))
and this succeeded in defining c to be 4 in the global environment.
Why did my first step fail while the second step succeeded?
Finally, I attempted to write the desired macro
(defmacro sym-def [sym value] `(def ~sym ~value))
but this has a "bad" macroexpand
(macroexpand '(sym-def (symbol "c") 4)) => (def (symbol "c") 4)
so that
(sym-def (symbol "c") 4)
fails with the same error as my first step.
What is the correct way to write the macro?
def does not evaluate its first argument. Imagine the chaos if it did! You couldn't write
(def x 1)
because it would first try to evaluate x, and fail because x is not yet defined! Now, since it doesn't evaluate its arguments, clearly it makes sense that
(def (symbol "c") 4)
doesn't work, just as
(def 'c 4)
wouldn't. def requires its first argument to be a literal symbol. You don't have a literal symbol, so you can't use def.
But there is a lower-level mechanism to interact with the mappings in a namespace. In this case, you want clojure.core/intern:
(intern *ns* (symbol "c") 4)
intern is an ordinary function, so it evaluates all its arguments, meaning you can construct your var name in whatever crazy way you want. Then it adds to the given namespace a var mapping your symbol to its desired value.
A correct form for the macro you want to write is the following:
(defmacro sym-def [s v] `(def ~(eval s) ~v))
... or equivalently:
(defmacro sym-def [s v] (list `def (eval s) v))
You just need to evaluate the first argument inside the macro, because a macro's arguments are not evaluated when it is applied. If the only symbol-producing expressions you are going to use are calls to symbol, you may prefer the following macro:
(defmacro defsym [s v] (list `def (symbol s) v))
... and a companion to it:
(defmacro sym [s] (symbol s))
These macros translate a string or symbol to a symbol. Here are some examples of their use:
(defsym "the first natural number" 0)
;=> #'user/the first natural number
(sym "the first natural number")
;=> 0
(defsym pi 3.14159) ;same as: (def pi 3.14159)
;=> #'user/pi
(sym pi) ;same as: pi
;=> 3.14159
The variations given below might also be usefull:
(defmacro defsym* [s v] (list `def (symbol (eval s)) v))
(defmacro sym* [s] (symbol (eval s)))
They translate a string/symbol-producing expression to a symbol after evaluating it. Here are some examples of defsym*'s use:
(defsym* "abc" "xyz")
(defsym* (str \a \b \c) "xyz")
(defsym* (symbol "abc") "xyz")
(defsym* 'abc "xyz")
;all the previous are equivalent and what follows is valid for any of them
;=> #'user/abc
abc
;=> "xyz"
(defsym* abc 0)
;=> #'user/xyz
abc
;=> "xyz"
xyz
;=> 0
I'm writing an interface layer with pure functions in ClojureScript, but I'm getting undesired results after compile. Simple parameterized functions work fine. The example:
(defn rev [s]
(.. s (split "") (reverse) (join "")))
... will successfully generate function rev(s){return s.split("").reverse().join(""); }, while, with more complex parameters, like [s a & [b]], or with parametric polimorphism, it generates an anonymous function:
(defn substr
([s a b]
(.. s (substr a b)))
([s a]
(.. s (substr a))))
... generates: function (s,a,b){ switch(arguments.length){ case 2: return substr__2.call(this,s,a); case 3: return substr__3.call(this,s,a,b); } throw(new Error('Invalid arity: ' + arguments.length)); }.
It doesn't even work with (def substr (fn ... instead. How can I force a function to be named with defn?
Use ^:export on your function if you are going to be calling it from javascript.
(defn ^:export substr
([s a b]
(.. s (substr a b)))
([s a]
(.. s (substr a))))
The compiler should then generate the name so that you can call it from javascript.
I did it by exporting the function to the Window global object, of JavaScript:
(aset js/window "substr" stdlib.string/substr)
Environment: Clojure 1.4
I'm trying to pull function metadata dynamically from a vector of functions.
(defn #^{:tau-or-pi: :pi} funca "doc for func a" {:ans 42} [x] (* x x))
(defn #^{:tau-or-pi: :tau} funcb "doc for func b" {:ans 43} [x] (* x x x))
(def funcs [funca funcb])
Now, retrieving the metadata in the REPL is (somewhat) straight-forward:
user=>(:tau-or-pi (meta #'funca))
:pi
user=>(:ans (meta #'funca))
42
user=>(:tau-or-pi (meta #'funcb))
:tau
user=>(:ans (meta #'funcb))
43
However, when I try to do a map to get the :ans, :tau-or-pi, or basic :name from the metadata, I get the exception:
user=>(map #(meta #'%) funcs)
CompilerException java.lang.RuntimeException: Unable to resolve var: p1__1637# in this context, compiling:(NO_SOURCE_PATH:1)
After doing some more searching, I got the following idea from a posting in 2009 (https://groups.google.com/forum/?fromgroups=#!topic/clojure/VyDM0YAzF4o):
user=>(map #(meta (resolve %)) funcs)
ClassCastException user$funca cannot be cast to clojure.lang.Symbol clojure.core/ns-resolve (core.clj:3883)
I know that the defn macro (in Clojure 1.4) is putting the metadata on the Var in the def portion of the defn macro so that's why the simple (meta #'funca) is working, but is there a way to get the function metadata dynamically (like in the map example above)?
Maybe I'm missing something syntactically but if anyone could point me in the right direction or the right approach, that'd would be great.
Thanks.
the expression #(meta #'%) is a macro that expands to a call to defn (actually def) which has a parameter named p1__1637# which was produced with gensym and the call to meta on that is attempting to use this local parameter as a var, since no var exists with that name you get this error.
If you start with a vector of vars instead of a vector of functions then you can just map meta onto them. You can use a var (very nearly) anywhere you would use a function with a very very minor runtime cost of looking up the contents of the var each time it is called.
user> (def vector-of-functions [+ - *])
#'user/vector-of-functions
user> (def vector-of-symbols [#'+ #'- #'*])
#'user/vector-of-symbols
user> (map #(% 1 2) vector-of-functions)
(3 -1 2)
user> (map #(% 1 2) vector-of-symbols)
(3 -1 2)
user> (map #(:name (meta %)) vector-of-symbols)
(+ - *)
user>
so adding a couple #'s to your original code and removing an extra trailing : should do the trick:
user> (defn #^{:tau-or-pi :pi} funca "doc for func a" {:ans 42} [x] (* x x))
#'user/funca
user> (defn #^{:tau-or-pi :tau} funcb "doc for func b" {:ans 43} [x] (* x x x))
#'user/funcb
user> (def funcs [#'funca #'funcb])
#'user/funcs
user> (map #(meta %) funcs)
({:arglists ([x]), :ns #<Namespace user>, :name funca, :ans 42, :tau-or-pi :pi, :doc "doc for func a", :line 1, :file "NO_SOURCE_PATH"} {:arglists ([x]), :ns #<Namespace user>, :name funcb, :ans 43, :tau-or-pi :tau, :doc "doc for func b", :line 1, :file "NO_SOURCE_PATH"})
user> (map #(:tau-or-pi (meta %)) funcs)
(:pi :tau)
user>
Recently, I found it useful to attach metadata to the functions themselves rather than the vars as defn does.
You can do this with good ol' def:
(def funca ^{:tau-or-pi :pi} (fn [x] (* x x)))
(def funcb ^{:tau-or-pi :tau} (fn [x] (* x x x)))
Here, the metadata has been attached to the functions and then those metadata-laden functions are bound to the vars.
The nice thing about this is that you no longer need to worry about vars when considering the metadata. Since the functions contain metadata instead, you can pull it from them directly.
(def funcs [funca funcb])
(map (comp :tau-or-pi meta) funcs) ; [:pi :tau]
Obviously the syntax of def isn't quite as refined as defn for functions, so depending on your usage, you might be interested in re-implementing defn to attach metadata to the functions.
I'd like to elaborate on Beyamor's answer. For some code I'm writing, I am using this:
(def ^{:doc "put the-func docstring here" :arglists '([x])}
the-func
^{:some-key :some-value}
(fn [x] (* x x)))
Yes, it is a bit unwieldy to have two metadata maps. Here is why I do it:
The first metadata attaches to the the-func var. So you can use (doc the-func) which returns:
my-ns.core/the-func
([x])
put the-func docstring here
The second metadata attaches to the function itself. This lets you use (meta the-func) to return:
{:some-key :some-value}
In summary, this approach comes in handy when you want both docstrings in the REPL as well as dynamic access to the function's metadata.
I'd like to do this (in REPL or anywhere)
(defn (symbol "print-string") [k] (println k))
and then be able to do
(print-string "lol")
Or, if there is any other way to create defn from custom strings in macroses, could you push me into the right direction please?
(defmacro defn-with-str [string args & body]
`(defn ~(symbol string) ~args ~#body))
(defn-with-str "print-string" [k] (println k))
(print-string "lol")
dnolen's solution works at macro expansion time, Brian Carper's at read-time. Now, here's one for run-time:
(intern *ns* (symbol "a") (fn [k] (println k)))
I like dnolen's answer better, but you can do this too:
(defn #=(symbol "print-string") [k] (println k))
#=() is evaluated at read-time. I don't know how stable a feature of Clojure this is, I wouldn't rely on it not to change in the future. Macros are how I'd do it.
FYI - dnolen's answer will only work for literal strings, and not for strings in def'd or let'd variables.
(defmacro defn-with-str [string args & body]
`(defn ~(symbol string) ~args ~#body))
(def hisym "hi")
(defn-with-str hisym [] (println "hi"))
You now have a function called "hisym"
(hi) -> java.lang.Exception: Unable to resolve symbol: hi in this context (NO_SOURCE_FILE:6)
(hisym) -> prints "hi"
To avoid this, eval the function name string in the macro
(defmacro defn-with-str [string args & body]
`(defn ~(symbol (eval string)) ~args ~#body))