I want to get following results when I evaluate edit-url and (edit-url 1).
edit-url --> "/articles/:id/edit"
(edit-url 1) --> "/articles/1/edit"
Is it possible to define such a Var or something?
Now, I use following function, but I don't want to write (edit-url) to get const string.
(defn edit-url
([] "/articles/:id/edit")
([id] (str "/articles/" id "/edit")))
Thanks in advance.
If those behaviors are exactly what you want, print-method and tagged literals may be used to imitate them.
(defrecord Path [path]
clojure.lang.IFn
(invoke [this n]
(clojure.string/replace path ":id" (str n))))
(defmethod print-method Path [o ^java.io.Writer w]
(.write w (str "#path\"" (:path o) "\"")))
(set! *data-readers* (assoc *data-readers* 'path ->Path))
(comment
user=> (def p #path"/articles/:id/edit")
#'user/p
user=> p
#path"/articles/:id/edit"
user=> (p 1)
"/articles/1/edit"
user=>
)
edit-url will either have the value of an immutable string or function. Not both.
The problem will fade when you write a function with better abstraction that takes a string and a map of keywords to replace with words. It should work like this
(generate-url "/articles/:id/edit" {:id 1})
Clojure is a "Lisp 1" which means that is has a single namespace for all symbols, including both data scalars and functions. What you have written shows the functionally of both a string and a function but for a single name, which you can do in Common Lisp but not Clojure (not that a "Lisp 2" has its own inconveniences as well).
In general this type of "problem" is a non issue if you organize your vars better. Why not just make edit-url a function with variable arity? Without arguments it returns something, with arguments it returns something else. Really the possibilities are endless, even more so when you consider making a macro instead of a function (not that I'm advocating that).
Related
I am trying to print the documentation for all functions in a given namespace by invoking the following expression in a REPL:
(doseq
[f (dir-fn 'clojure.repl)]
(doc f))
However the invocation of this expression returns nil without printing the documentation to the REPL. I know this might have to do with doc being a macro, but I'm a Clojure novice and am not entirely sure how to understand the problem.
Why does this expression return nil without printing the documentation?
How can this expression be modified so that it prints the documentation for each function in a given namespace?
Thanks!
Update: Combined both provided answers:
(defn ns-docs [ns']
(doseq [[symbol var] (ns-interns ns')]
(newline)
(println symbol)
(print " ")
(println (:doc (meta var)))))
(ns-docs 'clojure.repl)
I would, instead, start here:
The Clojure CheatSheet
ClojureDocs.org
Clojure-Doc.org (similar name, but different)
The API & Reference sections at Clojure.org
Note that doc is in the namespace clojure.repl, which reflects its intended usage (by a human in a repl). Here is some code that will also iterate on a namespace & print doc strings (using a different technique):
(doseq [[fn-symbol fn-var] (ns-interns 'demo.core)]
(newline)
(println fn-symbol)
(println (:doc (meta fn-var))))
where demo.core is the namespace of interest.
Note that ns-interns gives you both a symbol and var like:
fn-symbol => <#clojure.lang.Symbol -main>
fn-var => <#clojure.lang.Var #'demo.core/-main>
The meta function has lots of other info you may want to use someday:
(meta fn-var) =>
<#clojure.lang.PersistentArrayMap
{ :arglists ([& args]),
:doc "The Main Man!",
:line 9, :column 1,
:file "demo/core.clj",
:name -main,
:ns #object[clojure.lang.Namespace 0x14c35a06 "demo.core"]}>
While this probably won't help you with answering your question, the problem of evaluating macro's comes up a lot when you are learning Clojure.
Macros are responsible for the evaluation of their arguments. In this case clojure.repl/doc will ignore the current lexical context and assume that the symbol f that you're giving it is the name of a function you want to see the documentation for. It does this because it's intended to be used at the REPL, and is assuming you wouldn't want to type quotes all the time.
As f doesn't exist, it prints nothing. Then doseq returns nil, since it exists to do something for side effects only - hence starting in do. In order to pass an argument to a macro that refuses to respect the lexical context like this, you need to write the code for each element in the list.
You can do this by hand, or by constructing the code as data, and passing it to eval to execute. You can do this in an imperative style, using doseq:
(doseq [f (ns-interns 'clojure.repl)]
(eval `(doc ~(symbol "clojure.repl" (str (first f))))))
or in a slightly more Clojurey way (which will allow you to see the code that it would execute by removing eval from the end and running it at the REPL):
(->> (ns-interns 'clojure.repl)
(map #(list 'clojure.repl/doc (symbol "clojure.repl" (str (first %)))))
(cons `do)
eval)
In both of these we use quote and syntax-quote to construct some code from the list of symbols reflected from the namespace, and pass it to eval to actually execute it. This page on Clojure's weird characters should point you in the right direction for understanding what's going on here.
This an example of why you shouldn't write macro's, unless you've got no other options. Macro's do not compose, and are often difficult to work with. For a more in depth discussion, Fogus's talk and Christophe Grand's talk are both good talks.
Why does this expression return nil without printing the documentation?
Because the doc macro is receiving the symbol f from your loop, instead of a function symbol directly.
How can this expression be modified so that it prints the documentation for each function in a given namespace?
(defn ns-docs [ns']
(let [metas (->> (ns-interns ns') (vals) (map meta) (sort-by :name))]
(for [m metas :when (:doc m)] ;; you could filter here if you want fns only
(select-keys m [:name :doc]))))
(ns-docs 'clojure.repl)
=>
({:name apropos,
:doc "Given a regular expression or stringable thing, return a seq of all
public definitions in all currently-loaded namespaces that match the
str-or-pattern."}
...
)
Then you can print those maps/strings if you want.
I am programming something that doesn't have side-effects, but my code is not very readable.
Consider the following piece of code:
(let [csv_data (if header_row (cons header_row data_rows) data_rows)]
)
I'm trying to use csv_data in a block of code. What is a clean way of conditioning on the presence of a header_row? I've looked at if-let, but couldn't see how that could help here.
I have run into similar situations with functional for-loops as well where I'm binding the result to a local variable, and the code looks like a pile of expressions.
Do I really have to create a separate helper function in so many cases?
What am I missing here?
Use the cond->> macro
(let [csv_data (cond->> data_rows
header_row (cons header-row)]
)
It works like the regular ->> macro, but before each threading form a test expression has to be placed that determines whether the threading form will be used.
There is also cond->. Read more about threading macros here: Official threading macros guide
First, don't use underscore, prefer dashes.
Second, there is nothing wrong with a little helper function; after all, this seems to be a requirement for handling your particular data format.
Third, if you can change your data so that you can skip those decisions and have a uniform representation for all corner cases, this is even better. A header row contains a different kind of data (column names?), so you might prefer to keep them separate:
(let [csv {:header header :rows rows}]
...)
Or maybe at some point you could have "headers" and "rows" be of the same type: sequences of rows. Then you can concat them directly.
The ensure-x idiom is a very common way to normalize your data:
(defn ensure-list [data]
(and data (list data)))
For example:
user=> (ensure-list "something")
("something")
user=> (ensure-list ())
(())
user=> (ensure-list nil)
nil
And thus:
(let [csv (concat (ensure-list header) rows)]
...)
i would propose an utility macro. Something like this:
(defmacro update-when [check val-to-update f & params]
`(if-let [x# ~check]
(~f x# ~val-to-update ~#params)
~val-to-update))
user> (let [header-row :header
data-rows [:data1 :data2]]
(let [csv-data (update-when header-row data-rows cons)]
csv-data))
;;=> (:header :data1 :data2)
user> (let [header-row nil
data-rows [:data1 :data2]]
(let [csv-data (update-when header-row data-rows cons)]
csv-data))
;;=> [:data1 :data2]
it is quite universal, and lets you fulfill more complex tasks then just simple consing. Like for example you want to reverse some coll if check is trueish, and concat another list...
user> (let [header-row :header
data-rows [:data1 :data2]]
(let [csv-data (update-when header-row data-rows
(fn [h d & params] (apply concat (reverse d) params))
[1 2 3] ['a 'b 'c])]
csv-data))
;;=> (:data2 :data1 1 2 3 a b c)
update
as noticed by #amalloy , this macro should be a function:
(defn update-when [check val-to-update f & params]
(if check
(apply f check val-to-update params)
val-to-update))
After thinking about the "cost" of a one-line helper function in the namespace I've came up with a local function instead:
(let [merge_header_fn (fn [header_row data_rows]
(if header_row
(cons header_row data_rows)
data_rows))
csv_data (merge_header_fn header_row data_rows) ]
...
<use csv_data>
...
)
Unless someone can suggest a more elegant way of handling this, I will keep this as an answer.
First, I have no experience with CS and Clojure is my first language, so pardon if the following problem has a solution, that is immediately apparent for a programmer.
The summary of the question is as follows: one needs to create atoms at will with unknown yet symbols at unknown times. My approach revolves around a) storing temporarily the names of the atoms as strings in an atom itself; b) changing those strings to symbols with a function; c) using a function to add and create new atoms. The problem pertains to step "c": calling the function does not create new atoms, but using its body does create them.
All steps taken in the REPL are below (comments follow code blocks):
user=> (def atom-pool
#_=> (atom ["a1" "a2"]))
#'user/atom-pool
'atom-pool is the atom that stores intermediate to-be atoms as strings.
user=> (defn atom-symbols []
#_=> (mapv symbol (deref atom-pool)))
#'user/atom-symbols
user=> (defmacro populate-atoms []
#_=> (let [qs (vec (remove #(resolve %) (atom-symbols)))]
#_=> `(do ~#(for [s qs]
#_=> `(def ~s (atom #{}))))))
#'user/populate-atoms
'populate-atoms is the macro, that defines those atoms. Note, the purpose of (remove #(resolve %) (atom-symbols)) is to create only yet non-existing atoms. 'atom-symbols reads 'atom-pool and turns its content to symbols.
user=> (for [s ['a1 'a2 'a-new]]
#_=> (resolve s))
(nil nil nil)
Here it is confirmed that there are no 'a1', 'a2', 'a-new' atoms as of yet.
user=> (defn new-atom [a]
#_=> (do
#_=> (swap! atom-pool conj a)
#_=> (populate-atoms)))
#'user/new-atom
'new-atom is the function, that first adds new to-be atom as string to `atom-pool. Then 'populate-atoms creates all the atoms from 'atom-symbols function.
user=> (for [s ['a1 'a2 'a-new]]
#_=> (resolve s))
(#'user/a1 #'user/a2 nil)
Here we see that 'a1 'a2 were created as clojure.lang.Var$Unbound just by defining a function, why?
user=> (new-atom "a-new")
#'user/a2
user=> (for [s ['a1 'a2 'a-new]]
#_=> (resolve s))
(#'user/a1 #'user/a2 nil)
Calling (new-atom "a-new") did not create the 'a-new atom!
user=> (do
#_=> (swap! atom-pool conj "a-new")
#_=> (populate-atoms))
#'user/a-new
user=> (for [s ['a1 'a2 'a-new]]
#_=> (resolve s))
(#'user/a1 #'user/a2 #'user/a-new)
user=>
Here we see that resorting explicitly to 'new-atom's body did create the 'a-new atom. 'a-new is a type of clojure.lang.Atom, but 'a1 and 'a2 were skipped due to already being present in the namespace as clojure.lang.Var$Unbound.
Appreciate any help how to make it work!
EDIT: Note, this is an example. In my project the 'atom-pool is actually a collection of maps (atom with maps). Those maps have keys {:name val}. If a new map is added, then I create a corresponding atom for this map by parsing its :name key.
"The summary of the question is as follows: one needs to create atoms at will with unknown yet symbols at unknown times. "
This sounds like a solution looking for a problem. I would generally suggest you try another way of achieving whatever the actual functionality is without generating vars at runtime, but if you must, you should use intern and leave out the macro stuff.
You cannot solve this with macros since macros are expanded at compile time, meaning that in
(defn new-atom [a]
(do
(swap! atom-pool conj a)
(populate-atoms)))
populate-atoms is expanded only once; when the (defn new-atom ...) form is compiled, but you're attempting to change its expansion when new-atom is called (which necessarily happens later).
#JoostDiepenmaat is right about why populate-atoms is not behaving as expected. You simply cannot do this using macros, and it is generally best to avoid generating vars at runtime. A better solution would be to define your atom-pool as a map of keywords to atoms:
(def atom-pool
(atom {:a1 (atom #{}) :a2 (atom #{})}))
Then you don't need atom-symbols or populate-atoms because you're not dealing with vars at compile-time, but typical data structures at run-time. Your new-atom function could look like this:
(defn new-atom [kw]
(swap! atom-pool assoc kw (atom #{})))
EDIT: If you don't want your new-atom function to override existing atoms which might contain actual data instead of just #{}, you can check first to see if the atom exists in the atom-pool:
(defn new-atom [kw]
(when-not (kw #atom-pool)
(swap! atom-pool assoc kw (atom #{}))))
I've already submitted one answer to this question, and I think that that answer is better, but here is a radically different approach based on eval:
(def atom-pool (atom ["a1" "a2"]))
(defn new-atom! [name]
(load-string (format "(def %s (atom #{}))" name)))
(defn populate-atoms! []
(doseq [x atom-pool]
(new-atom x)))
format builds up a string where %s is substituted with the name you're passing in. load-string reads the resulting string (def "name" (atom #{})) in as a data structure and evals it (this is equivalent to (eval (read-string "(def ...)
Of course, then we're stuck with the problem of only defining atoms that don't already exist. We could change the our new-atom! function to make it so that we only create an atom if it doesn't already exist:
(defn new-atom! [name]
(when-not (resolve (symbol name))
(load-string (format "(def %s (atom #{}))" name name))))
The Clojure community seems to be against using eval in most cases, as it is usually not needed (macros or functions will do what you want in 99% of cases*), and eval can be potentially unsafe, especially if user input is involved -- see Brian Carper's answer to this question.
*After attempting to solve this particular problem using macros, I came to the conclusion that it either cannot be done without relying on eval, or my macro-writing skills just aren't good enough to get the job done with a macro!
At any rate, I still think my other answer is a better solution here -- generally when you're getting way down into the nuts & bolts of writing macros or using eval, there is probably a simpler approach that doesn't involve metaprogramming.
I have the following line in my code:
(spit path (prn-str job-data))
It does it's work well execpt for one thing, every item in the list are put between double-quotes...
( ":a" ":b" ":a" )
the expected result that I'd like to have
( :a :b :a )
How to get the expected result?
Thanks in advance!
What's happening
The issue isn't that the items are being put in double quotes per se but that they're strings (as opposed to the keywords you're expecting).
prn-str, which is ultimately based on pr, prints objects "in a way that objects can be read by the reader". This means strings are printed in double-quotes - otherwise the reader wouldn't be able to tell strings from symbols, or read strings with whitespace in them. See here for more information on Clojure's reader.
println and print, on the other hand, are intended to "produce output for human consumption" and do not put strings in double-quotes. This is why you're seeing the difference in output between prn-str and println.
You can verify this with class. If you try (-> job-data first class) the answer will be either java.lang.String or clojure.lang.Keyword.
Here are some examples demonstrating the different behaviors of the printing functions when used with keywords and strings:
(def str-job-data '(":a" ":b" ":c"))
(def key-job-data '(:a :b :c))
;; `println` prints both keywords and strings without quotes
(with-out-str (println str-job-data)) ;=> "(:a :b :c)\n"
(with-out-str (println key-job-data)) ;=> "(:a :b :c)\n"
;; `prn-str` prints the strings in quotes but the keywords without quotes
(prn-str str-job-data) ;=> "(\":a\" \":b\" \":c\")\n"
(prn-str key-job-data) ;=> "(:a :b :c)\n"
How to change it
Now for possible solutions. If you were expecting job-data to contain keywords then the right fix is most likely to modify job-data. However, I can't offer much guidance here without knowing more about how job-data is produced.
If for some reason you can't modify job-data (for instance, if it's produced by code you don't control) and you want to write keywords wherever it contains keyword-like strings then something like #maxthoursie's suggestion is probably your best bet. (You could hypothetically just switch to print or println but that could have undesirable effects on how other objects are printed).
(defn keyword-string->keyword [s]
(keyword (subs s 1)))
(spit path (prn-str (map keyword-string->keyword job-data)))
If job-data might contain objects other than keyword-like strings you could apply the function only when appropriate.
(defn convert-job-data [obj]
(if (and (string? obj)
(= (.charAt obj 0) \:))
(keyword-string->keyword obj)
obj))
(spit path (prn-str (map convert-job-data job-data)))
Of course, if the file you're writing is for human consumption anyway and all this business about the reader is irrelevant you could trivially make your own println-str:
(defn println-str [& more]
(with-out-str (apply println more)))
(spit path (println-str job-data))
I'm guessing job-data is not what you expect it to be.
user=> (prn-str '(:a :b :c))
"(:a :b :c)\n"
If you do have a list with strings that looks like keywords, and you would like to convert it to keywords, you could use something like
(map (comp keyword #(subs % 1)) '(":a" ":b" ":c"))
Which skips the : of each element, and then converts it to a keyword.
user=> (prn-str (map (comp keyword #(subs % 1)) '(":a" ":b" ":c")))
"(:a :b :c)\n"
Given a list of names for variables, I want to set those variables to an expression.
I tried this:
(doall (for [x ["a" "b" "c"]] (def (symbol x) 666)))
...but this yields the error
java.lang.Exception: First argument to def must be a Symbol
Can anyone show me the right way to accomplish this, please?
Clojure's "intern" function is for this purpose:
(doseq [x ["a" "b" "c"]]
(intern *ns* (symbol x) 666))
(doall (for [x ["a" "b" "c"]] (eval `(def ~(symbol x) 666))))
In response to your comment:
There are no macros involved here. eval is a function that takes a list and returns the result of executing that list as code. ` and ~ are shortcuts to create a partially-quoted list.
` means the contents of the following lists shall be quoted unless preceded by a ~
~ the following list is a function call that shall be executed, not quoted.
So ``(def ~(symbol x) 666)is the list containing the symboldef, followed by the result of executingsymbol xfollowed by the number of the beast. I could as well have written(eval (list 'def (symbol x) 666))` to achieve the same effect.
Updated to take Stuart Sierra's comment (mentioning clojure.core/intern) into account.
Using eval here is fine, but it may be interesting to know that it is not necessary, regardless of whether the Vars are known to exist already. In fact, if they are known to exist, then I think the alter-var-root solution below is cleaner; if they might not exist, then I wouldn't insist on my alternative proposition being much cleaner, but it seems to make for the shortest code (if we disregard the overhead of three lines for a function definition), so I'll just post it for your consideration.
If the Var is known to exist:
(alter-var-root (resolve (symbol "foo")) (constantly new-value))
So you could do
(dorun
(map #(-> %1 symbol resolve (alter-var-root %2))
["x" "y" "z"]
[value-for-x value-for-y value-for z]))
(If the same value was to be used for all Vars, you could use (repeat value) for the final argument to map or just put it in the anonymous function.)
If the Vars might need to be created, then you can actually write a function to do this (once again, I wouldn't necessarily claim this to be cleaner than eval, but anyway -- just for the interest of it):
(defn create-var
;; I used clojure.lang.Var/intern in the original answer,
;; but as Stuart Sierra has pointed out in a comment,
;; a Clojure built-in is available to accomplish the same
;; thing
([sym] (intern *ns* sym))
([sym val] (intern *ns* sym val)))
Note that if a Var turns out to have already been interned with the given name in the given namespace, then this changes nothing in the single argument case or just resets the Var to the given new value in the two argument case. With this, you can solve the original problem like so:
(dorun (map #(create-var (symbol %) 666) ["x" "y" "z"]))
Some additional examples:
user> (create-var 'bar (fn [_] :bar))
#'user/bar
user> (bar :foo)
:bar
user> (create-var 'baz)
#'user/baz
user> baz
; Evaluation aborted. ; java.lang.IllegalStateException:
; Var user/baz is unbound.
; It does exist, though!
;; if you really wanted to do things like this, you'd
;; actually use the clojure.contrib.with-ns/with-ns macro
user> (binding [*ns* (the-ns 'quux)]
(create-var 'foobar 5))
#'quux/foobar
user> quux/foobar
5
Evaluation rules for normal function calls are to evaluate all the items of the list, and call the first item in the list as a function with the rest of the items in the list as parameters.
But you can't make any assumptions about the evaluation rules for special forms or macros. A special form or the code produced by a macro call could evaluate all the arguments, or never evaluate them, or evaluate them multiple times, or evaluate some arguments and not others. def is a special form, and it doesn't evaluate its first argument. If it did, it couldn't work. Evaluating the foo in (def foo 123) would result in a "no such var 'foo'" error most of the time (if foo was already defined, you probably wouldn't be defining it yourself).
I'm not sure what you're using this for, but it doesn't seem very idiomatic. Using def anywhere but at the toplevel of your program usually means you're doing something wrong.
(Note: doall + for = doseq.)