I'm new to Clojure and building a web app using the Noir framework (very similar to Compojure, in fact I think it's Compojure with a different request handler layer). I'm getting a warning when I import the JDBC library:
WARNING: resultset-seq already refers to: #'clojure.core/resultset-seq in namespace: webapp.models.database, being replaced by: #'clojure.java.jdbc/resultset-seq
Do I have to live with this warning or is there a way around it? I'm importing the JDBC library using:
(use 'clojure.java.jdbc)
You can avoid the problem by specifying the exact bindings to be imported:
(use '[clojure.java.jdbc :only [insert-values transaction]])
(transaction
(insert-values ...))
Another option is to :exclude the offending binding:
(use '[clojure.java.jdbc :exclude [resultset-seq]])
(transaction
(insert-values ...))
You can also just use require instead:
(require '[clojure.java.jdbc :as db])
(db/transaction
(db/insert-values ...))
With regard to forward compatibility, require is arguably safest. Using :only is just slightly less clean but still a pretty good approach (and easy to fix when it breaks). Excluding the currently offending bindings is probably the least future-proof way of fixing the problem since other conflicting bindings can appear at any time and tracking down what is imported from where can be tricky.
There are lots of options. What this warning means is, that you are replacing an already defined symbol with a definition from different package. In this case, it looks like this is a variable that you've defined, right? If so the easiest solution might be to just rename it in your code.
Or if you don't need the resultset-seq from clojure.java.jdbc package you can exclude it:
(use '[clojure.java.jdbc :exclude (resultset-seq)])
or better yet,
(use '[clojure.java.jdbc :only (f1 f2 f3)])
where f1, f2, f3 are the things you're actually need.
(use '[clojure.java.jdbc :as jdbc])
and then use jdbc/resultset-seq
Or you can just:
(require 'clojure.java.jdbc)
and then use clojure.java.jdbc/reusltset-seq
In addition to the other excellent answers, if you want the jdbc resultset-seq instead of the core one, you can exclude the latter from being brought into the current ns:
(ns foo
(:refer-clojure :exclude [resultset-seq])
(:use clojure.java.jdbc))
Related
I am totally new to clojure. I would like to start writing simple scripts, and I came across lein-exec as a means of doing so even if a script contains dependencies. Although I can run an example I found online, I don't know how to require clojure.java.io.
(require 'leiningen.exec)
;places the dependency on the classpath
(leiningen.exec/deps '[[enlive/enlive "1.1.4"]])
(require '[net.cgrand.enlive-html :as html])
How would I require something like clojure.java.io using lein exec?
EDIT: adding more detail
(require 'leiningen.exec)
(leiningen.exec/deps '[[clojure.java.io]])
(require 'clojure.java.io)
(defn Example []
(.exists (clojure.java.io "Example.txt")))
(Example)
gives me
Caused by: java.lang.IllegalArgumentException: Provided artifact is missing a version: [clojure.java.io]
and this
(require 'leiningen.exec)
;(leiningen.exec/deps '[[clojure.java.io]])
(require 'clojure.java.io)
(defn Example []
(.exists (clojure.java.io "Example.txt")))
(Example)
gives me
Caused by: java.lang.ClassNotFoundException: clojure.java.io
Well, personally, I'd recommend using inlein instead of lein-exec for doing scripting with Clojure:
http://inlein.org/
https://github.com/hypirion/inlein
I find it nicer, and it uses standard lein dependency map for defining your dependencies.
Secondly, this: (require 'clojure.java.io) IS the right way to require clojure.java.io. That namespace is included with Clojure itself, so you don't need to declare an additional dependency on any other library to use it.
Your problem is that you are not using it correctly:
(.exists (clojure.java.io "Example.txt"))
In the above code, you are calling clojure.java.io as if it was a function, but it is not a function, it is a namespace. You need to choose a function inside of it to call, such as the file function. If this was java, you could kinda think of the namespace as the class, and the function as a method on the class. If you do:
(.exists (clojure.java.io/file "Example.txt"))
It should work now. Notice how in Clojure, the syntax is: namespace/function. This is different the some other languages like Java or Python, where you would have: some.location.class.method instead, basically the function is also separated by a .. This is not the case in Clojure, in Clojure, the function part is separated by a /.
I have number of useful helper functions that I use at the REPL during Clojure development. This includes some built-in functions like doc and pprint, but also some custom ones of my own from my user.clj. The default clojure experience seems to be:
dev> (doc +)
;; works
dev> (in-ns 'project.core)
project.core> (doc +)
;; what is this "doc" thing you're talking about!?!?
which is pretty irritating (I'm aware that I can refer to clojure.repl/doc here). Is there an easy way to ensure that something is available during development regardless of which namespace I'm currently operating in?
One option is to add a :repl-options to your project map in project.clj:
(defproject myproj "1.0"
:dependencies [[org.clojure/clojure "1.9.0-alpha15"]]
:repl-options { :init-ns myproj.core
:init (require '[clojure.repl :refer :all]) })
...
$ lein repl
myproj.core=> (doc +) ; works
As Alex Gherega mentions, it's probably a bad idea to automatically and indiscriminately import stuff into every namespace. However, you could create a macro (or maybe a function would work?) to help import development-related namespaced when/if they are needed. Add it to your user.clj. Then if you are working in another namespace and decide it would be useful, you can just run (user/import-useful-dev-stuff) (or whatever you decide to call it).
For what it's worth, I pretty much do all my REPL sessions within the user namespace and require all the namespaces I need from there. I use Emacs, and if I need to change something in another namespace, I just change the source, then do C-c C-k (after initiating a cider session) to reload the file. If you don't use Emacs, it's only a little more work to reload from the REPL via require with the :reload option.
Unless you use boot or lein or some other form that would "magically" add this namespace to your REPL development session then I'm afraid you're just have to require it by hand every time.
The thing is namespaces have their well intended role of avoiding name conflicts. If you would have another way of telling a current namespace to use/require another namespace without doing it then you would have a whole lot of other problems.
If you're not using lein/boot you could just do as Alex Miller responded. For a more controlled way of what you need from the clojure.repl or some other ns API just have a require & use declaration in you namespace of interest e.g.:
(ns 'project.core
(:require [clojure.repl :refer [doc]]))
If you get yourself in this situation, do:
(clojure.core/refer-clojure)
to require and load everything from clojure.core.
Or an alternate approach, if project.core exists, is to instead:
(require 'project.core)
(in-ns 'project.core)
Loading the namespace via require will refer clojure.core.
Or if project.core does not exist, do:
(ns project.core)
The ns macro will do both refer-clojure and in-ns.
I want to use the overtone and seesaw namespaces in one project. However when i load them there seems to be a shared function named (select) which exists in both seesaw.core and overtone.core ((seesaw.core/select) (overtone.core/select)), why i can't load both of the namespaces in the project. How could i come up with this? Will using refer with the :exclude keyword an option?
While many tutorials and examples use the use function or the :use clause in the ns form, in real code it is better to never do this. The preferable form is as follows:
(ns foo.bar
(:require [a.something :as some]
[b.another :as a]))
(some/f)
(a/f)
The functions are fully distinct and unambiguous, and also this makes the code easier to read and refactor - you have a clear indication of where the definitions you are using come from.
Is there any way to import a record type, that works in Clojure as well as ClojureScript?
As far as I can tell it's (ns x (:import y [A B])) for Clojure, (ns x (:require y :refer [A B])) for ClojureScript, and each is invalid for the respective other.
Ignoring the specifics on the syntax of requiring records, there are two main ways to write ns declarations (or any platform specific code) for multiple Clojure variants while sharing the majority of your code.
CLJX is a Clojure preprocessor that runs before the Clojure compiler. You write platform specific code prefixed with #+clj or #+cljs in a .cljx file. It can run on pretty much any Clojure code, and will spit out multiple platform specific files which the respective Clojure Compilers can handle.
Reader Conditionals are a feature in Clojure 1.7 and are available in recent releases of ClojureScript. This is similar in spirit to cljx, but is integrated into the Clojure Compiler. You write code with reader conditionals like #?(:clj 1 :cljs 2) in files with a .cljc extension.
Now back to your specific question, you can achieve this with Reader Conditionals like so:
(ns myapp.music-store
(:require #?(:clj [myapp.cool-music]
:cljs [myapp.cool-music :refer [Vinyl]]))
#?(:clj
(:import [myapp.cool_music Vinyl])))
I wrote a longer blog post about this too: Requiring records in Clojure and ClojureScript
This question already has an answer here:
Closed 10 years ago.
Possible Duplicate:
Why does require in the ns form behave different from the require function
I am dabbling in clojure and have run into a problem with importing. From the REPL
clojure.core=>(import '(java.io FileReader))
clojure.core=>(import 'java.io.FileReader)
clojure.core=>(import java.io FileReader)
each work perfectly, but from file only the following works:
(ns project.core
(import java.io.FileReader))
These each fail
(ns project.core
(import 'java.io.FileReader))
(ns project.core
(import '(java.io FileReader))
with the following errors:
ClassNotFoundException quote.java.io.FileReader java.net.URLClassLoader$1.run (URLClassLoader.java:366)
ClassNotFoundException quote.(java.io FileReader) java.net.URLClassLoader$1.run (URLClassLoader.java:366)
respectively.
In reality I need to import more than just java.io.FileReader, but this is a distilled version of the problem.
Any ideas what might be going wrong? I can't seem to find the problem anywhere else
The solution is:
(ns project.core
(:import [java.io FileReader BufferedReader FooBar]))
As to why this is necessary, the ns macro accepts several directives, including :import. Due to the way the macro is implemented, it expects arguments to the import directive to be in a list-like form - hence, [java.io ...]. Incidentally, a list such as (java.io ...) would work just as well.
For a complete rundown of how to use ns, please take a look at the ClojureDocs page: http://clojuredocs.org/clojure_core/clojure.core/ns. ClojureDocs is an invaluable resource for these sorts of problems.