I saw the this question and its answers but they slightly miss the point of how best to author core.clj to make a library easily shared and understood.
What is the purpose of core.clj file?
One answer said we could do anything we want but probably shouldn't. :) I am looking for the path of least resistance, so core.clj it is.
But another answer said users would be using that path to get access to my library. That makes me think I have it backwards, with core being a few fundamentals. Instead it sounds like core.clj (its namespace) should pull in my other paths and effectively serve as its API.
Like the accepted answer to your linked question says, the core namespace typically holds the entry point(s) to an application or to your library. That being said, this is only a convention, not a necessity, you technically don't need to have a core module at all.
If your core modules holds only a few fundamentals that are used by other code fragments and you expect users of your library to call functions foo and bar from the namespace yourlib.foobar as entry points, you would probably be better of to rename your core module to something else to avoid confusing people checking out the code of your library. However, one notable pattern I've seen is that some people put the entry points / functions in a yourlib.clj module (yourlib namespace, not yourlib.yourlib), which serve as a thin facade to calls to functions in yourlib.core. So, in a sense, yourlib.core is really the "core" of your library, but doesn't provide the entry points itself.
Related
My current project has two libraries libAAA and libBBB.
The former, libAAA, contains basic math and mechanics for my project (e.g. the rules for chess). The latter, libBBB, depends on libAAA but adds GUI elements (e.g. how single figures are drawn).
The executable depends on both, libAAA and libBBB. E.g. libBBB is used to setup the chess game while libBBB is used to define how it looks (console, qt, gtk,...).
Is it OK to have such redundant dependencies or is it probably bad practice?
edit:
I ask that because I like how easy it is to decouple a GUI representation from the mechanics. E.g. if I would like to change the look or the toolkit, I could just add libHHH, make another executable and that's it.
This is fine.
Let's look at some alternatives:
Pull in all the code from libAAA into libBBB: This is alright if libAAA isn't intended to be used in other contexts, and their respective sizes allow this to be done in a practical manner. In many contexts though, e.g. if libBBB is much larger and libAAA is meant as a small reusable library for many projects independently of the UI framework used in libBBB, this won't work.
Copy the code used by libBBB into it directly: Please don't. This would mean two separate libraries that do a lot of the same things, two places to update in the future, more confusion for you in the future or for anyone else trying to understand it, etc.
Bring the libBBB code into the application itself: This is not always possible or practical, depending on your development stack and environment. Many applications may also need libraries that were programmed and compiled in a different language for performance or other reasons.
I'm probably missing a couple other alternative options, but keeping both dependencies despite the redundancy seems the best option to me.
As you stated in the edit, this also allows you to swap out libBBB for another UI library that also uses libAAA(or another lib if need be) without tearing down any of your core application code or invalidating any other code in your dependencies.
I am just started on clojure and have a very basic question about core.clj.
What is a convention about it? What code should go there? Public API?
It is generated by leiningen when creating a project.
I have looked into source code of some libraries and this file is also present there (per package?).
Thanks
There's no defined meaning for it, but in an application it's often where you'll find the entry point, the main- function. For libraries, foo.core is often the namespace users will import to get your main functionality.
You don't have to do it that way, but it's a semi-predictable place to have the 'central bit' of whatever it is you're writing - even if your actual logic and algorithmic code is somewhere else.
Leiningen generates foo.core because it needs to pick a name and core is generic enough that it probably wont' be wrong. It's a style decision, but I typically opt to rename core.clj to a name that is actually meaningful for my project.
It's just a template file emitted by Leiningen. If I run lein new foo there is no specific or standard meaning to the namespace foo.core. It is entirely legal for the namespace foo to be the main API container for a project. It's just a free file for you to start working in.
That said, if you have a project foo, it is expected although not enforced that all your code for that one project exist in the foo.* namespace. Leiningen will allow you to build a project with the files src/foo.clj and src/bar.clj and it is entirely reasonable for foo to require bar, however when packaging your code for distribution this is probably a bad idea.
Apologies in advance for the somewhat discursive nature of this clump of related questions; I hope the answers will be a useful resource for newcomers to Clojure.
I have just begun to learn Clojure, motivated in part by this essay. I'm not a professional developer but I have several decades of programming experience (ARexx, VB/VBScript/VBA, then Perl and daily use of R starting in 2011). My platform is Windows 7 64-bit. I'm using Emacs 24.3, cider 20131221 and Leiningen 2.3.3 on Java 1.7.0_45 Java Hotspot 64-bit server. I have bought Clojure Programming and the Clojure Data Analysis Cookbook and dipped into both. I have found them promising but I am getting lost in the detail.
Obviously the thing to do is to get stuck in and experiment with code exercises and small tasks, but the immediate problem for me has been the complexity of structuring, organising and even just plain running projects in Clojure. With R I can get away with a file of plain text containing the bulk of the code, perhaps with one or two others containing common functions for larger projects.
Clojure is very different and with no experience in Java I am struggling to put the pieces together. Clojure Programming has a whole chapter on organising and building projects, but it is so comprehensive that conversely I'm finding it difficult to tease out the information relevant to me now. I guess I'm looking for something like this answer on Swank, but the tools seem to have moved on since then. So here goes.
Leiningen produces amongst other things a project.clj file that contains the project definition and dependencies. I think I get this. Can I use this file for code not related to the definition, below the defproject, or is it best to leave this untouched and have the code itself in different clj file(s)?
If the answer is to leave the project.clj file alone, how is the relationship between that and other files established? Is it simply that all the clj files in the project folder are counted part of the project?
How do I define the main code file, the 'entry point' of the project? Let's say I have project.clj and main.clj with some helper functions in common.clj - how are the relations between these three files defined? I can call functions from main.clj but how does the project know that main is the core of the project if/when I package the project into an uberjar?
If I have a number of clj files, what is the best way to import functions? I have read about require and use (and import and refer and...) but I don't fully understand the difference and those two keywords are difficult to search for. The examples for REPL in the Clojure Data Analysis Cookbook most often opt for use. I found a similar question but it was a little over my head.
This is more tool-specific, but as Emacs seems to be widely used it seems fair to ask: what's a good workflow to run small bits of code given (say) the main.clj example given above? Currently I just open the main.clj file in Emacs, do an M-x cider-jack-in to establish the REPL, experiment in the REPL, then when I want to try something I select the whole buffer and select Eval region from the CIDER menu (C-c C-R). Is this standard operating procedure or utterly misguided?
Is there a convention for defining namespaces? I think I understand that namespaces can cover multiple clj files and that ns is used to define the namespace. Should I explicitly define the namespace (at the beginning of) every file of code? Clojure Programming has some recommendations but I'm interested in input from other users.
Clojure programming says to "Use underscores in filenames when namespaces contain dashes. Very simply, if your namespace is to be com.my-project.foo, the source code for that namespace should be in a file located at com/my_project/foo.clj". (EDIT as explained in this useful answer and also this one). This restriction would never have occured to me. Are there any other gotchas with regard to naming namespaces and variables? R frequently uses dots in variable names but I guess given the Java connection that dots should generally be avoided?
No, don't put actual code in there unless you know what you are doing (e. g. generate the version number for defproject from the local git repository like in the repositories of juxt)
The project.clj is simply one big parameter to Clojures build tool leiningen. See an example here https://github.com/technomancy/leiningen/blob/master/sample.project.clj. For example, you could specifiy a different source directory than src in the :source-path.
Default is the -main function in project.core, but you can specify various different configurations in the project.clj.
require is preferred. :use imports all publics of a namespace unless you use it in conjunction with :only. Require let's you use an alias for an entire namespace with :as, but you can have the same effect from use with :only using :refer. Notice that in ClojureScript :use without :only is not even allowed.
This is normal. There are other combos like e.g. C-c C-k to reload the entire file of the buffer. If you find yourself entering too many forms into a REPL and would rather edit them in a separate buffer https://www.refheap.com/22235.
I like to experiment trying to name namespaces in in verbs rather than nouns, e. g. I prefer myproject.parse, myproject.interpret, over myproject.parser, myproject.interpreter etc. But that's a question of personal style. EDIT: Yes, explicitly define the naming of the namespace by its filename and the ns form at the beginning of the source file. It is unusual to have multiple source-file defining one namespace.
Afaic this is the only caveat regarding naming of namespace. You can hardly know it in advance.
I like your "worried" approach. You will (hopefully) find out that Clojure and especially Leiningen are almost nonsense-free in terms of these questions.
Regarding REPL use: I saw your comment under #Mars answer that you want to use a REPL in a fashion that you can re-use what you are entering. Two things:
Dynamic development is awesome, allowing you to test small components or functions interactively without the need to run an entire program written for that purpose.
If you find your self entering huge forms at the REPL that you intend to de-/recompose into functions or tests later, I recommend editing them in a seperate clj file that is not part of the project source (i. e. not in a namespace). You can then use this Emacs hack to eval forms from a Clojure buffer in the REPL. Ideally split your Emacs in two windows (C-x 3) with the nrepl buffer on one side and your .clj on the other side. Then use C-x C-. from within the clj file to have the form at point pasted into the nrepl and be evaluated. Installation instructions are at the link (and your .emacs file usually resides in the home directory).
#Igrapenthin's answers are great. Here are a few other thoughts.
On namespaces, this tutorial is great.
Just to clarify re #2: No, don't just put the .clj files anywhere under the project. They have to be under src/, or in whatever directories are listed (as strings) in the vector after :source-paths in project.clj, if that entry exists. Then strip off that initial path when you're making your namespace names. This drove me crazy until I figured it out. (People who know better, please correct me if something here isn't right.)
One #3, you need Igraphenthin's answer, but why not just start by evaluating expressions in the REPL? I've been working on a project on and off for weeks, and it does a lot, but my -main function still doesn't do anything. I just run whatever parts I'm working on. Well, you're used to languages with fully operational prompts--you decide.
EDIT: Whether or not you define the -main function to do anything, you can also put :use or :require keywords in the ns statement that defines the namespace for that same file. These will automatically get invoked when you start the REPL with lein repl, and so whatever you have made available through the ns keywords will be available at the REPL. That way, you have your previous work available, but you can play around with it in different ways in the REPL. (Also, if you don't like the default name for the file that's automatically loaded, you can redefine it in project.clj with :main. Igraphenthin alluded to that.)
I am trying to make little web framework in clojure. I have a bunch of clojure files in /handlers directory and I want to require all of them in my project's .core namespace. Every file defines its own namespace. For example: project.handlers.home. The idea behind this is when I add new handler, I don't want to modify namespace declaration in my core file to include it. The only solution I came up is to find all files in directory and load them with clojure.core/load, but it is far from beautiful and idiomatic. Is there a idiomatic way to do this?
Is there a idiomatic way to do this?
IMO, no. The idioms in Clojure usually favor being explicit over "doing magic", especially when it comes to naming global objects (which auto-loading namespaces clearly is).
I don't know why you wouldn't want to modify your "core" file when adding new handlers but you might consider introducing an additional namespace that loads both "core" and your handlers and hooks them together.
Noir included some functionality like this, making it an explicit API call to load namespaces under a particular directory. See load-views for an example of this. It used this to auto-load paths.
However, the Noir approach didn't feel idiomatic due to the amount of magic involved, as well as additional complications from the approach (e.g. lingering path definitions).
If you need to find namespaces from a tooling, framework, or library perspective, I would use find-namespaces in tools.namespace and then require/load them. This approach could be useful in terms of providing user level pluggability, where a user can drop a handler into a directory and then see new options in the code, though again being explicit tends to be significantly cleaner.
I have read in several places that single segment namespaces are discouraged in clojure.
Indeed almost every library I've seen has (require 'lib.core) instead of (require 'lib).
Why?
Edit: I am a bit stupid. Extra credit will be given for a concise example of how a single segment namespace might be a bad thing.
Java discourages the use of the default package because you can't refer to anything in it from any other package. If you pre-compile a one-segment Clojure namespace, you'll get a Java class in the default package. If anyone at any time wants to use your library from Java, he will be stopped dead by this triviality. As there is no good reason in favor of using a single-segment namespace, and there is this one well-defined reason not to, it is fair to say that single-segment namespaces should be a discouraged practice in Clojure.
Single segment namespaces have issues with Java interop. Some things may break in unexpected ways. See also https://groups.google.com/forum/?fromgroups=#!topic/clojure/gOffhotk25Y