CODEWITHSUNDEEP
clojurecommentsmarginalia
pauldoo
pauldoo

Reputation: 18615

What is the difference between ; and ;; in Clojure code comments?

What is the difference between ; and ;; when starting a comment in Clojure? I see that my text editor colours them differently, so I'm assuming there is notionally some difference.

I also see that Marginalia treats them differently:

; Stripped entirely
;; Appears in text section of marginalia
(defn foobar []
   ; Appears in code section of marginalia output
   ;; Again, appears in code section of marginalia output
   6)

Upvotes: 75

Views: 17996

Answers (7)

G__
G__

Reputation: 7111

There is no difference as far as the interpreter is concerned. Think of ; ;; ;;; and ;;;; as different heading levels.

Here is my personal use convention:

;;;; Top-of-file level comments, such as a description of the whole file/module/namespace

;;; Documentation for major code sections (i.e. groups of functions) within the file.

;; Documentation for single functions that extends beyond the doc string (e.g. an explanation of the algorithm within the function)

; In-line comments possibly on a single line, and possibly tailing a line of code

Upvotes: 89

amalloy
amalloy

Reputation: 91857

Check out the official description of the meaning of ; vs ;; in elisp: since the Clojure indenter is basically the same, it will treat them similarly. Basically, use ; if you are writing a long sentence/description "in the margins" that will span multiple lines but should be considered a single entity. Their example is:

(setq base-version-list                 ; there was a base
      (assoc (substring fn 0 start-vn)  ; version to which
             file-version-assoc-list))  ; this looks like
                                        ; a subversion

The indenter will make sure those stay lined up next to each other. If, instead, you want to make several unrelated single-line comments next to each other, use ;;.

(let [x 99 ;; as per ticket #425
      y "test"] ;; remember to test this
  (str x y)) ;; TODO actually write this function

Upvotes: 35

Alex Miller
Alex Miller

Reputation: 70201

Emacs ; to be used for end-of-line comments and will indent in surprising ways if that is not your intent. ;; does not so I usually use ;;.

Clojure doesn't care - any line is ignored from the ; to EOL.

I believe there is a tradition in CL of using increasing numbers of ; to indicate more important comments/sections.

Upvotes: 16

rplevy
rplevy

Reputation: 5463

In emacs lisp modes including clojure-mode, ;; is formatted with the convention of being at the beginning of a line, and indented as as any other line, based on the context. ; is expected to be used at the end of a line, so emacs will not do what you want it to if you put a single-semicolon comment at the beginning of a line expecting it to tab to the indentation for the present context.

Example:

(let [foo 1]
  ;; a comment
  foo) ; a comment

Upvotes: 6

fogus
fogus

Reputation: 6236

There is no difference from a Clojure-perspective. I find that ;; stands out a little better than ;, but that's only my opinion.

Marginalia on the other hand treats them differently because there are times when a comment should remain in the code section (e.g. license) and those are flagged with ;. This is an arbitrary decision and may change in the future.

Upvotes: 6

Arthur Ulfeldt
Arthur Ulfeldt

Reputation: 91534

no meaning for the language. ; is a reader macro for comment perhaps other tools parse them but "within clojure" they are the same.

Upvotes: 7

Owen
Owen

Reputation: 1551

I'm not sure (not used Clojure and never heard of this before), but this thread might help.

Upvotes: 1

Related Questions