You can use ;; to comment a whole line (; is the comment but ;; is Clojure convention).

(comment ) function can wrap other expressions to comment that code, returning nil when evaluated.

#_ is the comment reader macro that comments out specific forms in Clojure, so you can get really specific with your comments.

comment function

Wrap any Clojure code inside the comment function to prevent it from being executed. Using your editor, you can manually evaluate the code inside a comment function.

The comment function returns nil so its advised not to use it inside another form. For example:

(map + [1 2 3] (comment [1 2 3]))

This will fail as it tries to use the + function to add 1 to nil

Comment forms with the comment reader macro

#_ is the comment reader macro and is used to comment a Clojure form (Clojure code that can be evaluated). #_ is not just a line comment.

You can place #_ before the start of a form and everything inside that form will be commented

#_(def my-data [1 2 3 4 5])

#_ will comment forms that span multiple lines, for example function definitions

#_(defn my-function
        (str "I am an experiment, so not always needed"))

#_ can also be put on the line before the comment (possibly many lines - to test). This approach can make your code more readable and keep alignment of your code consistent.

comment nested forms

#_ tells the reader to ignore the next form, it is therefore never evaluated and neither is the #_. This means that #_ can be used inside a nested form to comment just a part of the expression

In this example the third vector of values is not read by the Clojure reader and therefore is not passed as an argument to + function by map

(map + [1 2 3] [4 5 6] #_[7 8 9])

Stacking comments

The comment reader macro has the ability to stack these comments on forms, so using #_#_ will comment out two successive forms.

In a let form we can comment out a name binding that is causing problems. As the name and value are both forms, then we use a stacked #_ to comment both out. We also do the same in the body of the let, so as to not include the evaluation of the string or name2 local name in the str form.

(let [name1 "value"
       #_#_name2 "another-value]
   (str "First name is: " name1 #_#_" second name is: " name2

Comments in threading macros

The comment reader macro can also be used in a threading macro, so provide a simple way to apply or skip a particular forms used within the threading macro.

(->> book
     (re-seq #"[a-zA-Z0-9|']+" ,,,)
     (map #(clojure.string/lower-case %))
     #_(remove common-english-words)
     #_(sort-by val)

Watch episode #13 of Practicalli Clojure study group to see this in practice.

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