· clojure

Clojure: Forgetting the brackets

I’ve been playing around with macros over the last few days and while writing a simple one forgot to include the brackets to make it evaluate correctly:

(defmacro say-hello [person]
  println "Hello" person)

This macro doesn’t even expand like I thought it would:

user=> (macroexpand-1 '(say-hello blah))

That seemed a bit strange to me but I eventually realised that I’d missed off the brackets around ‘println’ and the arguments following it which would have resulted in ‘println’ being evaluated with those arguments.

I was a bit curious as to why that happened so I tried the following expression without any brackets to see what would happen:

user=> println "hello" "mark"
#<core$println__5440 clojure.core$println__5440@681ff4>

It seems to just evaluate each thing individually and when we put this type of expression into a function definition the function will do the same thing but also return the last thing evaluated:

(defn say-hello [] println "hello" "mark")

user=> (say-hello)

A. J. Lopez pointed out that this is quite like progn in other LISPs and is the same as doing the following:

user=> (do println "hello" "mark")

do is defined as follows:

(do exprs*) Evaluates the expressions in order and returns the value of the last. If no expressions are supplied, returns nil.

The way to write a function which passes those two arguments to ‘println’ is of course to put brackets around the statement:

(defn say-hello [] (println "hello" "mark"))

user=> (say-hello) 
hello mark
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