Question

I defined an unless macro as follows:

user=> (defmacro unless [expr body] (list 'if expr nil body))
#'user/unless
user=> (unless (= 1 2) (println "Yo"))
Yo

As you can see it works fine.

Now, in Clojure a list can be defined in two ways:

; create a list
(list 1 2 3)

; shorter notation
'(1 2 3)

This means that the unless macro can be written without the list keyword. However, this results in a Java exception being thrown:

user=> (unless (= 1 2) (println "Yo"))
java.lang.Exception: Unable to resolve symbol: expr in this context

Can someone explain why this fails?

Answer 1

'(foo bar baz) is not a shortcut for (list foo bar baz), it’s a shortcut for (quote (foo bar baz)). While the list version will return a list containing the values of the variables foo, bar and baz, the version with ' will return a list containing the symbols foo, bar and baz. (In other words '(if expr nil body) is the same as (list 'if 'expr 'nil 'body).

This leads to an error because with the quoted version the macro expands to (if expr nil body) instead of (if (= 1 2) nil (println "Yo")) (because instead of substituting the macro’s arguments for expr and body, it just returns the name expr and body (which are then seen as non-existent variables in the expanded code).

A shortcut that’s useful in macro definitions is using `. ` works like ' (i.e. it quotes the expression following it), but it allows you to evaluate some subexpressions unquoted by using ~. For example your macro could be rewritten as (defmacro unless [expr body] `(if ~expr nil ~body)). The important thing here is that expr and body are unquoted with ~. This way the expansion will contain their values instead of literally containing the names expr and body.