What is the use of `,` and `,@` in Racket?

I'm new to Racket and I was hoping to get more insights in the these two operators: , & ,@. There's very little documentation of these new operators, however, to my understanding the former (,) unquotes everything if its is followed by a list. And the latter (,@) splices the values.

For example if the following is typed in the Dr. Racket interpreter:

(define scores '(1 3 2))
(define pets '(dog cat))

and then the following query is made:

`(,scores ,@pets)

this would yield : '((1 3 2) dog cat)

It would be appreciated if I could get more details, definitions and more examples about these operators. Thanks in advance.


  • A single quote followed by the written representation of a value will produce that value:

    Example: '(1 x "foo") will produce a value that prints as (1 x "foo").

    Suppose now that I don't want a literal symbol x in the list. I have a variable x in my program, and I want to insert the value to which x is bound.

    To mark that I want the value of x rather than the symbol x, I insert a comma before x:

    '(1 ,x "foo")

    It won't work as-is though - I now get a value that has a literal comma as well as a symbol x. The problem is that quote does not know about the comma convention.

    Backtick or backquote knows about the comma-convention, so that will give the correct result:

    > `(1 ,x "foo")
    (1 3 "foo")          ; if the value of x is 3

    Now let's say x is the list (a b).

    > `(1 ,x "foo")
    (1 (a b) "foo")          ; if the value of x is (a b)

    This looks as expected. But what if I wanted (1 a b "foo") as the result? We need a way so show "insert the elements of a list". That's where ,@ comes into the picture.

    > `(1 ,@x "foo")
    (1 a b "foo")          ; if the value of x is (a b)