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)