In Racket, if an unquoted pair is constructed with the dot notation, is it possible to use a variable or an expression value for the second element?

Question

In Racket, the following works:

(+ . [1 2]) ; => 3

{ define a + }
(a . [1 2]) ; => 3

However, i see no way to define b to be the (1 2) list so as to get (+ . b) and (a . b) to return 3. Is it possible?

Answer 1

Sure, just use apply:

(define a +)
(define b '(1 2))
(apply a b)       ; => 3
(apply + b)       ; => 3

Answer 2

How about this ... without using apply but using eval. But seriously, using apply is a better idea in this case, there's nothing wrong with it (eval is evil though, see the documentation to understand the last part with the namespace):

(define a +)
(define b '(1 2))

; executing in the evaluation window

(eval `(+ ,@b))
=> 3
(eval `(a ,@b))
=> 3

; executing from the definitions window

(define-namespace-anchor an)
(define ns (namespace-anchor->namespace an))    
(eval `(+ ,@b) ns)
=> 3
(eval `(a ,@b) ns)
=> 3