I can't seem to understand the output of the following code:

General
1 
#lang racket

;; Use '() instead of the 'empty' alias
(define board #("X" "0" empty empty))

(for/vector ([val board]
             #:when (empty? val))
  val) ; this returns '#()

I would expect it to return '#(empty empty)

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2 
(for/vector ([val board]
             #:when (eq? val 'empty))
  val)

this works, why? I am a bit confused

3

this works:

(define board (vector "X" "0" empty empty))
(for/vector ((val board) #:when (empty? val)) val)
'#(() ())
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4

Literal vectors are self-quoting, so you have a vector with two strings, and two symbols (Both 'empty). If you want to use empty so it gets treated as a variable and its contents used, you have to use vector:

(define board (vector "X" "O" empty empty))

;;; this works too:
;;; (define board #("X" "0" () ()))

;;; or quasiquote shenanigans
;;; (define board `#("X" "0" ,empty ,empty))
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