define: expected a variable, but found a part - racket

Question

I am getting define error from the function neighbours-in-grid below, do I need to use "let" to declare from inside the define???
;; Purpose: Produce a maze, being a list of whole numbers (cells)
;; between 0 and the square of grid-size (grid-size^2)
;; The list prepresenting the maze will start with 0 and be expanded
;; by randomly picking a cell already in the maze
;; then randomly selecting a neighbouring cell (horz and vert axis
;; only) following tests to ensure that the new neighbouring cell is:
; a) within the grid/maze area
; b) not already in the maze
; c) not adjacent to a cell already in the maze (otherwide could block the maze path)
; d) will likely add tests here to make sure maze does not consume itself
; Grid will lay out as follows (assuming grid-size is 3;
; 0 1 2
; 3 4 5
; 6 7 8
(define grid-size 15)
(define a-maze (list 0))
;Is returned value part of the existing a-maze
(check-expect (member? (random-cell a-maze) a-maze)
#true)
;Is returned vale a number?
(check-expect (number? (random-cell a-maze))
#true)
;Is returned value within the limits of the grid
(check-expect (<= 0 (random-cell a-maze) (sqr grid-size))
#true)
;random-cell: non-empty list -> random number from list
(define (random-cell a-list)
(list-ref a-list (random (length a-list))))
(check-expect (member? 15 (neighbours (random-cell a-maze))) #true)
(check-expect (member? 1 (neighbours (random-cell a-maze))) #true)
(check-expect (member? -1 (neighbours (random-cell a-maze))) #true)
(check-expect (member? -15 (neighbours (random-cell a-maze))) #true)
(check-expect (= (length (neighbours-in-grid (random-cell a-maze))) 2) #true)
;neighbours: non-empty whole number representing a cell -> list of
;neighbouring cells - use horz and vert axis only (no diagonals)
(define (neighbours member-cell)
(list (+ member-cell grid-size) ;cell below
(+ member-cell 1) ;cell immediate right
(- member-cell 1) ;cell immediate left
(- member-cell grid-size) ;cell above
)
)
;neighbours-in-grid: non-empty list of potential neighbours -> narrowed list of validated neighbours that are in the grid
(define (neighbours-in-grid (neighbours (random-cell a-maze)))
(cond [(< 0 (first neighbours) > grid-size) (remove (first neighbours))]
[(< 0 (second neighbours) > grid-size) (remove (second neighbours))]
[(< 0 (third neighbours) > grid-size) (remove (third neighbours))]
[(< 0 (fourth neighbours) > grid-size) (remove (fourth neighbours))]
))

Answer 1

The problem here has to do with the first line of your neighbours-in-grid function.
Specifically, a function is defined using this shape:
(define (<name-of-function> <name-of-argument> ...)
<body-of-function>)
Your function's first line looks like this:
(define (neighbours-in-grid (neighbours (random-cell a-maze)))
(cond ...))
This doesn't fit the pattern. What are the names of the arguments? Based on the code that follows this line, it looks to me like you want a single argument, named neighbours. If so, this code should instead read:
(define (neighbours-in-grid neighbours)
(cond ...))
I think part of the confusion here may stem from the fact that separately, you have a function named neighbours.