Question

In Haskell, inside a list comprehension, i can bind expressions to a variable every iteration:

[a | x <- xs, let a = x ^ 2, a >= 25]

How do i bind lexical variables in Racket’s for comprehension?

Currently i have this code:

(define (euler4)
  (apply max
         (for*/list ([i (in-range 100 1000)]
                     [j (in-range i 1000)]
                     #:when (string=? (number->string (* i j))
                                      (string-reverse (number->string (* i j)))))
           (* i j))))

I want to bound (* i j) to a variable and replace the expression to it everywhere in function.

Answer 1

Use the in-value form to have a loop variable that is bound to a single value.

In your example:

(define (euler4)
  (apply max
         (for*/list ([i (in-range 100 1000)]
                     [j (in-range i 1000)]
                     [ij (in-value (* i j))]
                     #:when (string=? (number->string ij)
                                      (string-reverse (number->string ij))))
           (* i j))))