在 Racket 中使用 λ 定义函数

Question

Following function works:

(define (testfn)
  (define (contains sl item)    (ormap (λ(x)(equal? item x)) sl))
  (if (contains (list 1 2 3) 2)    "yes" "no"))

(testfn)

Output:

"yes"

But following, which uses λ symbol, does not:

(define (testfn2)
  (λ (contains sl item)    (ormap (λ(x)(equal? item x)) sl))
  (if (contains (list 1 2 3) 2)    "yes" "no"))

Error is:

contains: unbound identifier in module in: contains

Can λ symbol be used to define inner (or general) functions which may be called at multiple places?

Answer 1

Yes, but you need to define it like you would with any other identifier.

(define (testfn2)
  (define contains (λ (sl item) (ormap (λ(x)(equal? item x)) sl)))
  (if (contains (list 1 2 3) 2)    "yes" "no"))

Your code creates a function but (1) it is not bound to anything and (2) it actually takes three arguments, the first of which is "contains".