CODEWITHSUNDEEP
scheme
cpd1
cpd1

Reputation: 789

Null use in a small Scheme script

I'm just learning how to write in Scheme and hitting a road block with the use of Null. I have the script below from this video series...

https://www.youtube.com/watch?v=Qqext1NwmqM&index=2&list=PLgyU3jNA6VjRMB-LXXR9ZWcU3-GCzJPm0

Script...

(define (my-map fn lst)
   (if (null? lst)
    null
   (cons (fn (car lst)) (my-map fn (cdr lst)))))

This is just redefining the map function in Scheme. It fails at the third line as "null: undefined. cannot reference undefined identifier" when I pass a function and a list.

I just copied what the video shows so not sure why it's failing. If I switch out null for '(), that works. Anyone know why?

Upvotes: 0

Views: 43

Answers (1)

Óscar López
Óscar López

Reputation: 236124

It's quite possible that your Scheme interpreter doesn't have the null symbol bound to the '() value, as this is not required by the language specification. Either do this:

(define null '())

Or equivalently this, as you already guessed:

(define (my-map fn lst)
  (if (null? lst)
      '()
      (cons (fn (car lst))
            (my-map fn (cdr lst)))))

Notice that the latest Scheme report states that (null? obj) should return #t

if obj is the empty list, otherwise returns #f

So it's ok to define null as '().

Upvotes: 1

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