CODEWITHSUNDEEP
listschemelet
Daniel H
Daniel H

Reputation: 31

Append new item to end of list in scheme

I need to append a new item to the end of a list. Here is what I tried:

(define my-list (list '1 2 3 4))
(let ((new-list (append my-list (list 5)))))
new-list

I expect to see:

(1 2 3 4 5)

But I receive:

let: bad syntax (missing binding pair5s or body) in (let ((new-list (append my-list (list 5)))))

Upvotes: 3

Views: 1296

Answers (2)

Sylwester
Sylwester

Reputation: 48745

A let makes a local variable that exists for the duration of the let form. Thus

(let ((new-list (append my-list '(5))))
  (display new-list) ; prints the new list
  ) ; ends the let, new-list no longer exist

new-list ; error since new-list is not bound

Your error message tells you that you have no expression in the let body which is a requirement. Since I added one in my code it's allowed. However now you get an error when you try evaluating new-list since it doesn't exist outside the let form.

Here is the Algol equivalent (specifically in JS)

const myList = [1,2,3,4];
{ // this makes a new scope
   const newList = myList.concat([5]);
   // at the end of the scope newList stops existing
}
newList;
// throws RefereceError since newList doesn't exist outside the block it was created.

Since you already tried using define you can do that instead and it will create a variable in the global or function scope:

(define my-list '(1 2 3 4))
(define new-list (append my-list (list 5)))
new-list ; ==> (1 2 3 4 5)

Upvotes: 0

NlightNFotis
NlightNFotis

Reputation: 9805

Your problem is mostly of syntactical nature. The let expression in Scheme has the form (let (binding pairs) body). In your example code, while you do have a binding that should work, you don't have a body. For it to work, you need to change it to 

(let ((new-list (append my-list (list 5))))
    new-list)

In my DrRacket 6.3 this evaluates to what you would expect it to: '(1 2 3 4 5).

Upvotes: 1

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