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in scheme (I'm using racket R5RS) when I call this procedure 

(map display '(1 2 3 4 5))

it returns

12345(#<void> #<void> #<void> #<void> #<void>)

Why is that? What's up with the void?

Arash Saidi
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3 Answers3

3

You said:

"it returns 12345(#<void> #<void> #<void> #<void> #<void>)"

which isn't precisely correct. That is what it prints/displays; not what it returns. You get the return value with:

> (define test (map display '(1 2 3 4 5)))
12345> test
(#<void> #<void> #<void> #<void> #<void>)
>

Here it is clearer: '12345' was printed; the return value, bound to test, is your 'void' list.

The reason for the 'void' values is that map applies its function to each element in the list and constructs a new list with the value returned by function. For display the return value is 'void'. As such:

> (define test2 (display 1))
1> (list test2)
(#<void>)
>
GoZoner
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1

You need to use for-each instead of map, in your case. for-each is non-collecting, unlike map.

Also, for-each is guaranteed to pass the list items from left to right to your function. map makes no such guarantee, and is allowed to pass the list items in any order. (Although, in the specific case of Racket, map does use a left-to-right order. But you cannot depend on this in other Scheme implementations; some use a right-to-left order, and theoretically, other orderings are possible.)

C. K. Young
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0

map collects the results of each invocation of the function on each element of the list and returns the list of these results.

display returns an unspecified value. #<void> is just something that Racket is using. It could have returned 42 as well.

You probably have a typo. It should have returned

12345(#<void> #<void> #<void> #<void> #<void>)

without the leading open parenthesis. I.e. the five values are isplayed, and then the return value.

Will Ness
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  • But map sends each element of a list to display, which then should not return anything. How would I get display to return void in racket myself? And you are right, the leading open parenthesis is a typo, will fix it. Thanks! – Arash Saidi May 30 '13 at 12:51
  • @HatoriSanso you can read the specs at the link that I provided. It says "returns unspecified value". I.e. `display` indeed returns some value - to the function that called it. That at the same time some output also appeared at some console window, is just another part of its operation. You see it because you call it at the REPL, which also prints the last returned value. – Will Ness May 30 '13 at 14:12