map, for-each

map, for-each

[Eric J. Van der Velden]

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Hello,

I don't understand what the manual says about when there are more then two
arguments to map or for-each.

With two arguments, the last one must be a list, so OK is

(for-each display '(1 3))

But the following are ERR,

(for-each display 1 3)

(for-each display '(1 3) '(1 3))

Thanks,

Eric J.

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Re: map, for-each

[Paul Emsley]

On 26/08/10 12:17, Eric J. Van der Velden wrote:
> Hello,
>
> I don't understand what the manual says about when there are more then 
> two arguments to map or for-each.
>
> With two arguments, the last one must be a list, so OK is
>
> (for-each display '(1 3))
>
> But the following are ERR,
>
> (for-each display 1 3)
>
> (for-each display '(1 3) '(1 3))

The function, in this case `display', needs to be able to take 2 args, 
for display it's the variable and the port:

(for-each display '(1 3) (list (current-output-port) (current-output-port)))

also consider

(map + '(1 3) '(1 3))

Paul.

Re: map, for-each

[Thien-Thi Nguyen]

() "Eric J. Van der Velden" <ericjvandervelden@gmail.com>
() Thu, 26 Aug 2010 13:17:38 +0200

   But the following are ERR,

   (for-each display 1 3)                     ; A

   (for-each display '(1 3) '(1 3))           ; B

The arity of the proc (specifically, the "required" portion)
must concord with the number of lists passed as arg2... to
‘for-each’.  In this case, the arity of ‘display’ is 1.

In A, there are two errors:
  - arg2... are not lists
  - (count arg2...) => 2, which is not 1

In B, there is only one error:
  - (count arg2...) => 2, which is not 1

If you:

(define (display2 a b)
  (display a) (display #\space) (display b) (newline))

then substituting it for ‘display’ in A produces only
one error, and in B none.

Actually, the above is a simplification: ‘display’ accepts
an optional second arg, a port to send its output to, so one
could say that B has an additional error: arg3 is not a
list of output ports (this is a type error in some schools).
For example, this does not produce an error:

 (define CO (current-output-port))
 (for-each display '(1 3) (list CO CO))         ; C

For C, (count arg2...) => 2, which is not 1, but that's ok.

Re: map, for-each

[Andy Wingo]

On Thu 26 Aug 2010 04:17, "Eric J. Van der Velden" <ericjvandervelden@gmail.com> writes:

> I don't understand what the manual says about when there are more then two arguments to map or for-each.
>
> With two arguments, the last one must be a list, so OK is
>
> (for-each display '(1 3))

Consider:

  (map + '(1 3))
    = (list (+ 1) (+ 3))
    = (list 1 3)
    => (1 3)

  (map + '(1 3) '(5 7))
    = (list (+ 1 5) (+ 3 7))
    = (list 6 10)
    => (6 10)

After the function, all args need to be lists. The `display' example you
gave, even if you had given for-each lists, still doesn't make sense:

  (for-each display '(1 3) '(5 7))
    = (begin (display 1 5) (display 3 7))
    => ERROR: in (display 1 5): 5 is not a port

Hope that helps,

Andy
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