[CM] dopairs

[Orm Finnendahl]

Hi Rick,

using your dopairs macro I noticed the following behaviour:

    (dopairs (x y '(1 2 3 4 5))
             (begin
              (display (list x y))
              (newline)))

    guile> (1 2)
    (3 4)
    <unnamed port>: In procedure cadr in expression (cadr #{\ g75}#):
    <unnamed port>: Wrong type argument in position 1: ()
    ABORT: (wrong-type-arg)

    Type "(backtrace)" to get more information or "(debug)" to enter the debugger.

I think, the error of an uneven pair list should get caught and assume
it's due to a typo in the fourth last line of the dopairs definition.

To fix this, change that line:

-   (if (null? ,a)

into

+   (if (null? (cdr ,a))

After replacement:

    (dopairs (x y '(1 2 3 4 5))
             (begin
              (display (list x y))
              (newline)))

    guile> (1 2)
    (3 4)
    <unnamed port>: In procedure error in expression (error str):
    <unnamed port>: Uneven pair list: (1 2 3 4 5)
    ABORT: (misc-error)
    guile> 

--
Orm