Re: conditionaly setting characteristics

[Richard Light <richard@xxxxxxxxxxxxxxxxx>]

In message <34EF2511.526C39F8@xxxxxxxxxx>, Charlie & Barbara Bozeman
<cbozeman@xxxxxxxxxx> writes
>Is there a method to construct flow objects and conditionally set their
>characteristics depending on the value or presence of attributes? An
>example of what I want to do is:
>
>(element cell
>  (let ((col-num attribute-string "col-num")
>        (align attribute-string "align"))
>     (make table-cell
>           (if col-num
>               column-number:  col-num)
>           (if align
>               cell-row-alignment: align))))

In principle there is no problem doing this.  You need to 'bracket'
attribute-string, since it is a procedure, and you need to take care
with the object it returns, which is #f if it fails to produce an
attribute value for any reason, and a string representation of that
value if it succeeds.  

This can give rise to some strange logic - I don't think that you can
test for (attribute-string) returning #t as you do, since it only
returns #f or a string.  You will see a similar case in the (mapcolnum)
procedure below, where I have tried to cope with the similar behaviour
of (string->number) by testing for the #f case:

    (if (not (string->number colnum))
        ... #f returned, so give default value
        ... else number returned, so use it

Finally, since attribute-string returns a string literal, you need to
map this to a symbol or integer so it can be a valid characteristic
value.  Write sub-procedures to check 'col-num' and 'align' and return a
suitable characteristic value:

(element cell
  (make table-cell
        column-number: (mapcolnum (attribute-string "col-num"))
        cell-row-alignment: (mapalign (attribute-string "align"))))

(define (mapcolnum colnum)
  (if (string? colnum)
    (if (not (string->number colnum))
        1
        (string-number colnum))
    1))

(define (mapalign align)
  (if (string? align)
    (case (STRING-DOWNCASE align)
          (("start") 'start)
          (("end") 'end)
          (("center") 'center)
          (("centre") 'center)
          (else 'start))
    'start))

[Note that I have chosen not to use (string->symbol) to convert the
attribute (string) values to symbols.  Providing an explicit set of
options in a case statement lets you guard against invalid settings in
the instance which could give DSSSL processing errors.  It also lets you
deal with reasonable variations, e.g. 'centre'.]

Richard Light.

Richard Light
SGML/XML and Museum Information Consultancy
richard@xxxxxxxxxxxxxxxxx


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