string conversion, format functions

[Jay Nelson]

Chris wrote:

 > It's kind of strange, when you think about it, have there
 > even been any significant advances in "output formatting
 > technology" since, what, FORTRAN? None that have
 > caught peoples' imagination enough to be adopted,

Hate to bring it up again, but I do remember coding in LISP
and using the "format language" as we called in.  You could
write a format string that included looping and conditional
branching constructs in the *execution* of the format string.
So you could pass a list of arguments or functions, and then
there were characters like colon or ~ or something that
delimited a loop and caused iteration.  You could skip over
arguments or back up and use them again.  And who hasn't
used the automatic pluralizer!!?

<aside>
I just had to look it up in CLTL2*, so here are a few examples
(the format options take up 30 pages of text to describe):


Simple conditional branching with pluralization (0, 1, n cases):

(format nil "~R dog~:*~[s are~; is~:;s are~] here." n)
    =>  "three dogs are here."

(format nil "Here ~[are~;is~:;are~] ~:*~R pupp~:@P." n)
    =>  "Here are three puppies."
    =>  "Here is one puppy."

Makes your output look soo much more intelligent...


~R with no parameters = arg as a cardinal English number: four
~:R prints arg as an ordinal English number: fourth
~@R prints arg as a Roman numeral: IV
~:@R prints arg as an old Roman numeral: IIII

Standard English nomenclature for fairly large integers (up to
10 ^6 at least) based on appending the suffix -illion to Latin names
of integers.

This worked for bignums and an example is given of the text
for 2 ^200 which encompasses a paragraph of 10 lines!

There is also a MacLisp format string that is given as an example
of the hairiest one seen in real code.  It requires 3/4 page of text
to explain each of the elements of the format string.  A simpler case
is the following:

"~%;; ~{~<~%;; ~1:; ~S~>~^,~}.~%"

which naturally "can be used to print a list of items separated by
commas without breaking items over line boundaries, beginning
each line with ;; .  The prefix parameter 1 in ~1:; accounts for the
width of the comma that will follow the justified item if it is not the
last element in the list, or the period if it is.  If ~:; has a second
prefix parameter, then it is used as the width of the line, thus
overriding the natural line width of the output stream."

Hmm, I forgot the tilde (too much C / Perl).  Wonder if that's
where it came from for erlang?
</aside>

Of course, the standard example was the format
string that evaluated an s-expression and if it had an error
would print out the statement with an arrow pointing at the
point of syntax failure.  Now that was a tricky format string!
Was just about a whole REPL in a single format.

Not that this was highly recommended, but sometimes it
was just the most concise way to do something complicated.

I guess ~i is so that you can construct
an io:format string on the fly and combine it with a list of
stuff that was generated elsewhere to get a nice little dynamic
formatter.

 > anyway - Perl's 'reports' are pretty nice, but I don't
 > think I've ever seen a Perl program that actually
 > *uses* them...

I've only written packed tags to files to communicate with
other programs, or HTML which isn't positional so I
haven't done the pretty formatting, except wait... no, even
my email formatter didn't need to do anything fancy.

Can't imagine too many inventory systems written in
Perl, but why not, eh?

jay



*Common LISP The Language, Second Edition, Guy L. Steele Jr.
(also called "The Bible" or just "Steele")
-- of course