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Re: Lightness vs. Largeness

   Date: Wed, 05 Dec 2001 13:08:12 -0500
   From: Scott McKay <swm@hotdispatch.com>

(Not to dispute the greater point from which I wrested the following

   I've written 100,0000-line Lisp applications that don't have any
   type declarations.

Me too, but my current opinion is that the next time I write a
100,000-line application, I'm going to use type declarations
everywhere I can, if the langauge makes them available (whether
mandatory or optional).  I've really changed my mind on this over the
years.  Partly it's because type declarations sometimes catch
programming mistakes statically, and partly it's because they make
programs easier to read.

How's that for a provocative statement?  :-) Yes, easier to read.
It's easier to understand a program if you can see, right on its face,
important invariants.  Assertions are helpful, and type declarations
are an important special case of assertions.

In the code I work on, another useful assertion at a many points is
"There must (or must not) be a transaction in progress for the current
thread at this point"; this doesn't provide static checking, only
dynamic checking, but it makes the program easier to read in a similar

Right now, when I am reading through a large Java program that I have
never read before, one of the first things I do is scan through the
code in order to figure out, for each "Map" and "List" and "Set" field
( instance variable), what the actual types are (the types of the
elements of the sets, or the keys and values of the map, etc), so that
I can draw a UML-like picture of the data structures.  I'm looking
forward to the new Java in which these classes will be parameterized
so that I won't have to go through this step.  (Maybe someday I'll get
for Xmas a tool that creates the UML diagrams...)

Sure, it's good for the type declarations to be optional, particularly
when you're writing a very small program.  If I were in a team writing
a large program in a declarations-optional language, though, I'd think
carefully about team coding standards regarding where it's OK to omit
type declarations.

(In another discussion, I learned an interesting thing about the Curl
language.  In Java, there is one type for every class, but in Curl
there are two: a type that allows any object of that class or null,
and a type that allows any object of that class but not null.  That
sounds useful.  Of course there are important tradeoffs about how
expressive you make your type system.)

Do you think it's true that, for any language, if you want type
declarations to be optional, there must be some type that is the union
of all types?  (There is no such type in Java because of the
"primitive" types; otherwise you could imagine omitted type
declarations "defaulting" to "Object", although perhaps the grammar
would need some adjusting...)