RE: dynamic vs. static typing

["Anton van Straaten" <address@hidden>]

Responding belatedly - been busy:

On 30 Nov 2003, at 4:46 PM, Pascal Costanza wrote:
> On 26 Nov 2003, at 20:08, Anton van Straaten wrote:
>
> >> One of the things I don't like about Smalltalk is that there
> >> is quite some information that I know when I'm coding, but
> >> don't (can't) tell the program about.
> >
> > This is one of my big objections to "pure" dynamic typing.  If
> > there's information that I want to be able to express in my program,
> > that I can't reasonably express, that's a limitation that I don't
> > like.
>
> ...but you can always add assertions to express your intentions, can't
> you?

You're right that assertions will express intentions, but there are a number
of problems with them.  First, in some languages - particularly in dynamic
languages - they can't be disabled, so you may take a runtime performance
hit for using them.  This can inhibit their use.  Certainly, DT programs
tend not to make nearly as many assertions about types as they could.

Secondly, in languages where you can disable assertions at runtime, or if
you decide to use a global flag to determine at runtime whether an assertion
should be checked, you run into a potentially worse problem: deciding which
assertions to disable, and which to leave active.  It's quite possible to
have an assertion for some condition, decide to disable it because it
doesn't trigger during testing, and then experience subtle bugs that would
have been detected and reported by an assertion, had it not been turned off.
Leaving all assertions on, all the time, may not be an option, for
performance reasons.

Finally, of course, assertions in a DT language don't give you any choice
about when they can be run.  I'd be quite happy with a DT language which
could analyze at least some of my assertions statically and tell me things
about them.  In fact, that's what I've been advocating.

Anton