Re: scripting/extension language for Ada (was : Re: tagged types extensions)

[Brian Rogoff <bpr@shell5.ba.best.com>]

On Sat, 5 Feb 2000 blaak@infomatch.com wrote:
> Robert A Duff <bobduff@world.std.com> wrote:
> > Ray Blaak <blaak@infomatch.com> writes:
> > > Scripting languages, on the other hand, tend to be used for
> > > small, quickly written programs, and good ones tend to have a
> > > succinct powerful notation.
> >
> > I've seen way too much long-lived software written in Perl,
> > csh, awk, make, etc.

Me too.

> There is always room for a better language. Is Ada right for a
> scripting language, though?

IMO, no. That's why I suggested embedding a language in Ada, with that 
languages implementation being startlingly clear Ada code.

> > Anyway, "quickly written" is irrelevant.  The only time it can
> > possibly make sense to use languages that are difficult to
> > maintain is when you're going to quickly throw away the code
> > after writing it.

I've never observed the phenomena described, IME code is never quickly
thrown away.

> Nobody (at least not me) is advocating the use of scripting
> languages that are difficult to maintain.
> 
> The usefulness of a good scripting language is that it is well
> designed for invoking OS services (expressing the piping of a
> chain of processes, for example), interactive and easy to debug,
> and tends to have small programs.  E.g. a few K at most for a
> reasonably complex script -- that's it! No build steps, object
> files, link times. Just run it.

All languages are pushed outside their original design limitations, so 
a scripting language had better be capable of growing beyond "a few KLOC".

> One tends to use scripting languages to implement small little
> tasks (munge a few files, spit out some email, walk a few
> websites,...)
> 
> Scripting languages should encourage programs that are easy to
> write, easy to understand, and easy to change.
> 
> Most scripting languages are atrocious (esp. sh and
> friends). They don't have to be.
> 
> What I like about scsh is that one has a real programming
> language (Scheme) focused on tasks such that the limitations of
> Scheme don't apply (i.e. the lack of a module or package system
> for programming "in the large").

Scsh uses Scheme48, which has a fascinating and powerful module system
IMO.

> Now Ada is a fine language, and my language of choice for real
> software systems. The problem with Ada for scripting purposes,
> however, is that there is too much declarative infrastructure
> needed to do simple tasks. 

OK so far...

> Ada's strong static typing requires ones abstractions to be carefully
> specified. 

Wrong. *Explicit* static typing, *explicit* instantiation, and a
heavyweight (some say more readable :-) syntax are what make Ada painful 
as a scripting language. See ML, Haskell, or the DrScheme environment for 
counterexamples. 

> Dynamic typing in a scripting language, on the other hand, allows one
> to just "whip off" a solution, and yet still have strong type checking
> for good runtime error detection.

There are good arguments for dynamic typing, but this isn't one of them.

> Consider this scsh snippet:
> 
>   ;; Mail myself files matching some criteria
>   (with-cwd "~/myfiles"
>     (for-each (lambda (file)
>                 (if (run (grep "some pattern" ,file))
>                     (run (mail -s "found it" blaak@home) (< ,file))))
>               (directory-files)))

ML or Haskell would require a few more lines to make the types right, like 
checking that the result of your grep isn' a null string.

-- Brian