Re: Why would anyone want opacity?

[Jeff Dalton <jeff@aiai.ed.ac.uk>]

> I simply don't think there are any abilities that are removed.  It is
> trivial to transliterate *any* Scheme program into an ML program. 

It would be at least a bit harder, or a bit less direct, if Scheme had
the ability to define new, disjoint, record tpes.  ^_^

> No, this is not the correct metaphor.  Lexical and syntax analysis
> (which amount to spell-checking) are there even in Scheme.  What Gerry
> objects to is a set of rules that let you decide statically (by just
> looking at the sentence), whether it makes sense or not.  He prefers a
> system where we find out by `running' the sentence.  He would find out
> that `At night it is cooler than outside.' doesn't make much sense
> only when his audience starts making funny faces.

Actually, "At night it is cooler than outside" could well make
sense, in the right context, which I will leave as an exercise
for the reader.  (After all, you can't expect to fully understand
such a sentence in isolation, becuase you need some idea of what
"it" refers to.)

BTW, I think it's true that ML-style (and other!) type systems
can be valuable and effective aids to thinking.  But such type
systems tend to rule out some perfectly valid Lisp-style programs
and hence do "take away" something.  (I have to say something
like "Lisp-style" because Scheme has only a small number of
types, all built-in, and so can easily be "implemented" in ML.)

-- jd