Why would anyone want opacity?

[Gerald Jay Sussman <gjs@martigny.ai.mit.edu>]

     > 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.

I don't object to any such rules.  You may have any such rules you
please, for your own code.  I am pleased to admit features into the
language that allow us to express type ideas and to make tools to
check types.  If you provide a convenient means of declaring and tools
for checking types that I may use on my own code I will probably use
it, because helpful tools are always welcome, and I have often desired
the ability to declare and check types.

However, what I object to is the ENFORCEMENT of your rules as part of
the language.  I cannot and I will not use a language that REQUIRES me
to make any type system happy.  If you wish to make such a language,
you are free to do so, but don't inflict it on me.