Path: utzoo!utgpu!watserv1!watmath!att!att!pacbell.com!ucsd!ucbvax!UM.CC.UMICH.EDU!Paul_Abrahams%Wayne-MTS
From: Paul_Abrahams%Wayne-MTS@UM.CC.UMICH.EDU
Newsgroups: comp.lang.icon
Subject: Dynamic typing revisited
Message-ID: <270830@Wayne-MTS>
Date: 17 Nov 90 20:45:32 GMT
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Thanks to all for your replies to my query on this subject.
 
Re Ralph's comment: yes, I was being sloppy in saying that the type of a
variable can change.  Viewing variables as untyped containers of arbitrary
values is both cleaner and and the correct depiction of reality.
 
Yes, you can use Icon's flexible typing structure to construct heterogeneous
lists.  But what isn't clear to me is what such lists are good for, particu-
larly when they contain values of truly arbitrary types rather than of
related types.
 
In SPLASH I have a `box' type that can designate variables of any type.  That
provides an escape from strong typing---the second approach that Dave Gudeman
suggested.  I'm still wrestling with the semantics of boxes, though.  What
prompted my query was wondering what operations you might want on a box in
addition to the image operation (in the sense of the Icon `image' procedure). 
In particular, the image operation would suffice for the trapping that Gregg
gave as an example.
 
You might think that an obvious operation on boxes would be to get a string
giving the type of the box.  But that turns out to be remarkably difficult in
the presence of recursive types.  That problem doesn't come up in Icon
because Icon doesn't have recursive types---with dynamic typing, you don't
need them.
 
To clarify: in Pascalish languages you can have something like:
	type t is record v: integer; link: ref t end
How then would you display the definition of t in a form that id independent
of the name you give the type?  In Icon the type would simply be `t' (which
actually isn't very revealing if you don't have the definition of `t' handy). 	
 
By the way, Modula-3 has a `refany' type which provides an escape from strong
typing in that language.  But you can only dereference a `refany' value using
a `typecase' construction, which forces you to enumerate all the
possibilities.  That's less general than being able to apply `image' without
knowing the type.
 
Paul
 
abrahams%wayne-mts@um.cc.umich.edu