🚀 go-pugleaf

In <1991Mar22.210725.29448@neon.Stanford.EDU>
hoelzle@neon.Stanford.EDU (Urs Hoelzle) writes:

>But you aren't convinced and I've heard this argument often enough,
>so let me give an example.  Please show me how you would use the many
>features of Ada to write this in Ada (or any other statically-typed
>language):

>[example of heterogeneous list of "unrelated" objects deleted]

>To avoid misunderstandings, please note that the above example *is*
>type-safe in an abstract sense: no 'message not understood' error can
>occur because all elements of listC understand both 'foo' (which may
>be sent by someProc) and 'display'.  However, I claim that this piece
>of code won't type-check in Ada, C++, Eiffel, ...

Sooner or later, opponents of static typing bring out the
heterogeneous collection.  There's no question that it isn't possible
with most traditional language type-systems to express the type of the
list in Urs's example.  Type systems that can express union types
(such as, "an A or a B") or purely extensional types (such as,
"anything that understands 'foo'") [is that the right name, type
theory mavens?]  are not common (but not unknown, either---I think
Ralph Johnson's Typed Smalltalk can do this).

The problem is, no one has ever come up with a convincing reason why I
should want my type system to handle the heterogeneous collection.
This is because no one has come up with a convincing reason why I
should want to write programs that contain heterogeneous collections.
Needing collections of otherwise-unrelated objects generally signals a
flaw in the design, not a failing of the type system.

joe