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From: diamond@jit533.swstokyo.dec.com (Norman Diamond)
Newsgroups: comp.lang.misc
Subject: Re: Run-time Type Errors in Smalltalk
Message-ID: <1991May16.011804.21042@tkou02.enet.dec.com>
Date: 16 May 91 01:18:04 GMT
References: <1991May9.023313.28308@tkou02.enet.dec.com> <1991May13.061520.11992@daffy.cs.wisc.edu> <1991May13.082152.29294@tkou02.enet.dec.com> <1991May15.141839.20603@daffy.cs.wisc.edu>
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Reply-To: diamond@jit533.enet@tkou02.enet.dec.com (Norman Diamond)
Organization: Digital Equipment Corporation Japan , Tokyo
Lines: 73

In article <1991May15.141839.20603@daffy.cs.wisc.edu> quale@saavik.cs.wisc.edu (Douglas E. Quale) writes:
>In article <1991May13.082152.29294@tkou02.enet.dec.com> diamond@jit533.enet@tkou02.enet.dec.com (Norman Diamond) writes:
>>In article <1991May13.061520.11992@daffy.cs.wisc.edu> quale@khan.cs.wisc.edu (Douglas E. Quale) writes:
>>>  XtVaGetValues(widget, XtNwidth, &width, XtNheight, &height, NULL);
>>>Unfortunately width and height are type XtArgVal which is a union which
>>>completely defeats any typechecking.
>>>This kind of type error CANNOT occur in a dynamically typed language.
>>
>>It could occur in a dynamically typed language, if the programmer can tell
>>the language to throw away tagging and type-checking.  A competent programmer
>>would put a tag in the union type.
>
>In lisp, the most ``common'' dynamically typed language, you cannot throw away
>tagging.

I did say "if".

>But the fact is, this particular error does NOT occur in a dynamically typed
>language.

It could be made to occur.  (Not that I would recommend it, of course.)

>If you or anyone else reading this is familiar with the X Toolkit Intrinsics
>and can see a way to avoid this problem I'd like to hear about it.

As I said, C is not an encouraging language for it, but a competent programmer
could put a tag in.  (C allows a programmer to support dynamic typing, though
C itself does not support it, and in fact cumbersome syntax discourages it.)

typedef enum XtArgTag {
        XtArgCaddr,
        XtArgCharPtr,
        XtArgLong,
        XtArgIntPtr,
        XtArgProcPtr,
} XtArgTag;
typedef union XtArgUnion {
        caddr_t     Caddr;
        char       *CharPtr;
        long        Long;
        int        *IntPtr;
        proc       *ProcPtr;
} XtArgUnion;
typedef struct XtArgVal {
        XtArgTag    Tag;
        XtArgUnion  Val;
} XtArgVal;

(I have tried to maintain the same style as the typename "XtArgVal".
My personal naming style conflicts with a Posix standard.)

>The designers of the Intrinsics found that C's static typing wasn't adequate

Of course.

>and built an unchecked dynamic type system on top of it.

That's their problem.  And then they made life difficult for their clients.

>Now C's type system is very primitive, but I'm not certain
>that any static type system is flexible enough to handle this problem.

Pascal handled this problem -- and the syntax deliberately encouraged this
kind of checking.

>most promising approach might be a statically typed object oriented language.

Of course, object orientation is useful, but object orientation is really
orthogonal (or nearly so) to issues of static vs. dynamic typing.
--
Norman Diamond       diamond@tkov50.enet.dec.com
If this were the company's opinion, I wouldn't be allowed to post it.
Permission is granted to feel this signature, but not to look at it.