Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!sunic!news.funet.fi!tukki.jyu.fi!sakkinen
From: sakkinen@jyu.fi (Markku Sakkinen)
Newsgroups: comp.object
Subject: Re: A Hard Problem for Static Type Systems
Message-ID: <1991May9.052645.29470@jyu.fi>
Date: 9 May 91 05:26:45 GMT
References: <2479@m1.cs.man.ac.uk> <1158@tetrauk.UUCP> <2500@m1.cs.man.ac.uk>
Reply-To: sakkinen@jytko.jyu.fi (Markku Sakkinen)
Organization: University of Jyvaskyla, Finland
Lines: 22

In article <2500@m1.cs.man.ac.uk> mario@cs.man.ac.uk (Mario Wolczko) writes:
> ...
>I will make only two further observations:
> ...
>2. The most common "type error" in Smalltalk is sending a message to
>   nil, ie forgetting to initialize a variable.  Static typing doesn't
>   help here at all.

Right, with type systems defined as they usually are, trying to access
'nil' cannot be caught as a type error even in statically typed languages.
The story becomes different if pointer types as such are defined as "strict",
i.e. 'nil' not allowed, and conventional (possibly 'nil') pointers are
then regarded as union types.  Of course, one could not very well build
typical dynamic, recursive data structures using only strict pointers.

Markku Sakkinen
Department of Computer Science and Information Systems
University of Jyvaskyla (a's with umlauts)
PL 35
SF-40351 Jyvaskyla (umlauts again)
Finland
          SAKKINEN@FINJYU.bitnet (alternative network address)