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From: aipdc@castle.ed.ac.uk (Paul Crowley)
Newsgroups: comp.lang.misc
Subject: Re: Dynamic typing (part 3)
Message-ID: <9106@castle.ed.ac.uk>
Date: 12 Mar 91 23:50:48 GMT
References: <602@optima.cs.arizona.edu>
Organization: Edinburgh University
Lines: 33



I'm going to split languages in two by type extensibility.  In C and ML,
you can explicitly make up a new type from old types: a widget is made
up of two foos and a bar.  In Logo, there are only three types: words,
numbers, and lists.  If you want to define an imaginary number, you
could use a list of two integers.  If you want to define a UNIX-style
time as a list of two integers [secs, usecs], you can do that too.  If
you accidentally feed an imaginary number to a function that wants a
date, the language won't say a word.  Prolog behaves this way too. 

What are the words for these two? (I know that if you really wanted to,
you could preface every imaginary list with a typename, and write
functions that checked that the elements of these lists were the types
they were supposed to be.  This means that the types of all the elements
of a large structure are checked often.  Doubleplusungood.)

Are these two strong and weak typing?

Also, some languages do type-checking at compile-time, and some at
run-time.  Some (ML and others) typecheck at compile-time _but_ it does
all the work itself.  Run-time typecheckers maintain the type of a value
with the value itself, and on each operation checks that the operation
is an appropriate thing to do.

Are these two static and dynamic typechecking?

Thanks, and sorry for asking here but many people have said "X is the
wrong word for Y" that I felt that the only people who could tell me
what the words mean in the debate were the participants.
                                         ____
\/ o\ Paul Crowley aipdc@uk.ac.ed.castle \  /
/\__/ Part straight. Part gay. All queer. \/