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From: gudeman@cs.arizona.edu (David Gudeman)
Newsgroups: comp.lang.misc
Subject: definitions
Message-ID: <2494@optima.cs.arizona.edu>
Date: 26 Apr 91 21:37:28 GMT
Article-I.D.: optima.2494
Sender: news@cs.arizona.edu
Lines: 68

There seems to be a certain entropy operating in comp.lang.misc: first
a lot of the hot air gets generated over misunderstandings because
people are using different definitions for terms.  Then some
definitions get posted and everyone uses the same terminology for a
month or so.  Then new posters start appearing who haven't seen the
original defs and recreate the misunderstandings.  This seems to have
happened in the typing debates, so I'm going to post definitions
again.

I've added definitions for binding as well as typing since there seems
to have been some confusion over that as well.  I for one didn't even
know that binding was part of the debate :-).  Each definition is
given in a formal version, followed by an implementation description
for those who don't like formalism.  It should be remembered though
that the formalism is the definition and the implementation
description is only one way (the most common way) to implement the
formalism.

I've decided to be a little more explicit about the fact that typing
refers to machine representations, since that seems to be the major
difference between the two extreme views.  In other words, if typing
only refers to an arbitrary set of values, then most of the
differences of opinion disappear.

Typing: the machine representation of a value.  You must know the
typing of a memory location to know how to interpret the bits as a
value.

Static typing: the assignment of a typing to all the values that a
given syntactic expression might produce.  This is generally
implemented by having the programmer declare the typings of the
variables, and then inferring the typing of compound expressions by the
types of the operations.

Dynamic typing: a programming language feature where the typing of a
value can be determined from the value alone, without reference to the
way in which the value is used.  This is generally implemented by
adding a type tag to all values at runtime.  Note that since the sizes
of values are not known statically, some form of automatic storage
management is needed to implement dynamic typing.

Strong typing: the feature that an operation cannot return an
undefined result because it did not know the typing of one of its
operands.  Strong static typing is implemented by only allowing
operators to be applied to operands whose typing can be statically
inferred to be correct for that operator.  Strong dynamic typing is
implemented by having each operator dynamically check the type tags of
its operands to make sure they are correct.

Weak typing: the lack of strong typing.  Generally, this means that
when you call an operation on a value with an unrecognized typing, the
operator will return some strange result or core dump.

Static binding: the operation represented by a given name or symbol is
determined by the program text alone.  The simplest way to implement
this is to make sure each token represents a unique operation.  Some
languages allow a token to represent different operations as long as
the correct operation can be uniquely determined by considering the
static typing.

Dynamic binding: the operation represented by a given expression
cannot be determined by text alone -- it depends on the execution of
the program.  Usually the dynamic choice of operation is made based on
dynamic typing tags.
--
					David Gudeman
gudeman@cs.arizona.edu
noao!arizona!gudeman