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From: lgm@cbnewsc.ATT.COM (lawrence.g.mayka)
Newsgroups: comp.lang.misc,comp.software-eng
Subject: Re: An Interesting View of "Strong" Vs. "Weak" Typing
Keywords: typing, Ada, Lisp, definitions, evidence
Message-ID: <12608@cbnewsc.ATT.COM>
Date: 5 Jan 90 23:25:33 GMT
References: <1990Jan5.084746.17836@cs.eur.nl>
Reply-To: lgm@cbnewsc.ATT.COM (lawrence.g.mayka,ihp,)
Organization: AT&T Bell Laboratories
Lines: 43

In article <1990Jan5.084746.17836@cs.eur.nl> reino@cs.eur.nl (Reino de Boer) writes:
>compile-time evaluation of possible types. Although something can be
>said in favor of the statement
>	all interpreters (CL, Smalltalk, etc.) by definition do only
>	run-time checking (not being able to compile at all).

Common Lisp programs are typically compiled into native machine
instructions.  For example, one typical implementation on the
SPARC architecture compiles the following function

        (defun add (x y)
          (+ x y)
          )

into exactly 11 machine instructions.  Such a function properly
computes the sum of any two numbers - integers of any size,
floating-point, rational, or complex.  (Only 5 instructions are
executed if each argument happens to be an integer that fits into
a single machine word.)  If one annotates this definition with
optional type declarations thus:

        (defun add (x y)
          (declare (fixnum x y))
          (the fixnum (+ x y))
          )

the compiler emits exactly 4 machine instructions.  If this latter
function is passed arguments of unexpected types - e.g., rational
numbers 4/5 and 7/8 - the run-time system traps the attempt and
asks the user if it should proceed with the addition anyway.  This
exception handling behavior can of course be overridden by the
programmer.)

An interpreter is usually also available but most CL programmers
use it primarily to execute top-level environment commands such as
ED, COMPILE-FILE, LOAD, TRACE, DESCRIBE, INSPECT, and DISASSEMBLE.


	Lawrence G. Mayka
	AT&T Bell Laboratories
	lgm@ihlpf.att.com

Standard disclaimer.