``` Subject: Re: eta conversion From: rpw3@rigden.engr.sgi.com (Rob Warnock) Date: 2000/05/29 Newsgroups: comp.lang.lisp Message-ID: <8gsmve\$ga0ke\$1@fido.engr.sgi.com> ```
```Joe Marshall  <jmarshall@alum.mit.edu> wrote:
+---------------
| Fernando <spamers@must.die> writes:
| > What is this eta conversion thing?
|
| In lambda calculus, the expression (lambda (x) (F x)) is equivalent to F
| That's eta conversion.
+---------------

Provided that "x" is not a free variable in "F", that is.

+---------------
| Unfortunately, eta conversion doesn't work in
| a typed lambda calculus (in general, anyway).
+---------------

Yup. The simplest example[*] I know of is "(lambda (x) (3 x))" versus "3".
The predicate "integer?" (INTEGERP in CL) will give different answers when
applied to those two [and so will "procedure?" (FUNCTIONP)].

-Rob

[*] Sabry & Felleisen, "Reasoning about Programs in Continuation-Passing
Style" [1992 Conf. on Lisp and Functional Programming], page 3.
<URL:http://www.cs.rice.edu/CS/PLT/Publications/lfp92-sf.ps.gz>

-----
Rob Warnock, 41L-955		rpw3@sgi.com
Applied Networking		http://reality.sgi.com/rpw3/
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```