Subject: Re: copying a vector? (Stalin 0.9?)
From: (Rob Warnock)
Date: 5 Jan 2001 05:01:49 GMT
Newsgroups: comp.lang.scheme
Message-ID: <933kft$lfq5b$>
Dorai Sitaram <> wrote:
| Alfred J. Reich <> wrote:
| >SICP (2nd Ed.) has a footnote on the origin of the word (p. 401).
| >Basically, it's slang for a past tense form of "thought".  A thought (noun)
| >that has already been thought (verb) is a "thunk".
| SICP seems to have changed its etymology for the word
| _thunk_ when it went into its second edition.
| The first edition advanced an onomatopoeic origin: the
| sound made by an activation record as it landed on the
| control stack.

Well, the ALGOL-10 compiler for the DEC PDP-10 (~1970) used thunks
(their term) for the call-by-name argument accessor code fragments
they passed around, and IIRC (though it's been ~30yr, so I could
have it slightly wrong), those thunks weren't "on the stack", nor
were they "activation records" -- they were fixed code fragments
whose addresses were passed as arguments.

That is, for each argument (all of which were call-by-name, of course,
this being Algol), a subroutine was passed the addresses of two thunks,
one for fetching that argument and one for storing it back again. [O.k.,
so by Scheme's modern definition of "thunk" == "arity-0 proc" the
second one was actually a 1-arg closure.]  A subroutine call like
"foo(a, (a**2)*(b+c))" actually generated code that in Scheme might
look somewhat like this:

	(foo (vector (lambda () a)
		     (lambda (x) (set! a x)))
	     (vector (lambda () (* (expt a 2) (+ b c)))
	             (lambda (x) (error "Can't mutate argument expression!"))))

allowing weird call-by-name stuff like Jensen's Device to work properly.

So "thunk" as "pre-thought" is more consistent with the ALGOL-10 usage.


p.s. Yes, I know that one way to do the above would be for the caller to
JIT-compile the thunks onto the stack at call time, but IIRC that's not
what the ALGOL-10 compiler did. The thunk code was read-only (rather,
reentrant, since on the KA-10 it did live in the low seg), and it walked
back up the display link to get at whatever lexical variables it needed.

p.p.s. The only reason I remember as much of the above as I do is that I
happened to have spent some time reverse-engineering the interface between
the compiled code and the run-time system -- using only the executables
of the compiler and the RTS (and the binary output from test compiles) --
because we were missing the library that connected the two!! [And couldn't
ask DEC for it, but that's another story...]

Rob Warnock, 31-2-510
SGI Network Engineering
1600 Amphitheatre Pkwy.		Phone: 650-933-1673
Mountain View, CA  94043	PP-ASEL-IA