... Mass.¹

Putting this paper in L^ATEXpartly supported by ARPA (ONR) grant N00014-94-1-0775 to Stanford University where John McCarthy has been since 1962. Copied with minor notational changes from CACM, April 1960. If you want the exact typography, look there. Current address, John McCarthy, Computer Science Department, Stanford, CA 94305, (email: jmc@cs.stanford.edu), (URL: http://www-formal.stanford.edu/jmc/ )

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... new²

reference Kleene

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....'' ³

I sent a proposal for conditional expressions to a CACM forum on what should be included in Algol 60. Because the item was short, the editor demoted it to a letter to the editor, for which CACM subsequently apologized. The notation given here was rejected for Algol 60, because it had been decided that no new mathematical notation should be allowed in Algol 60, and everything new had to be English. The ${\bf if} \ldots {\bf then} \ldots {\bf else}$ that Algol 60 adopted was suggested by John Backus.

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... blanks.⁴

1995 remark: Imbedded blanks could be allowed within symbols, because lists were then written with commas between elements.

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... considerably.⁵

1995: More characters were made available on SAIL and later on the Lisp machines. Alas, the world went back to inferior character sets again--though not as far back as when this paper was written in early 1959.

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... definitions.⁶

1995: This version isn't quite right. A comparison of this and other versions of $eval$ including what was actually implemented (and debugged) is given in ``The Influence of the Designer on the Design'' by Herbert Stoyan and included in Artificial Intelligence and Mathematical Theory of Computation: Papers in Honor of John McCarthy, Vladimir Lifschitz (ed.), Academic Press, 1991

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... Lists⁷

1995: These were later called property lists.

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... reclamation⁸

We already called this process ``garbage collection'', but I guess I chickened out of using it in the paper--or else the Research Laboratory of Electronics grammar ladies wouldn't let me.

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... list⁹

1995: now called a stack

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