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From: Alan Bawden <alan@csail.mit.edu>
Newsgroups: comp.lang.lisp
Subject: Re: Lisp history: IF, etc.
Date: Wed, 03 Apr 2024 17:54:51 -0400
Organization: ITS Preservation Society
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Paul Rubin <no.email@nospam.invalid> writes:

   Alan Bawden <alan@csail.mit.edu> writes:
   > McCarthy invented the conditional expression and thus created the
   > first expression-only programming language.

   I think Church's lambda calculus also had this.

   https://en.wikipedia.org/wiki/Church_encoding#Church_Booleans

Yeah, I though about that while I was composing my previous message, but
my life is finite, so I didn't go there.  But this is Usenet, so we are
forced to explore every side issue until we all drop dead of exhaustion.

So does Lambda Calculus qualify as a programming language?  Well _today_
we would all say yes, because we have examples of purely functional
programming languages where evaluation is lazy and so we don't need a
special operator for conditional evaluation.  But when McCarthy was
inventing Lisp, nobody thought that pure Lambda Calculus was anything
like a programming language.  It's only since then that we've done the
work to make that practical.

So I _could_ argue that in 1958 Lambda Calculus was _not_ a programming
language, but today in 2024 Lambda Calculus _is_ a programming language,
even though Lambda Calculus didn't change in any way!

But given the way being a "programming language" was understood in 1958,
nobody was going to invent an expression-only programming language until
they first invented the conditional expression.  So thank you John
McCarthy.

- Alan