On Sep 14, 2015, at 6:16 PM, John Carmack <jo...@oculus.com> wrote:
> Is there a deep reason why defines aren’t allowed in the branches of an if,
> but are for cond / while / unless?
>
> Wrapping code in (let() …) instead of (begin …) works fine, but it is a
> strange quirk to explain to someone else.
It exposes our history. Traditionally there are no extra markers in LISP-style
syntax to introduce phrases within a grammatical sentence. Thus, an
if-expression obeys the grammar
(if test-expression
then-expression
else-expression)
not an Algol-style variant with additional keywords:
(if test-expression
#:then then-expression
#:else else-expression)
If we had those markers (not just in if-expressions but wherever we may allow
sequences of expressions), we could mixin definitions wherever we wanted.
In cond, it is obvious which phrase is a branch (due to the
parentheses/brackets) and that the first part of each branch is a
test-expression. The extra pair of parens play the role of Algol-style markers.
Now, if Racket had emerged as an alternative attempt to address all of Lisp's
weaknesses, we might have overcome this problem, too. But Racket grew
organically and we value backward compatibility very highly for our
programmers. You might say we are the C++ of the LISP world, but I hope we can
do better in the long run.
-- Matthias
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