On 6/30/2015 5:34 PM, 'John Clements' via Racket Users wrote:
> On Jun 30, 2015, at 8:10 AM, Alexey Cherkaev <alexey.cherk...@gmail.com>
wrote:
>
> ... wouldn't it be beneficial to have such a generalised 'set!' system-wide?
I understand that Racket focusses more on immutable structures, but there are
still vectors and hash-tables which are inherently mutable and still have their
niche.
This style of set! seems like a bad idea to me.
Specifically, one of the basic ideas of algebraic languages is that programs
are compositional. Specifically, if I write (a (b x) c), then the meaning of
this term depends on the meanings of a, (b x), and c. That is, I can combine
these three values to get the result. Generalized set! breaks this intuition.
Specifically, (set! (aref x 0 1) 13) does not depend on the value of (aref x 0
1). Rather, it pulls apart this term and uses its subterm’s meanings. Put
differently, this is lvalues.
I know, I know, I sound like a pure-functional snooty-poo.
Don't confuse source syntax with what goes on under the hood - compilers
for imperative languages often go to great lengths to transform programs
into more "functional" forms. Search for "value numbering" and "static
single assignment" for more information. [Those aren't the only
functionalizing transformations, but they are the most commonly used.]
Moreover, assignment per se is not incompatible with composition -
that's just semantics. On real hardware, assignment is all you have:
"binding" is implemented as assignment to a (possibly) new location.
Tail recursion is an important case of invisibly "binding" new values to
the same locations (i.e. assigning to the control variables of an
imperative loop).
George
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