Re: [Scheme-reports] Mutable Pairs

[Eli Barzilay <eli@x>]

On Jun 14, David Rush wrote:
> On 14 June 2010 06:56, Eli Barzilay <eli@x> wrote:
> 
> > The bottom line is that this:
> >
> >  (define (map f l)
> >    (define (loop l)
> >      (if (null? l) '() (cons (f (car l)) (loop (cdr l)))))
> >    (if (list? l)
> >      (loop l)
> >      (error "poof!")))
> >
> > can be proved as a correct implementation of map *assuming* no changes
> > to the list ever occur in the call to `f', or more generally during
> > the dynamic extent of the call to `map'.  If you're one of these
> > theoretical people, there's not much else to see here.
> >
> > The problem is on the side of the *working* programmer
> 
> Your "*working*" programmer is either made of straw or demonstrably
> incompetent in building semantically sound interfaces. If the list
> structure is part of the invariants in the interface, then there
> should be no other piece of code touching it.

So ... you're suggesting that `map' is never used with 3rd party
functions, or more generally, HOF and lists go together only as long
as they're my functions?  I guess that would be a solution too.  It
shows even better why this is a straitjacket.


> > given only mutable pairs, the only way to guarantee that your own
> > pair-related invariant (such as `list?'ness) is not broken is to copy
> > it:
> >
> >  (define (map f l)
> >    (define (loop f l)
> >      (if (null? l) '() (cons (f (car l)) (loop f (cdr l)))))
> >    (if (list? l)
> >      (loop f (loop values l))
> >      (error "poof!")))
> >
> > In almost two decades of reading and writing Scheme code, I have not
> > seen this done even once.
> 
> If this was *that* big of a problem, or worse - as big as the _goto_
> problem - I think we'd have seen a larger reaction before this,
> hmmm?  SET-CAR! and SET-CDR! and their predecessors RPLACA and
> RPLACD have been around a lot longer than two decades even.

Sure, and the bugs were (and are) still there.


> > To put this differently: Schemes (and Lispers) have been dreaming
> > for a really long time about Scheme/Lisp operating systems and
> > other large scale systems.
> 
> And, interestingly enough, they are still almost entirely written in
> C, assembly, and other *even less safe* languages. So what does that
> tell us, hmm?

It tells us that it's possible, which I've never disputed.  The
question is the amount of pain suffered on writing such a thing.  And
I agree that it's "less pain than C and assembly" -- but that bar was
not that high to begin with.


> [...]
> 
> The bottom line is that mutation is harder than it first looks.

Yes, exactly.


> The bottom line is that every example thus far has been more full of
> design flaws than an undergrad senior project. If you want Haskell,
> go program in it.

Please show me your definition of `map'.  (But if the above is correct
and you never use it with any functions except your own then it
doesn't matter -- but then what's the point of using scheme.)


> > Bugs involving state will almost always be found in some code that
> > is syntactically unrelated to the buggy code.  I've spent too many
> > nights tracking such bugs to just let myself go with the "assuming
> > no bad mutation" flow and hope for the best.
> 
> My day job is about 80% debugging (read that as "reasoning about")
> code written in dialects of C by other people, accumulated over the
> course of 20 years, without documentation or even any institutional
> memory or its original provenance. I assume *nothing* because nearly
> every invariant has been smashed as this program evolved the *BIG*
> ball of mud pattern all on its own. [...]

(Yes, I'm sure everyone expects C to be more painful.)


> It's a poor workman who doesn't know when to get better tools. And a
> better saw won't help you measure the right length before you
> cut. Mutable pairs is most emphatically *not* the problem here.

(Same as above: show me your `map'.  Or any robust code that uses
pairs.)


> > But *why* lump mutable data together with something as fundamental
> > as pairs?  Where's the "jewel-like"ness of that?
> 
> Because Scheme allows mutability as a top-level entity without
> reference to a state monad. Its a foundational part of the design
> space.
> [...]
> Haskell is over -> there.

You too failed to quote the part of my post where I explicitly said
*explicitly* that I'm not advocating abolishing mutation.  (Not only
do I use mutation, I very often criticize Haskell for wrapping
mutation-like code in a monad, only to write code that is as
imperative as any other C code.)


> > IMO, features that can be separated into N sub-features should
> > always be separated
> 
> So are you ready to take on the notion that all functions should
> only have one argument and deal with the necessity of re-defining
> the procedure call semantics in a way that is syntactically
> consistent with legacy code, while supporting a proper decomposition
> of the Scheme calling sequence, thus making CALL-WITH-VALUES
> obsolete? No, I didn't think so.

(Huh?)


> > So the whole dilemma (if I had one, that is) boils down to the
> > question of preserving legacy code.
> 
> Specifically, it boils down to the ability to bring in code which
> uses a-lists as a mutable lookup table. I can't do that with R6RS or
> PLT anymore without (admittedly minor) surgical alterations.
                       ^^^^^^^^^^^^^^^^

Yes.  And in the next sentence (which you didn't quote) I said "At
this point things become subjective".

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                    http://barzilay.org/                   Maze is Life!

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