[PRL] annoying types ARGH! -> structure-shy

[Karl Lieberherr]

Hi Pengcheng:

Your back or parent link calls for an intersection with:

"from * bypassing -> *,parent,* to *"

You are right that such expressiveness has to be used carefully. But the
enhanced code is still more flexible than Matthias' hard-wired
traversal.

-- Karl

> -----Original Message-----
> From: Pengcheng Wu [mailto:wupc at ccs.neu.edu]
> Sent: Thursday, May 12, 2005 10:51 AM
> To: Karl Lieberherr
> Cc: 'Matthias Felleisen'; 'PRL Mailing List'
> Subject: RE: [PRL] annoying types ARGH! -> structure-shy
> 
> 
> On Thu, 12 May 2005, Karl Lieberherr wrote:
> 
> > And the point is that the details of the data model are not
important,
> > BOTH to the program and to the test input.
> 
> Well, sometimes, the details of the data model does matter. For
example,
> in your class structure, if each Shape object happens to have a back
link
> to the Compound object in which the Shape object is stored (this is a
> common situation in programs), then your program may enter an infinite
> loop and never terminates.
> 
> I would like to say this way: in the Adaptive Programming world, when
one
> writes a traversal strategy, she has to think carefully to which data
> model this strategy will be applied, sometimes even including the
details;
> and the strength of Adaptive Programming is that when in situations
> where the details are not important, the generator can automatically
> generate traversal code, which otherwise will be tedious and
error-prone.
> 
> --Pengcheng
> 
> 
> > Consider this class dictionary:
> >
> > Compound = "(" <first> Shape <second> Shape ")".
> > Shape : Simple | Compound.
> > Simple : Rectangle | Circle common "ref" Point.
> > Rectangle = "rectangle" "length" <length> int "width" <width> int.
> > Circle = "circle" "radius" <radius> int.
> >
> > Point = <x> int <y> int.
> >
> > Both program and test cases also work for this class dictionary:
> >
> > Compound = "(" <elements> List(Shape) ")".
> > Shape : Simple | Compound.
> > Simple : Rectangle | Circle common "ref" Point.
> > Rectangle = "rectangle" "length" <length> int "width" <width> int.
> > Circle = "circle" "radius" <radius> int.
> >
> > Point = <x> int <y> int.
> > List(S) ~ {S}.
> >
> > Matthias solution does not have this flexibility.
> >
> > -- Karl
> >
> >
> > > -----Original Message-----
> > > From: prl-bounces at lists.ccs.neu.edu
> > [mailto:prl-bounces at lists.ccs.neu.edu]
> > > On Behalf Of Matthias Felleisen
> > > Sent: Thursday, September 01, 2005 9:43 AM
> > > To: PRL Mailing List
> > > Subject: [PRL] annoying types ARGH!
> > >
> > > I just wrote this little program for Karl:
> > >
> > > > type posn = float * float;;
> > > >
> > > > type shape =
> > > >        Circle of posn * float
> > > >      | Square of posn * float * float
> > > >      | Union of shape * shape;;
> > > >
> > > > let distance((x1,y1),(x2,y2)) =
> > > >     let dx = x1 -. x2 in
> > > >     let dy = y1 -. y2 in
> > > >       sqrt(dx *. dx +. dy *. dy)
> > > >
> > > > let rec inShape(s,p)  = match s with
> > > >     Circle(c,r) -> distance(c,p) <= r
> > > >   | Square(nw,width,height) -> false
> > > >   | Union(top,bot) -> inShape(top,p) || inShape(bot,p);;
> > >
> > > Before I finished, I had_10_ type errors, one was real. Can you
guess
> > > what went wrong?
> > >
> > > Next I had one logical error, which would have launched
anti-aircraft
> > > rockets at anything remotely on the planet. (This is an excerpt
from
> > > the Hudak-documented DoD programming contest, reformulated a in A
> > > Little Java for peaceful purposes :-)
> > >
> > > Yes, I am on record saying that I'd use OCAML if I worked in
industry,
> > > but boy this makes me reconsider this one.
> > >
> > > -- Matthias
> > >
> > >
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