[Haskell-cafe] RE: Re[4]: [Haskell] Dynamic binding

Ralf Lammel ralfla at microsoft.com
Fri Jun 24 03:44:10 EDT 2005

I guess we just need to agree that we disagree.

Of course, you are right that I am just a newbie when it comes to

How did you know that I am actually a Cobol programmer?
Did you google?

Seriously (?):


I never intended to impose any particular OO style on Haskeller's.
Its more about the *intellectual* question whether we can take
code and do a structure-preserving conversion to Haskell without
starting to argue on paradigm conversion.

Or as Alistair Bayley  argues (Alistair, thanks for helping out!)


"... it might be that you need to interface with external OO code, or
you are porting an existing program and need to remain faithful to the
original design, if only because you don't understand it well enough to
convert it to a functional design. ..."

Bulat, why not read all of this? Also see the end of that Wiki page,
where a reference to a paper is given, which seems to solve all these
problems, but my brain is to small to really be sure. ;-)


No matter how much you try, the code that you showed
was not very close to a faithful Shapes implementation.
If you dare to read the spec:


You made two proposals that don't solve the problem no matter of

1st proposal of Bulat:

> just create list of draw functions itself:
> [drawCircle (10,10) 5, drawSquare (20,20) 10]
> you are not expected that this problem can be solved with one line of
code? :)

My reply: you are not building a list of objects.
You are not organizing a loop whose body iterates over a list and
executes functionality.

2nd proposal of Bulat:

> for more complex tasks - declare interface as a structure:
> data ShapeInterface = Shape { draw :: IO (),
>                               moveTo :: Point -> IO (),
>                               calcArea :: Float
>                             }
> and return this structures from "constructor" functions:
> circle x y r = Shape { draw = drawCircle center r,
>                        moveTo newCenter = ....,
>                        calcArea = pi*r*r
>                      }
>                where center = Point x y
> square x y size = Shape { draw = ....,
>                           moveTo newCenter = ....,
>                           calcArea = size*szie
>                         }
> figures = [circle 1 2 3, square 4 5 6, circle 7 8 9]

The tragedy here is that this stops to work once circles and squares
do have different interfaces. You solution does not scale. Subtyping
allows for interface extension, and the actual benchmark exploits
this, but your code avoids it.

Anyway the solution is trivial: ex. quantification, you didn't propose

Ralf (back to Cobol)

> -----Original Message-----
> From: Bulat Ziganshin [mailto:bulatz at HotPOP.com]
> Sent: Thursday, June 23, 2005 11:37 PM
> To: Ralf Lammel
> Cc: Pal-Kristian Engstad; haskell-cafe at haskell.org
> Subject: Re: [Haskell-cafe] RE: Re[4]: [Haskell] Dynamic binding
> Hello Ralf,
> Thursday, June 23, 2005, 11:40:13 PM, you wrote:
> RL> a) building (i) a list of data is fundamentally different
> RL> from building (ii) a list of anticipated results of each datum.
> RL> I would be surprised to hear that this counts as a valid
> RL> BTW, you can do the *same* in a lazy OO language. (No lazy OO
> RL> at hand -- well lazyness can be simulated.)
> sorry, "valid" technique is technique that work :)  i use in my own
> program all the three ways i mentioned to solve problems of different
> complexity
> afair, you are relatively new to Haskell (relatively to my 1-year
> experience :) and, i think, you are not share FP thinking style. when
> programming in Haskell, i think in terms "what thing i will need in
> this point of program?". if i need, for example, a possibility to
> string against regular expression, then i will pass a function which
> does this check, not original RE. if i need a possibility to
> draw a shape, i will pass action which draws this shape. if i need
> several functions, i just combine them in a tuple
> there is one interesting example in my program. i have a list of
> wildcards and list of filenames and for each filename i need to know
> number of first wildcard to which this filename matched. in early
> stages of my program development i just passed list of wildcards to
> file-finding routine (as [Wildcard]). then, i changed list of
> to list of functions which check match against each wildcard
> ([Filename->Bool]). and after that, i changed this to one function
> just finds first True answer (Filename->Int). it was also more
> effective to compute this function only one time (it was something
> compiled
> on moment of computing and worked as fast as hand-written analyzer for
> given set of wildcards)
> as you see, i slowly migrated from traditional way of solving this
> problem to perfectly functional way and it was required several months
> RL> a) building (i) a list of data is fundamentally different
> RL> from building (ii) a list of anticipated results of each datum.
> i think that you don't have "Haskell brain" ;)  and therefore don't
> "trust" functions, which are passed as parameters, collected in lists,
> saved in data structures and so on. you are prefer to "put hands on"
> some data, preferably an object, which can be manipulated with any
> method declared in his interface. i'm right? ;)
> FP encourage another way - passing functions which will then be
applied to
> some additional arguments, as in my program, where file-finding
> function absolutely don't need list of wildcards. it just need to map
> filename to wildcard number, so a Filename->Int parameter is just
> enough
> in the draw example, each elment in a list was an action (having type
> IO() ), so i don't create list of anticipated results, i created list
> of actions which can be performed, for example, by sequence_
> RL> Anyway, even if people end up coding as you propose,
> RL> it won't work in general. Think of mutation methods that
> RL> change the state but preserve the type. Then your list will
> RL> still be heterogonous. NO?
> my second example was just of this type. it uses IORefs to hold
> current state, but this IORefs don't need to appear in interface
> see method moveTo, which changes state variable center, and method
> draw, which uses this variable
> >> this state is just don't need to appear in interface definition :)
> >> circle x y r
> RL> You are not talking about state but constructor arguments.
> "interface definition" is a structure ShapeInterface, which have only
> fields for exposed object methods. so it supports any figures in
> universe :)
> RL> In OO, mutable state tends to leak to the interface,
> RL> perhaps not as public fields, perhaps not even as public
> only in C++ and other languages which need to calculate object size :)
> declaration of _non-public_ fields in _interface_ is something strange
> ;)
> >> this state is just don't need to appear in interface definition :)
> >>
> >> if you need to maintain mutable state, this is also not a problem:
> >>
> >> data ShapeInterface = Shape { draw :: IO (),
> >>                               moveTo :: (Int,Int) -> IO (),
> >>                               calcArea :: Float
> >>                             }
> >> circle x y r = do
> >>   center <- ref (x,y)
> >>   return Shape { draw     = val center >>= drawCircle r
> >>                , moveTo   = (center=:)
> >>                , calcArea = pi*r*r
> >>                }
> >> main = do
> >>   figures <- sequence [circle 1 2 3, square 4 5 6, circle 7 8 9]
> >>   mapM_ draw figures
> >>   mapM_ (moveTo (0,0)) figures
> >>   mapM_ draw figures
> >>
> >> ref=newIORef
> >> val=readIORef
> >> (=:)=writeIORef
> --
> Best regards,
>  Bulat                            mailto:bulatz at HotPOP.com

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