[Haskell-beginners] Boilerplate Code

[Kyle Murphy]

I was going to argue this point but then it occurred to me that writing a
whole bunch of functions like:

isAFoo :: FooBarBaz -> FooBarBaz -> Bool
isAFoo x = typeChecker (Foo undefined) x

isn't really in any way better than:

isAFoo :: FooBarBaz -> FooBarBaz -> Bool
isAFoo (Foo _) = True
isAFoo _ = False

Although it did give me a chance to play around with Data and Typeable a
bit.

-R. Kyle Murphy
--
Curiosity was framed, Ignorance killed the cat.


On Tue, Aug 3, 2010 at 15:25, Brent Yorgey <byorgey at seas.upenn.edu> wrote:

> On Tue, Aug 03, 2010 at 04:19:58PM +0300, Alex Rozenshteyn wrote:
> > I have never used Data.Typeable, but maybe it could be made relevant
> here?
>
> Not really.  Data.Typeable lets you pass (representations of) types
> around at runtime, and thus do things like type-safe casts.  So it's
> useful for things like serialization, extracting things out of
> existential wrappers, and so on.  It isn't really relevant in this
> situation, as Matt already has all the type information he could want.
>
> -Brent
>
> >
> > On Tue, Aug 3, 2010 at 4:18 PM, Brent Yorgey <byorgey at seas.upenn.edu>
> wrote:
> >
> > > On Tue, Aug 03, 2010 at 09:51:45PM +1000, Matt Andrew wrote:
> > > > Hi all,
> > > >
> > > > I am in the process of writing a Scheme interpreter/compiler in
> Haskell
> > > as my first serious project after learning the basics of Haskell. The
> goal
> > > is to really get a feel for Haskell. I am trying to accomplish this as
> much
> > > as I can on my own, but am referring to Jonathan Tang's 'Write Yourself
> a
> > > Scheme in 48 hours' whenever I get really stuck.
> > > >
> > > > I have a question regarding a pattern that I have found within my
> code
> > > for which I cannot seem to find an abstraction.
> > > >
> > > > I am implementing some of the primitive Scheme type-checker functions
> > > with the following code:
> > > >
> > > > numberP :: SchemeVal -> SchemeVal
> > > > numberP (Number _) = Bool True
> > > > numberP _          = Bool False
> > > >
> > > > boolP :: SchemeVal -> SchemeVal
> > > > boolP (Bool _) = Bool True
> > > > boolP _        = Bool False
> > > >
> > > > symbolP :: SchemeVal -> SchemeVal
> > > > symbolP (Atom _) = Bool True
> > > > symbolP _        = Bool False
> > > >
> > > > This is a pattern that I could easily provide an abstraction for with
> a
> > > Lisp macro, but I'm having trouble discovering if/how it's possible to
> do so
> > > elegantly in Haskell. The closest (but obviously incorrect) code to
> what I'm
> > > trying to accomplish would be:
> > >
> > > It isn't really possible to abstract this any further in Haskell.
> > > Constructors are rather magical functions, but they are still
> > > functions, and like other functions cannot be compared for equality
> > > directly.  Pattern-matching them is the only sort of equality
> > > comparison you get.
> > >
> > > With that said, your intuition to use Lisp macros is a good one.
> > > Haskell has a similar metaprogramming facility called Template
> > > Haskell, which could easily be used to automatically generate these
> > > sorts of functions.  Of course, it's a little more complicated than
> > > Lisp macros since Haskell syntax is so much more complex than Lisp's
> > > -- but given that, on the whole it's not so bad.  I wouldn't use TH to
> > > generate just the three functions you showed -- but I would certainly
> > > consider it for ten.
> > >
> > > -Brent
> > > _______________________________________________
> > > Beginners mailing list
> > > Beginners at haskell.org
> > > http://www.haskell.org/mailman/listinfo/beginners
> > >
> >
> >
> >
> > --
> > ()  ascii ribbon campaign - against html e-mail
> > /\  www.asciiribbon.org   - against proprietary attachments
> >
> >           Alex R
>
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