I'm not advocating existential types in this case. I rarely use them myself.
I was just pointing out that the mechanism for doing the OO thing
exists in Haskell too, albeit looking a little different.
I don't think there's anything weird about existential types, except
an unfamiliar name.
Perhaps, it helps to make Lennart's point about composable building
blocks made more explicit (simplifying slightly):
- a function 'f :: a -> IO a' has to accept any type 'a', so it can't know
anything about those 'a's
- a function 'f :: Interface a => a -> IO a' only has to accept types 'a'
that *at least* implement 'Interface', so there's a *lower bound* on the
information required about those 'a's, and 'f' can't use any more than
that (but since 'a' is exposed to 'f's calling context, it might be fixed
there)
- a constructor of type '(forall a . Interface a => a) -> SomeType' can only
be applied to objects of type 'a' that *at least* implement 'Interface',
but if 'SomeType' doesn't mention 'a', the result *at most* implements
'Interface' and 'a' itself gets hidden, so there's an *upper bound* on
the information being provided
The reference that comes to mind
On understanding types, data abstraction, and polymorphism.
Luca Cardelli and Peter Wegner.
©1985 Computing Surveys, 17(4):471-522, 1985.
http://lucacardelli.name/Bibliography.htm
predates Haskell, so any 'splodin' heads can't be blamed on Haskell!-)
I wouldn't expose beginning programmers to this level of detail about
types, but if you're going to talk about interfaces and types at all, and
about type classes in particular, to programmers who are beginning
their switch to Haskell, it can't hurt them much to read it. It might
even help, and if not, it is at least a concrete basis for more Haskell
specific explanations when they start running into those questions.
Claus
On Wed, Oct 15, 2008 at 1:15 AM, John Lato <[EMAIL PROTECTED]> wrote:
Are you advocating introducing existential types to beginning
Haskellers? I think something with the scary name "existential
quantification" would greatly increase the head'splodin' on the
learnin' slope. Certainly there's a place for them, but I wouldn't
want to see new Haskell programmers habitually approach problems with
a "first create a type class, then make an existential wrapper"
mentality. Which is exactly what I fear is the current situation.
Although my list example is far to shallow to make this point, it
seems to me that it's fairly likely that somebody faced with this
problem has had something go severely wrong at some earlier time.
Existentials are certainly useful, but isn't it also possible that,
for many cases, an alternative design exists which fits a functional
idiom better and doesn't face this issue at all?
John
On Tue, Oct 14, 2008 at 5:03 PM, Lennart Augustsson
<[EMAIL PROTECTED]> wrote:
You can do equivalent of
// List and MyList are different classes
if (something) { return new List(); }
else { return new MyList(); }
in Haskell as well. But to do that you have to introduce an
existential wrapper in the return type.
In OO languages the existential wrapper is built in to OO constructs,
but in Haskell you have smaller building blocks that you have to
assemble to make the thing you want.
So to extend your example, you can do
data IList = forall a . (Foldable a, Indexable a) => IList a
getListOfData :: IO IList
and now you can return different kinds of types from getListOfData
depending on circumstances.
-- Lennart
On Tue, Oct 14, 2008 at 1:11 PM, John Lato <[EMAIL PROTECTED]> wrote:
I was just thinking about what I wish someone had told me when I
started working with Haskell (not that long ago). It would have saved
me a great deal of trouble.
The Difference Between Interfaces and Type Classes.
Many "Introduction to Haskell for the OOper" style tutorials claim
that type classes are like Interfaces (for languages with that
feature). I now think that, although this is technically true, it's
fundamentally misleading. Here's a simple example demonstrating my
line of thought:
In C# (and presumably Java), this sort of function is common:
public IList GetListOfData()
In Haskell, a similar function may be
GetListOfData :: (Foldable a, Indexable a) => IO a
In C#, it doesn't matter what the actual type returned by
GetListOfData is, as long is it supports the IList interface. It's
not uncommon for GetListOfData to make a choice between several
different implementations, depending on the nature of the data to be
returned. The following code is perfectly reasonable in C# :
// List and MyList are different classes
if (something) { return new List(); }
else { return new MyList(); }
The equivalent won't compile in Haskell, because the actual return
type does matter, and *is determined by the calling code*. Our
fictional GetListOfData can't return a List or a Mylist depending on
some conditional, in fact it can't explicitly return either one at
all, because the actual type of the result, as determined by the
caller, could be either one, or something else entirely (ignoring the
IO bit for the time being).
So I've come to the conclusion that stating type classes are like
interfaces is misleading to newcomers to Haskell, because it leads
people to think they should use type classes for the same sorts of
problems OO-languages solve with interfaces. In turn this means new
programmers are encouraged to use OO-style architecture in programs,
reassured that they're in a "functional" idiom because they're using
the "functionally-approved" feature of type classes. I think that if
I had understood this earlier, I would have embraced functional idioms
more quickly.
Incidentally, I'm still horrible at designing functional APIs and
modules, but at least now I know it, and I'm no longer trying to force
OO interfaces into Haskell. So I've made progress.
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