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).

I have had an unresolved issue on my stack of Haskell vs Java that I wonder if your observation explains.

If you notice java generics has all sort of gotchas (e.g. http://www.ibm.com/developerworks/java/library/j-jtp01255.html). I somehow don't see this discussion in Haskell. I wonder if haskell's model of letting the caller determine the result type has advantage in that you don't have all the complexity you would have if you let the API determine their types.

daryoush

On Wed, Oct 15, 2008 at 11:45 AM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:

> The instance selection for an interface is done at run-time and this is
> inherently necessary. The instance (in a different sense) selection for
> type classes is almost always resolvable statically. In Haskell 98

In both cases, the dispatch is inherently dynamic, and in both cases,
most dispatches can be resolved at compile-time with sufficient effort.
The actual percentage may be quite different, tho.
Implementation techniques are also fairly different, and the resulting
coding style is also very different, but the two concepts are
fundamentally very close to each other.

Stefan

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