[Haskell-cafe] Re: What I wish someone had told me...

[Daryoush Mehrtash]

Would you please explain this a bit more:

the various
unfortunate consequences of type erasure in Java are avoided by the fact
that Haskell types lack constructors, so the user never expects to be
able to conjure up a value of an unknown type.

Thanks,

daryoush


On Wed, Oct 15, 2008 at 12:04 PM, Jonathan Cast
<jonathanccast at fastmail.fm>wrote:

> On Wed, 2008-10-15 at 11:56 -0700, Daryoush Mehrtash wrote:
> >         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.
>
> These look more like unfortunate interactions between generics and the
> pre-existing Java language than anything else.  Covariance isn't really
> an issue in Haskell, since Haskell lacks sub-typing; the various
> unfortunate consequences of type erasure in Java are avoided by the fact
> that Haskell types lack constructors, so the user never expects to be
> able to conjure up a value of an unknown type.
>
> jcc
>
>
>
>
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