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

[Jonathan Cast]

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