GHC 6.12.1 and impredicative polymorphism

[Ben Moseley]

Is there any difference between "-XImpredicativePolymorphism" and "- 
XImpredicativeTypes"?

Hyena uses the latter, and we're using Hyena somewhat "in anger".

--Ben

On 30 Oct 2009, at 09:51, Simon Peyton-Jones wrote:

> Friends
>
> One more update about GHC 6.12, concerning impredicative polymorphism.
>
> GHC has had an experimental implementation of impredicative  
> polymorphism for a year or two now (flag - 
> XImpredicativePolymorphism). But
>
>  a) The implementation is ridiculously complicated, and the complexity
>     is pervasive (in the type checker) rather than localized.
>     I'm very unhappy about this, especially as we add more stuff to
>     the type checker for type families.
>
>  b) The specification (type system) is well-defined [1], but is also  
> pretty
>     complicated, and it's just too hard to predict which programs will
>     typecheck and which will not.
>
> So it's time for a re-think.  I propose to deprecate it in 6.12, and  
> remove it altogether in 6.14.  We may by then have something else to  
> put in its place.  (There is no lack of candidates [2,3,4]!)
>
> Fortunately, I don't think a lot of people use the feature in  
> anger.  Please yell if you *are* using impredicative polymorphism  
> for something serious.  But if you are, we need to think of a  
> workaround.  The current situation seems unsustainable.
>
> Simon
>
> [1] http://research.microsoft.com/en-us/um/people/simonpj/papers/boxy/
> [2] http://research.microsoft.com/en-us/um/people/crusso/qml/
> [3] http://research.microsoft.com/en-us/um/people/daan/pubs.html
> [4] http://gallium.inria.fr/~remy/mlf/
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