Re: [Haskell-cafe] ContT and ST stack

On 11 March 2011 11:15, Max Bolingbroke wrote:

On 10 March 2011 17:55, Bas van Dijk wrote:

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On 10 March 2011 18:24, Yves Parès wrote:

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Why has the operator (.) troubles with a type like (forall s. ST s a)?

Why can't it match the type 'b' in (.) definition?

As explained by the email from SPJ that I linked to, instantiating a type variable (like 'b') with a polymorphic type (like 'forall s. ST s a' ) is called impredicative polymorphism. Since GHC-7 this is not supported any more because it was to complicated.

AFAIK this decision was reversed because SPJ found a simple way to support them. Indeed, they work fine in 7.0.2 and generate warnings. Try it out:

Great! Unfortunately foo still doesn't type check in 7.0.2: foo :: (forall s. ST s a) -> a foo st = ($) runST st For the same reason I still need this ugly hack in usb-safe: http://hackage.haskell.org/packages/archive/usb-safe/0.12/doc/html/src/Syste... Bas