Ah -- you used an *existential* there! Yes, existentially-bound type variables are rigid. They stand for themselves, as it were. That resolves the mystery -- but it existentials admittedly introduce a new complication How should this be clarified? S | -----Original Message----- | From: Mitchell, Neil [mailto:neil.mitchell.2@credit-suisse.com] | Sent: 15 September 2008 13:56 | To: Simon Peyton-Jones; glasgow-haskell-users@haskell.org | Subject: RE: GADT problems | | > | > | (case undefined of Foo GadtValue -> ()) :: () -- is rigid | > ... | > | | > | But the first compiles fine, so it seems that the scrutinee doesn't | > | have to always be rigid? | > | > Not for me! Either with 6.8.3 or HEAD. What compiler are you using? | | HEAD from last Thursday. The code I'm using is slightly different to | your code, and is attached at the end of the message. Is matching Foo | causing its argument to become rigid? | | Thanks | | Neil | | {-# LANGUAGE GADTs, ScopedTypeVariables #-} | module Test where | | data E x = E x | | data Foo where | Foo :: Gadt a -> Foo | | data Gadt a where | GadtValue :: Gadt (E a) | | g :: () | g = case undefined of | Foo GadtValue -> () | | ============================================================================== | Please access the attached hyperlink for an important electronic communications disclaimer: | | http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html | ============================================================================== |