Simon, I discovered that the following example also won't parse in ghc-6.12.1 because of the infix 'MayOpen' constraint: with ∷ (Resource resource, MonadCatchIO pr) ⇒ resource → (∀ s. s `MayOpen` resource ⇒ RegionalHandle resource (RegionT s pr) → RegionT s pr α) → pr α Is this also fixed by your previous patch? regards, Bas