GHC proposal #5 [*], which I drafted, was recently merged. I was asked to modify the proposal to restrict the type signatures of patterns and builders to be the same, except for their contexts. I would like to make one last plea to remove this restriction, unless there are fundamental reasons it cannot be done. The reasons are as follows. 1. Despite the restriction, it remains possible, at least in many cases, to achieve exactly the same effect, albeit in a manner that is considerably more difficult to read. For example, one could write the following (ill-advised) pattern: pattern Ha :: a ~ Int => Word -> a pattern Ha x <- (fromIntegral -> x) where Ha :: a ~ (Word -> Word) => Word -> a Ha x = (x +) which, after more thinking than one might wish to apply, turns out to mean pattern Ha :: Word -> Int pattern Ha x <- (fromIntegral -> x) where Ha :: Word -> Word -> Word Ha x = (x +) 2. It would be *extremely unpleasant* to make this trick work with RankNTypes, where such mismatched signatures seem likely to have the most practical value. For example, suppose I want to write import Control.Lens import Control.Lens.Reified pattern Package :: Lens s t a b -> ReifiedLens s t a b pattern Package f <- Lens f where Package :: ALens s t a b -> ReifiedLens s t a b Package f = Lens $ cloneLens f This convenience pattern would pack up an ALens into a ReifiedLens; when unpacked, I'd get back a full Lens (The type Lens s t a b subsumes the type ALens s t a b). Without being able to use those different type signatures, I'd have to write something like this monstrosity: import Control.Lens.Internal.Context -- additionally pattern Package :: c ~ Functor => (forall f. c f => LensLike f s t a b) -> ReifiedLens s t a b pattern Package f <- Lens f where Package :: c ~ ((~) (Pretext (->) a b)) => (forall f. c f => LensLike f s t a b) -> ReifiedLens s t a b Package f = Lens $ cloneLens f I pity the poor soul who has to try to reverse engineer what those signatures are supposed to mean! While it's certainly possible to implement the restricted version now and expand it later, that would add *yet another* PatternSynonyms change for users to hack around with CPP. I hope you'll reconsider that decision in light of these examples. Thank you for your time, David Feuer [*] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0005-bi...