[GHC Proposal #281: Visible forall in types of terms](https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0281-visible-forall.rst) is presented in two parts: 1. Part 1 introduces the `forall a ->` quantifier in types of terms but also requires a syntactic marker at use sites. 2. Part 2 specifies when it is permissible to omit the `type` herald. See also https://gitlab.haskell.org/ghc/ghc/-/wikis/DH-current-status. The first part was tracked at #22326 and implemented in 33b6850aaaa4b3a4760d4b384cd15611b159045b. This new ticket is to track the implementation of the remaining parts of the proposal, i.e. Part 2. To that end, I identified the following subtasks: * [x] #23738 T2T (term-to-type) transformation in expressions (at call sites) * [x] #23739 T2T (term-to-type) transformation in patterns (at binding sites) * [x] #24159 The types-in-terms syntax (`forall`, `->`, `=>`) * [x] #23719 Make `forall` a keyword * [x] #23740 Term variable capture and implicit quantification Here are a few examples of code that we aim to allow: * A variant of `id` that uses visible `forall`: ```haskell -- Definition: idv :: forall a -> a -> a idv a x = x :: a -- Usage: n = idv Double 42 ``` This is equivalent to `n = (42 :: Double)`. * A wrapper around `typeRep` that uses visible `forall`: ```haskell -- Definition: typeRepVis :: forall a -> Typeable a => TypeRep a typeRepVis a = typeRep @a -- Usage: t = typeRepVis (Maybe String) ``` * A wrapper around `sizeOf` that uses visible `forall` instead of `Proxy`: ```haskell -- Definition: sizeOfVis :: forall a -> Storable a => Int sizeOfVis a = sizeOf (Proxy :: Proxy a) -- Usage: n = sizeOfVis Int ``` * A wrapper around `symbolVal` that uses visible `forall` instead of `Proxy`: ```haskell -- Definition: symbolValVis :: forall s -> KnownSymbol s => String symbolValVis s = symbolVal (Proxy :: Proxy s) -- Usage str = symbolValVis "Hello, World" ```