[Haskell-cafe] On finding the right exposition...

[Anthony Clayden]

> ... I never understood why (->) is right associative in types, but (=>) is not and you are instead supposed to pack constraints in a tuple.


`-XRankNTypes` enables something called Rank 1 types, which accepts this:


>    foo :: Ord a => Num b => a -> Show b => b

>    -- foo :: (Ord a, Num b, Show b) => a -> b  -- inferred/canonical


We should also have:


>    bar :: forall a. forall {- empty -}. forall b. blah

>    -- bar :: forall a b. blah   -- equivalent


But `PatternSynonyms` drives a cart and horses through that: you must
have exactly 2 `... => ... => ...` and exactly 2 `forall`s (possibly
empty) -- that is, if you explicitly quantify at all. Or you can go:


>    MyPat :: Num a =>          a -> Baz a      -- which is shorthand for

>    MyPat :: Num a => ()    => a -> Baz a      -- which is equiv to

>    MyPat :: Num a => Num a => a -> Baz a

>    MyPat :: ()    => Num a => a -> Baz a      -- this means something different
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