On Mon, Dec 14, 2015 at 03:38:34PM +0100, martin wrote:
On 2015-12-09 22:20, Tom Ellis wrote:
while learning about all the type classes and their relationships I came across something I found weird. If I understand it correctly, banana brackets where originally developed for Applicatives. The intent was to enable us to write something like
I don't think Arrow banana brackets are related to these Applicative (or "Idiom") brackets.
Let me try to convince you. ;)
OK :)
Actually I have no idea if there is a historical relationship, but there is definitely a semantic one. Consider this:
1) - 3), all agreed.
4) An idiom bracket (| f x1 x2 ... xn |) translates very roughly to liftA (uncurryN f) (commaN x1 x2 ... xn) while a banana bracket (| f x1 x2 ... xn |) translates very roughly to liftA' (uncurryN f) (andAn x1 x2 ... xn)
But in an Applicative or Idiom bracket expression of the form (| f x1 ... xn |), f is a pure function. In the Applicative banana bracket, the expression that is in f's position is *not* pure, instead it's an operator on arrows. In the GHC users' guide we have https://downloads.haskell.org/~ghc/7.2.2/docs/html/users_guide/arrow-notatio... untilA :: ArrowChoice a => a e () -> a e Bool -> a e () ... ... (|untilA (increment -< x+y) (within 0.5 -< x)|) untilA is manifestly not a pure function. So, I don't think these brackets are the same thing. I may have been wrong to say they are not related, but I can't see that there's as close a correspondence as you are trying to make out. Tom