[Haskell-cafe] Arrows for invertible programming: Notation, question

Arjen A.vanWeelden at cs.ru.nl
Fri Dec 23 05:53:00 EST 2005

On Fri, 23 Dec 2005, Joel Reymont wrote:

 > Folks,
 > I have been looking at the code for the "Arrows for invertible
 > programming" paper (http://www.cs.ru.nl/A.vanWeelden/bi-arrows/) and
 > I have a question about syntax. ghci surely does not like it.
I've updated the web page to say that is does not work with Hugs, GHCi 
or GHC -fgenerics, sorry for the confusion.
It really requires Generic Haskell (GH) 1.42. I never got GH working 
with GHC 6.4, so you also really need GHC 6.2.2. I think you can run the 
result of the GH preprocessor (using the right command line options) in 
GHCi or Hugs, but I'm not sure.

If anyone got GH working with GHC 6.4(+), please tell me how you did it.

 > What does this mean and how do I make it compile?
 > mapl{|a, b|arr|} :: (mapl{|a, b|arr|}, ArrowChoice arr, BiArrow arr)
 > => arr a b
In GH this means: mapl is a polytypic (type-indexed in a and b) function 
that yields an arr(ow) from a to b. It is overloaded in ArrowChoice and 
BiArrow on the arr(ow). Because it is polytypic, it is also overloaded 
`in itself'. Most of the time GH does not require the last part 
explicitly, but sometimes it triggers a bug if you don't specify it.
The semantics of mapl can be found in the paper.


 >       Thanks, Joel
 > --
 > http://wagerlabs.com/

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