status of rebindable syntax for arrows

[Ross Paterson]

On Tue, Oct 14, 2014 at 10:26:46PM +0200, S. Doaitse Swierstra wrote:
> The GHC manual already for quite a number of version states:
> 
> 	• Arrow notation (see Section 7.17, “Arrow notation ”) uses whatever arr, (>>>), first, app, (|||) and loop functions are in scope. 
>          But unlike the other constructs, the types of these functions must match the Prelude types very closely. Details are in flux; if you want to use this, ask! 
> 
> When using this feature we get the error:
> 
> "Var/Type length mismatch: 
>    [s{tv aVL} [tv]]
>    []
> ghc: panic! (the 'impossible' happened)
>  (GHC version 7.8.3 for x86_64-apple-darwin):
> 	tcTyVarDetails s{tv aVL} [tv]
> 
> Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug"
> 
> So our question is whether we should really report this as a bug, and/or what we can do about this. 

It already has a bug entry: #7828.  What would help is to know the kind
of use you have in mind, to see whether it fits with the proposed solution.