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.