Hi Iavor Diatchki wrote:
Hello,
I just tripped over the "Contexts differ in length" error message. I know it's not a new problem, but I thought I'd enquire as to its status.
I have run into that a number of times. There aren't any technical issues with solving it, in fact, depending on how one implements the type checker, it is more work to implement this extra restriction
Yes, that's what I would expect. It's an inevitable but not threatening consequence of polymorphic recursion. It rather surprised me that just telling the machine what my plan was (something I always do anyway) produced such an objection.
It is proposed for fixing in Haskell' but I was surprised that it did not seem to appear on the "definitely in" list (as far as I can recall---I might be wrong).
From what I can tell, it's "probably in". Strikes me as fairly essential. I don't expect artificial restrictions to be placed on the types of my programs when I am willing to make those types explicit.
Hopefully the report will be fixed though. Otherwise, as Ian mentioned, it works in GHC 6.6 with -fglasgow-exts. Also, there should be no problems with higher-ranked types etc.
Thanks for the tip, Ian. I guess I shouldn't put off installing 6.6 any longer. All the best Conor