Hi Simon and George.
All of generics-sop, basic-sop, pretty-sop, lens-sop, json-sop (all on
Hackage) have instances of such pattern matches. You can grep for
'error "inaccessible"' to find the places in question.
Cheers,
Andres
On Thu, Feb 19, 2015 at 7:21 PM, Simon Peyton Jones
I have some code that I could provide, but I don't seem to see George's email address in the CC (only Tom's). Perhaps GMail ate it for some reason - but either way, I don't know where to send my code :)
Second attempt, to try to include George’s email. If it’s not in cc, it is: george.karachalias@gmail.com
Strange. My “sent items” folder definitely shows him in cc.
Sorry for the spam.
Simon
From: Simon Peyton Jones Sent: 19 February 2015 17:38 To: Haskell Libraries (libraries@haskell.org); Haskell Cafe (haskell-cafe@haskell.org) Cc: George Karachalias; Tom Schrijvers; Dimitrios Vytiniotis (dimitris@microsoft.com); Simon Peyton-Jones Subject: Pattern match checking for GADTs
Friends
George Karachalas, Tom Schrijvers, Dimitrios Vytiniotis, and I are working on finally cracking the problem of accurately reporting pattern-match overlap and redundancy warnings, in the presence of GADTs. You know the problem; consider
vzip :: Vect n a -> Vect n b -> Vect n (a,b)
vzip VN VN = VN
vzip (VC x xs) (VC y ys) = VC (x,y) (vzip xs ys)
data Vect :: Nat -> * -> * where
VN :: Vect Zero a
VC :: a -> Vect n a -> Vect (Succ n) a
Are there any missing equations in vzip? No! But GHC complains anyway. We have lots of Trac tickets about this; e.g. https://ghc.haskell.org/trac/ghc/ticket/3927.
We now have a draft paper (wait a week) and a prototype implementation, that fixes the problem. But we need your help. We’d like to try our prototype on real code, not just toy examples.
So, could you send George a pointer to any packages you have, or know of, that
· use GADTS (or other fancy type features) and
· would benefit from accurate pattern-match overlap/redundancy warnings?
Specifically:
· Where you have had to add a catch-all
f _ _ = error “impossible”
to silence GHC from saying “missing patterns”
· Or where you have added {-# OPTIONS_GHC –fno-warn-missing-patterns #-} to silence the warnings.
· Or something else like that.
George’s email is in cc.
Time is short – the ICFP deadline is on 27 Feb. So sooner is better than later for us. But later is better than never.
Thank you!
Simon
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