Saw a workaround on IRC; posted it to the issue on github:
https://github.com/archhaskell/habs/issues/202#issuecomment-141550410
Basically, copying the libncursesw.so.5.9 file out of the old package to /usr/lib/libncursesw.so.5 seems to work.

On Fri, Sep 18, 2015 at 12:45 AM, stef204 <stef204@yandex.com> wrote:
 
 
17.09.2015, 17:10, "Leif Warner" <abimelech@gmail.com>:
I tried downgrading back to the last ncurses version, but maybe that was a bad idea. Now every time I try to run pacman I get "error: database 'haskell-core' is not valid (invalid or corrupted database (PGP signature))"

On Thu, Sep 17, 2015 at 4:05 PM, Leif Warner <abimelech@gmail.com> wrote:
I just did "pacman -Syu", and among other things, it included an ncurses update:
core/ncurses       5.9-7 -> 6.0-3
 
Now things like ghci, ghc-pkg, etc fail with "error while loading shared libraries: libncursesw.so.5: cannot open shared object file: No such file or directory"
There's a libncursesw.so.6 instead, now. I suspect the ArchHaskell-provided GHC needs rebuilding against the newer ncurses?
 
Same situation here, I believe ghc (and more packages?) should be rebuilt against the new ncurses but this fails.
 
Gentoo has some reference to this:
 
<https://forums.gentoo.org/viewtopic-p-7806492.html?sid=bf394ed6ab70b57c0806d371f6940965>
 
and a bug report here:
 
<https://bugs.gentoo.org/show_bug.cgi?id=557478>
 
I haven't applied the suggestions yet as I wanted to get feedback from this list and Magnus (perhaps?) first....
 
 

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