A friend of mine is trying to install my "timeplotters" (see email signature).
He's getting a weird error: one of the tools installs fine but the other one fails with the error "Couldn't read cabal file "hashable/
1.2.0.0/hashable.cabal""; curiously, so does running "cabal install cabal-install".
That's a symptom of too old cabal-install that crashes when it encounters certain newer cabal file features. I believe you need to upgrade cabal-install manually (since cabal-install itself will crash trying to do so...).
It's not opening a cabal file directly, it is trying to parse entries in the compressed package list.