cabal dependency resolution fail for 'lens'

Question

I just did a cabal update and tried to install 'lens' from hackage. That gave me the following error -

$ cabal install -j lens
Resolving dependencies...
Configuring dlist-0.7.0.1...
... <snip>
Configuring mtl-2.0.1.0...
Building mtl-2.0.1.0...
Failed to install mtl-2.0.1.0
Last 10 lines of the build log ( /home/aj/.cabal/logs/mtl-2.0.1.0.log ):
Building mtl-2.0.1.0...
Preprocessing library mtl-2.0.1.0...
[ 1 of 21] Compiling Control.Monad.Writer.Class (
Control/Monad/Writer/Class.hs, dist/build/Control/Monad/Writer/Class.o )
[ 2 of 21] Compiling Control.Monad.State.Class ( Control/Monad/State/Class.hs,
dist/build/Control/Monad/State/Class.o )
[ 3 of 21] Compiling Control.Monad.Reader.Class (
Control/Monad/Reader/Class.hs, dist/build/Control/Monad/Reader/Class.o )
[ 4 of 21] Compiling Control.Monad.RWS.Class ( Control/Monad/RWS/Class.hs,
dist/build/Control/Monad/RWS/Class.o )
[ 5 of 21] Compiling Control.Monad.Identity ( Control/Monad/Identity.hs,
dist/build/Control/Monad/Identity.o )
[ 6 of 21] Compiling Control.Monad.Error.Class ( Control/Monad/Error/Class.hs,
dist/build/Control/Monad/Error/Class.o )
Control/Monad/Error/Class.hs:93:18: Not in scope: `catch'
... <snip>
cabal: Error: some packages failed to install:
aeson-0.7.0.3 depends on mtl-2.0.1.0 which failed to install.
exceptions-0.6 depends on mtl-2.0.1.0 which failed to install.
free-4.7.1 depends on mtl-2.0.1.0 which failed to install.
lens-4.1.2 depends on mtl-2.0.1.0 which failed to install.
mtl-2.0.1.0 failed during the building phase. The exception was:
ExitFailure 1

So looks like mtl-2.0.1.0 is broken. However some more dependency tracking reveals that nothing in the lens package definition actually depends on that version of mtl. Infact, the next thing I tried was -

$ cabal install -j aeson
$ cabal install -j free
$ cabal install -j lens

And that succeeds without problems.

So why would cabal try to install mtl-2.0.1.0 when installing lens directly? I've tried removing my .ghc and .cabal directories and repeating the steps, with the same results. The cabal version I'm using is 1.16 -

$ cabal -V
cabal-install version 1.16.0.2
using version 1.16.0 of the Cabal library 

Answer 1

The workaround that I am using is to first install transformers-compat with the transformers 3 flag

$ cabal install -ftransformer3 transformers-compat

and then install lens with the transformers 0.3.0.0 constraint

$ cabal install --constraint="transformers==0.3.0.0" lens