I was installing leksah ( "cabal install leksah" ) & the it failed with a message similar to below one ( about lens 4.2 dependency )
I tried installing the lasted version instead but that didn't solved my problem . ( I have unregistered that - "ghc-pkg unregister lens" )
Note : I do -not- want to install leksah on a sandbox
$ cabal install lens-4.2
src/Control/Lens/Internal/Instances.hs:36:10:
Duplicate instance declarations:
instance Foldable ((,) b)
-- Defined at src/Control/Lens/Internal/Instances.hs:36:10
instance Foldable ((,) b)
-- Defined in `Data.Traversable.Instances'
src/Control/Lens/Internal/Instances.hs:39:10:
Duplicate instance declarations:
instance Traversable ((,) b)
-- Defined at src/Control/Lens/Internal/Instances.hs:39:10
instance Traversable ((,) b)
-- Defined in `Data.Traversable.Instances'
src/Control/Lens/Internal/Instances.hs:42:10:
Duplicate instance declarations:
instance Foldable (Either a)
-- Defined at src/Control/Lens/Internal/Instances.hs:42:10
instance Foldable (Either a)
-- Defined in `Data.Traversable.Failed to install lens-4.2
s/Internal/Instances.hs:46:10:
Duplicate instance declarations:
instance Traversable (Either a)
-- Defined at src/Control/Lens/Internal/Instances.hs:46:10
instance Traversable (Either a)
-- Defined in `Data.Traversable.Instances'
src/Control/Lens/Internal/Instances.hs:50:10:
Duplicate instance declarations:
instance Foldable (Const m)
-- Defined at src/Control/Lens/Internal/Instances.hs:50:10
instance Foldable (Const m)
-- Defined in `Data.Traversable.Instances'
src/Control/Lens/Internal/Instances.hs:53:10:
Duplicate instance declarations:
instance Traversable (Const m)
-- Defined at src/Control/Lens/Internal/Instances.hs:53:10
instance Traversable (Const m)
-- Defined in `Data.Traversable.Instances'
src/Control/Lens/Internal/Instances.hs:57:10:
Duplicate instance declarations:
instance Foldable1 ((,) b)
-- Defined at src/Control/Lens/Internal/Instances.hs:57:10
instance Foldable1 ((,) a) -- Defined in `Data.Semigroup.Foldable'
src/Control/Lens/Internal/Instances.hs:60:10:
Duplicate instance declarations:
instance Traversable1 ((,) b)
-- Defined at src/Control/Lens/Internal/Instances.hs:60:10
instance Traversable1 ((,) a)
-- Defined in `Data.Semigroup.Traversable'
cabal: Error: some packages failed to install:
lens-4.2 failed during the building phase. The exception was:
ExitFailure 1
output of "ghc-pkg list"
$ ghc-pkg list
/var/lib/ghc/package.conf.d:
Cabal-1.16.0
GLURaw-1.3.0.0
GLUT-2.4.0.0
HTTP-4000.2.8
HUnit-1.2.5.2
MonadCatchIO-mtl-0.3.0.5
OpenGL-2.8.0.0
OpenGLRaw-1.3.0.0
QuickCheck-2.6
array-0.4.0.1
async-2.0.1.4
attoparsec-0.10.4.0
base-4.6.0.1
bin-package-db-0.0.0.0
binary-0.5.1.1
bytestring-0.10.0.2
case-insensitive-1.1.0.2
cgi-3001.1.8.3
containers-0.5.0.0
deepseq-1.3.0.1
directory-1.2.0.1
extensible-exceptions-0.1.1.4
fgl-5.4.2.4
filepath-1.3.0.1
(ghc-7.6.3)
ghc-prim-0.3.0.0
hashable-1.2.1.0
haskell-src-1.0.1.5
(haskell2010-1.1.1.0)
(haskell98-2.0.0.2)
hoopl-3.9.0.0
hpc-0.6.0.0
html-1.0.1.2
integer-gmp-0.5.0.0
mtl-2.1.2
network-2.4.1.2
old-locale-1.0.0.5
old-time-1.1.0.1
parallel-3.2.0.3
parsec-3.1.3
pretty-1.1.1.0
primitive-0.5.0.1
process-1.1.0.2
random-1.0.1.1
regex-base-0.93.2
regex-compat-0.95.1
regex-posix-0.95.2
rts-1.0
split-0.2.2
stm-2.4.2
syb-0.4.0
template-haskell-2.8.0.0
text-0.11.3.1
time-1.4.0.1
transformers-0.3.0.0
unix-2.6.0.1
unordered-containers-0.2.3.0
vector-0.10.0.1
xhtml-3000.2.1
zlib-0.5.4.1
/home/bandbaz/.ghc/x86_64-linux-7.6.3/package.conf.d:
Cabal-1.20.0.2
aeson-0.7.0.6
attoparsec-0.11.3.4
attoparsec-conduit-1.0.1.2
bifunctors-4.1.1.1
binary-shared-0.8.3
blaze-builder-0.3.3.2
blaze-html-0.7.0.2
blaze-markup-0.6.1.0
cairo-0.12.5.3
cmdargs-0.10.9
comonad-4.2.2
conduit-1.0.17.1
contravariant-0.6.1.1
cpphs-1.18.5
darcs-2.8.4
dataenc-0.14.0.7
distributive-0.4.4
dlist-0.7.1
exceptions-0.6.1
executable-path-0.0.3
free-4.9
ghc-paths-0.1.0.9
ghcjs-codemirror-0.0.0.1
ghcjs-dom-0.0.10
gio-0.12.5.3
glib-0.12.5.4
gtk3-0.12.5.7
haddock-2.13.2.1
hamlet-1.2.0
hashed-storage-0.5.11
haskeline-0.7.1.3
haskell-lexer-1.0
haskell-src-exts-1.14.0.1
haskell-src-meta-0.6.0.5
hlint-1.8.61
hscolour-1.20.3
hslogger-1.2.4
hxt-9.3.1.4
hxt-charproperties-9.1.1.1
hxt-regex-xmlschema-9.1.0
hxt-unicode-9.0.2.2
jmacro-0.6.9
leksah-server-0.13.1.1
lifted-base-0.2.3.0
ltk-0.13.2.0
mmap-0.5.9
mmorph-1.0.3
monad-control-0.3.3.0
nats-0.2
pango-0.12.5.3
parseargs-0.1.5.2
polyparse-1.9
prelude-extras-0.4
pretty-show-1.6.8
profunctors-4.2.0.1
reflection-1.5.1
regex-tdfa-1.2.0
resourcet-0.4.10.2
safe-0.3.7
scientific-0.3.3.0
semigroupoids-4.2
semigroups-0.15.2
shakespeare-2.0.1.1
strict-0.3.2
system-fileio-0.3.14
system-filepath-0.4.12
tagged-0.7.2
tar-0.4.0.1
terminfo-0.3.2.6
text-stream-decode-0.1.0.5
th-lift-0.6.1
th-orphans-0.8.1
transformers-base-0.4.2
transformers-compat-0.3.3.4
uniplate-1.6.12
utf8-string-0.3.8
vado-0.0.1
vcsgui-0.0.4
vcswrapper-0.0.4
void-0.6.1
webkitgtk3-0.12.6.1
webkitgtk3-javascriptcore-0.12.5.1
wl-pprint-text-1.1.0.2
ghc-pkg list
? – user2407038