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shijiandeMacBook-Pro:language alvin$ cabal install
Warning: --root-cmd is no longer supported, see
haskell/cabal#3353 (if you didn't type --root-cmd,
comment out root-cmd in your ~/.cabal/config file)
Resolving dependencies...
Configuring language-lama-0.1...
Building language-lama-0.1...
Failed to install language-lama-0.1
Build log ( /Users/alvin/.cabal/logs/ghc-8.2.2/language-lama-0.1-KMwpHKY3lZSB1jgmMQSKz3.log ):
cabal: Entering directory '.'
Configuring language-lama-0.1...
Preprocessing library for language-lama-0.1..
Building library for language-lama-0.1..
[ 2 of 27] Compiling Data.Graph.Inductive.GenShow ( lib/Data/Graph/Inductive/GenShow.hs, dist/build/Data/Graph/Inductive/GenShow.o )

lib/Data/Graph/Inductive/GenShow.hs:9:10: error:
Duplicate instance declarations:
instance (Show a, Show b) => Show (Gr a b)
-- Defined at lib/Data/Graph/Inductive/GenShow.hs:9:10
instance (Show a, Show b) => Show (Gr a b)
-- Defined in ‘Data.Graph.Inductive.PatriciaTree’
|
9 | instance forall a b. (Show a, Show b) => Show (Gr a b) where
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cabal: Leaving directory '.'
cabal: Error: some packages failed to install:
language-lama-0.1-KMwpHKY3lZSB1jgmMQSKz3 failed during the building phase. The
exception was:
ExitFailure 1

I have a question that
why did I catch the error "Duplicate instance declarations:" ?

Thank you!