Resolving dependencies...
Downloading rfc3339-1.0.1...
Configuring rfc3339-1.0.1...
Building rfc3339-1.0.1...
Preprocessing library rfc3339-1.0.1...
[1 of 1] Compiling Data.Time.RFC3339 ( Data/Time/RFC3339.hs, dist/build/Data/Time/RFC3339.o )

Data/Time/RFC3339.hs:43:10:
    Illegal instance declaration for ‘RFC3339 String’
      (All instance types must be of the form (T a1 ... an)
       where a1 ... an are *distinct type variables*,
       and each type variable appears at most once in the instance head.
       Use FlexibleInstances if you want to disable this.)
    In the instance declaration for ‘RFC3339 String’
Failed to install rfc3339-1.0.1
cabal: Error: some packages failed to install:
rfc3339-1.0.1 failed during the building phase. The exception was:
ExitFailure 1