Resolving dependencies...
Downloading wtk-0.1...
Configuring wtk-0.1...
Building wtk-0.1...
Preprocessing library wtk-0.1...
[1 of 4] Compiling Text.PrettyShow  ( Text/PrettyShow.hs, dist/build/Text/PrettyShow.o )

Text/PrettyShow.hs:78:10:
    Illegal instance declaration for `PrettyShow 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 -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `PrettyShow String'
Failed to install wtk-0.1
cabal: Error: some packages failed to install:
wtk-0.1 failed during the building phase. The exception was:
ExitFailure 1