fix Tutorial doctest

Unfortunately this requires introducing singletons as dependencies.
I don't know how to best solve this.

Author: MMesch

Data/Vinyl/Tutorial/Overview.hs

@@ -7,6 +7,7 @@
 
     Let's work through a quick example. We'll need to enable some language
     extensions first:
+>>> :set -package lens
 
 >>> :set -XDataKinds
 >>> :set -XPolyKinds
@@ -15,6 +16,7 @@
 >>> :set -XTypeFamilies
 >>> :set -XFlexibleContexts
 >>> :set -XFlexibleInstances
+>>> :set -XStandaloneKindSignatures
 >>> :set -XNoMonomorphismRestriction
 >>> :set -XGADTs
 >>> :set -XTypeSynonymInstances
@@ -27,7 +29,6 @@
 >>> import Control.Lens hiding (Identity)
 >>> import Control.Lens.TH
 >>> import Data.Char
->>> import Test.DocTest
 >>> import Data.Singletons.TH (genSingletons)
 >>> import Data.Maybe
 
@@ -295,3 +296,4 @@ module Data.Vinyl.Tutorial.Overview where
 import Data.Vinyl.Core
 import Data.Vinyl.Functor
 import Data.Vinyl.Lens
+import Data.Singletons.TH (genSingletons)

vinyl.cabal

@@ -43,7 +43,8 @@ library
   build-depends:       base >= 4.11 && <= 5,
                        ghc-prim,
                        deepseq,
-                       array
+                       array,
+                       singletons == 2.7
   if impl (ghc < 8.6.0)
     build-depends: constraints >= 0.6.1
   default-language:    Haskell2010