Make default ToSample instances productive

The default Generics-based ToSample instance now uses Omega type
to productively produce distinct samples. The previous version
was based on lists and hence left-recursive. This means that with
previous versions the default toSamples for [Bool] would return an
infinite list like this:

[[],[False],[False,False],[False,False,False],...

As you can see it would never produce a list with True in it.
Omega handles this and produces a more diverse output:

[[],[False],[False,False],[True],...

This is still not the best possible case, but to do better we need
to use Omega not only in GToSample, but in ToSample as well since
GToSample uses ToSample instances recursively.

servant-docs/servant-docs.cabal

@@ -42,6 +42,7 @@ library
     , string-conversions
     , text
     , unordered-containers
+    , control-monad-omega == 0.3.*
   hs-source-dirs: src
   default-language: Haskell2010
   ghc-options: -Wall

servant-docs/src/Servant/Docs/Internal.hs

@@ -23,6 +23,7 @@ module Servant.Docs.Internal where
 import           Control.Applicative
 import           Control.Arrow              (second)
 import           Control.Lens               hiding (List, to, from)
+import qualified Control.Monad.Omega        as Omega
 import           Data.ByteString.Conversion (ToByteString, toByteString)
 import           Data.ByteString.Lazy.Char8 (ByteString)
 import qualified Data.CaseInsensitive       as CI
@@ -394,12 +395,12 @@ defaultSample :: forall a b. (Generic a, Generic b, GToSample (Rep a) (Rep b)) =
 defaultSample _ = to <$> gtoSample (Proxy :: Proxy (Rep a))
 
 defaultSamples :: forall a b. (Generic a, Generic b, GToSample (Rep a) (Rep b)) => Proxy a -> [(Text, b)]
-defaultSamples _ = second to <$> gtoSamples (Proxy :: Proxy (Rep a))
+defaultSamples _ = Omega.runOmega $ second to <$> gtoSamples (Proxy :: Proxy (Rep a))
 
 class GToSample t s where
   gtoSample  :: proxy t -> Maybe (s x)
-  gtoSample _ = snd <$> listToMaybe (gtoSamples (Proxy :: Proxy t))
-  gtoSamples :: proxy t -> [(Text, s x)]
+  gtoSample _ = snd <$> listToMaybe (Omega.runOmega (gtoSamples (Proxy :: Proxy t)))
+  gtoSamples :: proxy t -> Omega.Omega (Text, s x)
   gtoSamples _ = maybe empty (pure . ("",)) (gtoSample (Proxy :: Proxy t))
 
 instance GToSample U1 U1 where
@@ -426,7 +427,7 @@ instance (GToSample p p', GToSample q q') => GToSample (p :+: q) (p' :+: q') whe
 
 instance ToSample a b => GToSample (K1 i a) (K1 i b) where
   gtoSample  _ = K1 <$> toSample (Proxy :: Proxy a)
-  gtoSamples _ = second K1 <$> toSamples (Proxy :: Proxy a)
+  gtoSamples _ = second K1 <$> Omega.each (toSamples (Proxy :: Proxy a))
 
 instance (GToSample f g) => GToSample (M1 i a f) (M1 i a g) where
   gtoSample  _ = M1 <$> gtoSample (Proxy :: Proxy f)

stack.yaml

@@ -14,4 +14,5 @@ packages:
 - servant-server/
 extra-deps:
 - engine-io-wai-1.0.2
+- control-monad-omega-0.3.1
 resolver: nightly-2015-09-10