Add tests for compliance with lambda spec

src/Text/Mustache.hs

@@ -94,6 +94,22 @@ not an instance of 'ToMustache' but an instance of 'ToJSON' you can use the
       ]
 @
 
+To define a mustache lambda, provide a `Text -> Text` function. This function
+takes in the rendered text and returns text that should be displayed instead.
+This doesn't completely implement the mustache spec for functions, but it should
+suffice for most use cases.
+
+For more advanced use cases, look at any 'ToMustache' instances for other
+function types.
+
+@
+  -- outputs: "Hello Alice"
+  substitute [mustache|{{#greeting}}{{name}}{{/greeting}}|] $ object
+    [ "name" ~> "Alice"
+    , "greeting" ~> \\t -> "Hello " <> t <> "!"
+    ]
+@
+
 All operators are also provided in a unicode form, for those that, like me, enjoy
 unicode operators.
 
@@ -192,6 +208,7 @@ import           Text.Mustache.Types
 import qualified Data.Text as T
 
 
+{-# DEPRECATED overText "Use toMustache instead" #-}
 -- | Creates a 'Lambda' which first renders the contained section and then applies the supplied function
 overText :: (T.Text -> T.Text) -> Value
-overText f = toMustache $ fmap (f . snd) . catchSubstitute . substituteAST
+overText = toMustache

src/Text/Mustache/Render.hs

@@ -245,3 +245,8 @@ lambdaHelper conv f = Lambda $ (<$> askContext) . wrapper
 
 instance ToMustache (STree -> SubM Text) where
   toMustache f = Lambda (fmap (return . TextBlock) . f)
+
+instance ToMustache (Text -> Text) where
+  toMustache f = Lambda $ \tree -> do
+    (_, res) <- catchSubstitute $ substituteAST tree
+    return [TextBlock $ f res]