Merge pull request #47 from Bernardoow/data-in-session

Cookbook to put data in session

cookbook/data-in-session.md

@@ -0,0 +1,97 @@
+# Adding Data Type in Session 
+
+This is a example using simple YesodWeb Template.
+
+### Json Step
+
+My suggestion is to use Json string to save in session.
+
+```
+data MyResponse = MyResponse {
+      msg :: Text
+} deriving (Show, Eq)
+
+instance ToJSON MyResponse where
+    toJSON (MyResponse x) = object [ "msg" .= x ]
+
+data Person = Person 
+    { age :: Int
+    }   
+    deriving (Show, Eq)
+
+instance FromJSON Person where
+    parseJSON = withObject "Person" $ \v -> Person
+        <$> v .: "age"
+
+instance ToJSON Person where
+  toJSON (Person age) = object ["age" .= age]
+
+  toEncoding (Person age) = pairs ("age" .= age )
+```
+
+### Turning Json into Text
+
+If we try to encode (from Data.Aeson) direct (encode $ person) an error is raised because encode return a ByteText and session must be Text.
+So we need to work with this. I used encodeToLazyText :: ByteText -> Data.Text.Lazy and then toStrict :: Data.Text.Lazy -> Text.
+
+```
+setSession "object"  ((toStrict $ encodeToLazyText $ person) :: Text) 
+```
+
+### Turning Text into Json
+
+When we want to do the way back we need to use fromStrict :: Text -> Data.Text.Internal.Lazy.Text, TL.encodeUtf8 :: Data.Text.Internal.Lazy.Text -> ByteString and then decode :: ByteString -> Maybe a
+
+```
+decode $ TL.encodeUtf8 $ fromStrict objectInSession :: Maybe Person
+```
+
+### Full ApiExample
+```
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+module Handler.ApiExample where
+import Import
+import Data.Aeson (withObject, pairs, decode)
+import Data.Aeson.Text (encodeToLazyText)
+import qualified Data.Text.Lazy.Encoding as TL
+import qualified Data.Aeson.Types as J
+
+data MyResponse = MyResponse {
+      msg :: Text
+} deriving (Show, Eq)
+
+instance ToJSON MyResponse where
+    toJSON (MyResponse x) = object [ "msg" .= x ]
+
+data Person = Person 
+    { age :: Int
+    }   
+    deriving (Show, Eq)
+
+instance FromJSON Person where
+    parseJSON = withObject "Person" $ \v -> Person
+        <$> v .: "age"
+
+instance ToJSON Person where
+  toJSON (Person age) = object ["age" .= age]
+
+  toEncoding (Person age) = pairs ("age" .= age )
+
+postExampleSetObjectInSessionR :: Handler Value
+postExampleSetObjectInSessionR = do
+    mayeObjectInSession <- lookupSession "object"
+    case mayeObjectInSession of
+        Nothing -> do
+            let person = Person 29
+            setSession "object"  ((toStrict $ encodeToLazyText $ person) :: Text)
+            sendStatusJSON status201 person
+        Just objectInSession ->
+            case (decode $ TL.encodeUtf8 $ fromStrict objectInSession :: Maybe Person) of
+                Just object -> do
+                    liftIO $ print object
+                    sendStatusJSON status201 object
+                Nothing ->
+                    sendStatusJSON status400 (MyResponse { msg = "Some error happend in decode"})
+```