diff --git a/src/App/Messages.purs b/src/App/Messages.purs
new file mode 100644
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--- /dev/null
+++ b/src/App/Messages.purs
@@ -0,0 +1,49 @@
+module App.Messages where
+
+import Effect (Effect)
+
+import Data.Argonaut.Core as J
+import Data.Codec.Argonaut as CA
+import Data.Either
+import Data.Codec.Argonaut.Record as CAR
+
+-- Base types.
+type Email = String
+type Password = String
+type Login = { email :: Email, password :: Password }
+type Token = { token :: String }
+
+-- Their related JSON codecs.
+login_codec ∷ CA.JsonCodec Login
+login_codec = CA.object "Login" (CAR.record { email: CA.string, password: CA.string })
+token_codec ∷ CA.JsonCodec Token
+token_codec = CA.object "Token" (CAR.record { token: CA.string })
+
+-- All possible requests.
+data RequestMessage
+  = MessageLogin Login
+
+-- All possible answers.
+data AnswerMessage
+  = Logged Token
+
+encodeRequest ∷ RequestMessage → J.Json
+encodeRequest m = case m of
+    (MessageLogin login) -> CA.encode login_codec login
+
+-- TODO
+--rawSerialize :: RequestMessage -> ArrayBuffer
+--rawDeserialize :: ArrayBuffer -> AnswerMessage
+
+data DecodeError
+  = JSONERROR CA.JsonDecodeError
+  | UnknownNumber
+
+decodeAnswer :: Int -> J.Json -> Either DecodeError AnswerMessage
+decodeAnswer number json
+  = case number of
+    -- TODO: take right values
+    11 -> case (CA.decode token_codec json) of
+      (Left err) -> Left (JSONERROR err)
+      (Right v)  -> Right (Logged v)
+    _ -> Left UnknownNumber