Try out several things

src/TaskMachine/DateExpr.hs

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+{-# LANGUAGE OverloadedStrings #-}
+
+module TaskMachine.DateExpr
+  ( DateExpr
+  , parse
+  , save
+  ) where
+
+import Control.Exception
+
+import qualified Data.Text as T
+import qualified Database.SQLite.Simple           as DB
+import qualified Database.SQLite.Simple.FromField as DB
+import qualified Database.SQLite.Simple.Ok        as DB
+import qualified Database.SQLite.Simple.ToField   as DB
+
+data DateExpr = DummyValue
+
+parse :: T.Text -> Maybe DateExpr
+parse = const (Just DummyValue)
+
+save :: DateExpr -> T.Text
+save = const "dummy string"
+
+
+data DummyException = DummyException
+  deriving (Show)
+
+instance Exception DummyException
+
+instance DB.ToField DateExpr where
+  toField = DB.SQLText . save
+
+instance DB.FromField DateExpr where
+  fromField f = case DB.fromField f of
+    DB.Errors e -> DB.Errors e
+    DB.Ok text -> case parse text of
+      Nothing   -> DB.Errors [SomeException DummyException] -- TODO: Use proper exception
+      Just expr -> DB.Ok expr