Use the migration helper to remove boilerplate

lib/Guide.hs

@@ -90,7 +90,7 @@ This application doesn't use a database – instead, it uses acid-state. Acid-st
 
   * acid-state has a nasty feature – when the state hasn't changed, 'createCheckpoint' appends it to the previous checkpoint. When state doesn't change for a long time, it means that checkpoints can grow to 100 MB or more. So, we employ a dirty bit and use createCheckpoint' instead of createCheckpoint. The former only creates the checkpoint if the dirty bit is set, which is good.
 
-  * When any type is changed, we have to write a migration function that would read the old version of the type and turn it into the new version. It's enough to keep just one old version (and even that isn't needed after the migration happened and a new checkpoint has been created). For examples, look at “instance Migrate” in Types.hs. Also, all types involved in acid-state (whether migrate-able or not) have to have a SafeCopy instance, which is generated by 'deriveSafeCopySimple'.
+  * When any type is changed, we have to write a migration function that would read the old version of the type and turn it into the new version. This is done by 'changelog' – you only need to provide the list of differences between the old type and the new type.
 
   * There are actually ways to access the state directly (GetGlobalState and SetGlobalState), but the latter should only be used when doing something one-off (e.g. if you need to migrate all IDs to a different ID scheme).
 

lib/Types.hs

@@ -165,7 +165,7 @@ Types.hs
 
   1. Fix all warnings about uninitialised fields that might appear (by e.g. providing a default value).
 
-  2. Update the migration code; see Note [acid-state]. (Usually updating the migration code means simply copying and pasting the old version of the type and adding “_n” to all fields, where ‘n’ is ‘previous n’ + 1.)
+  2. Update the migration code; see Note [acid-state].
 
   3. If the field is user-editable: add a new constructor to 'Edit' and update the migration code for 'Edit'. Update 'isVacuousEdit', too.
 
@@ -216,30 +216,16 @@ data Trait = Trait {
   deriving (Show, Generic)
 
 -- See Note [acid-state]
-deriveSafeCopySorted 3 'extension ''Trait
+deriveSafeCopySorted 4 'extension ''Trait
 makeFields ''Trait
 
+changelog ''Trait 4 []
+deriveSafeCopySorted 3 'base ''Trait_v3
+
 instance A.ToJSON Trait where
   toJSON = A.genericToJSON A.defaultOptions {
     A.fieldLabelModifier = over _head toLower . drop (T.length "_trait") }
 
--- Old version, needed for safe migration. It can most likely be already
--- deleted (if a checkpoint has been created), but it's been left here as a
--- template for future migrations.
---
--- Again, see Note [acid-state].
-data Trait_v2 = Trait_v2 {
-  _traitUid_v2 :: Uid Trait,
-  _traitContent_v2 :: MarkdownInline }
-
-deriveSafeCopySimple 2 'base ''Trait_v2
-
-instance Migrate Trait where
-  type MigrateFrom Trait = Trait_v2
-  migrate Trait_v2{..} = Trait {
-    _traitUid = _traitUid_v2,
-    _traitContent = _traitContent_v2 }
-
 --
 
 data ItemKind
@@ -293,50 +279,16 @@ data Item = Item {
   _itemKind        :: ItemKind }
   deriving (Show, Generic)
 
-deriveSafeCopySorted 10 'extension ''Item
+deriveSafeCopySorted 11 'extension ''Item
 makeFields ''Item
 
+changelog ''Item 11 []
+deriveSafeCopySorted 10 'base ''Item_v10
+
 instance A.ToJSON Item where
   toJSON = A.genericToJSON A.defaultOptions {
     A.fieldLabelModifier = over _head toLower . drop (T.length "_item") }
 
--- Old version, needed for safe migration. It can most likely be already
--- deleted (if a checkpoint has been created), but it's been left here as a
--- template for future migrations.
-data Item_v9 = Item_v9 {
-  _itemUid_v9         :: Uid Item,
-  _itemName_v9        :: Text,
-  _itemCreated_v9     :: UTCTime,
-  _itemGroup__v9      :: Maybe Text,
-  _itemDescription_v9 :: MarkdownBlock,
-  _itemPros_v9        :: [Trait],
-  _itemProsDeleted_v9 :: [Trait],
-  _itemCons_v9        :: [Trait],
-  _itemConsDeleted_v9 :: [Trait],
-  _itemEcosystem_v9   :: MarkdownBlock,
-  _itemNotes_v9       :: MarkdownBlockWithTOC,
-  _itemLink_v9        :: Maybe Url,
-  _itemKind_v9        :: ItemKind }
-
-deriveSafeCopySimple 9 'base ''Item_v9
-
-instance Migrate Item where
-  type MigrateFrom Item = Item_v9
-  migrate Item_v9{..} = Item {
-    _itemUid = _itemUid_v9,
-    _itemName = _itemName_v9,
-    _itemCreated = _itemCreated_v9,
-    _itemGroup_ = _itemGroup__v9,
-    _itemDescription = _itemDescription_v9,
-    _itemPros = _itemPros_v9,
-    _itemProsDeleted = _itemProsDeleted_v9,
-    _itemCons = _itemCons_v9,
-    _itemConsDeleted = _itemConsDeleted_v9,
-    _itemEcosystem = _itemEcosystem_v9,
-    _itemNotes = _itemNotes_v9,
-    _itemLink = _itemLink_v9,
-    _itemKind = _itemKind_v9 }
-
 --
 
 data Hue = NoHue | Hue Int
@@ -448,9 +400,12 @@ data Category = Category {
   _categoryItemsDeleted :: [Item] }
   deriving (Show, Generic)
 
-deriveSafeCopySorted 8 'extension ''Category
+deriveSafeCopySorted 9 'extension ''Category
 makeFields ''Category
 
+changelog ''Category 9 []
+deriveSafeCopySorted 8 'base ''Category_v8
+
 instance A.ToJSON Category where
   toJSON = A.genericToJSON A.defaultOptions {
     A.fieldLabelModifier = over _head toLower . drop (T.length "_category") }
@@ -459,39 +414,6 @@ categorySlug :: Category -> Text
 categorySlug category =
   T.format "{}-{}" (makeSlug (category^.title), category^.uid)
 
--- Old version, needed for safe migration. It can most likely be already
--- deleted (if a checkpoint has been created), but it's been left here as a
--- template for future migrations.
-data Category_v7 = Category_v7 {
-  _categoryUid_v7 :: Uid Category,
-  _categoryTitle_v7 :: Text,
-  _categoryGroup__v7 :: Text,
-  _categoryProsConsEnabled_v7 :: Bool,
-  _categoryEcosystemEnabled_v7 :: Bool,
-  _categoryCreated_v7 :: UTCTime,
-  _categoryStatus_v7 :: CategoryStatus,
-  _categoryNotes_v7 :: MarkdownBlock,
-  _categoryGroups_v7 :: Map Text Hue,
-  _categoryItems_v7 :: [Item],
-  _categoryItemsDeleted_v7 :: [Item] }
-
-deriveSafeCopySimple 7 'base ''Category_v7
-
-instance Migrate Category where
-  type MigrateFrom Category = Category_v7
-  migrate Category_v7{..} = Category {
-    _categoryUid = _categoryUid_v7,
-    _categoryTitle = _categoryTitle_v7,
-    _categoryGroup_ = _categoryGroup__v7,
-    _categoryProsConsEnabled = _categoryProsConsEnabled_v7,
-    _categoryEcosystemEnabled = _categoryEcosystemEnabled_v7,
-    _categoryCreated = _categoryCreated_v7,
-    _categoryStatus = _categoryStatus_v7,
-    _categoryNotes = _categoryNotes_v7,
-    _categoryGroups = _categoryGroups_v7,
-    _categoryItems = _categoryItems_v7,
-    _categoryItemsDeleted = _categoryItemsDeleted_v7 }
-
 -- Edits
 
 -- | Edits made by users. It should always be possible to undo an edit.
@@ -714,21 +636,10 @@ data EditDetails = EditDetails {
   editId   :: Int }
   deriving (Eq, Show)
 
-deriveSafeCopySorted 3 'extension ''EditDetails
+deriveSafeCopySorted 4 'extension ''EditDetails
 
-data EditDetails_v2 = EditDetails_v2 {
-  editIP_v2   :: Maybe IP,
-  editDate_v2 :: UTCTime,
-  editId_v2   :: Int }
-
-deriveSafeCopySimple 2 'base ''EditDetails_v2
-
-instance Migrate EditDetails where
-  type MigrateFrom EditDetails = EditDetails_v2
-  migrate EditDetails_v2{..} = EditDetails {
-    editIP = editIP_v2,
-    editDate = editDate_v2,
-    editId = editId_v2 }
+changelog ''EditDetails 4 []
+deriveSafeCopySorted 3 'base ''EditDetails_v3
 
 data Action
   = Action'MainPageVisit
@@ -751,24 +662,10 @@ data ActionDetails = ActionDetails {
   actionUserAgent :: Maybe Text }
   deriving (Show)
 
-deriveSafeCopySorted 2 'extension ''ActionDetails
+deriveSafeCopySorted 3 'extension ''ActionDetails
 
-data ActionDetails_v1 = ActionDetails_v1 {
-  actionIP_v1        :: Maybe IP,
-  actionDate_v1      :: UTCTime,
-  actionReferrer_v1  :: Maybe Referrer,
-  actionUserAgent_v1 :: Maybe Text }
-  deriving (Show)
-
-deriveSafeCopySimple 1 'base ''ActionDetails_v1
-
-instance Migrate ActionDetails where
-  type MigrateFrom ActionDetails = ActionDetails_v1
-  migrate ActionDetails_v1{..} = ActionDetails {
-    actionIP = actionIP_v1,
-    actionDate = actionDate_v1,
-    actionReferrer = actionReferrer_v1,
-    actionUserAgent = actionUserAgent_v1 }
+changelog ''ActionDetails 3 []
+deriveSafeCopySorted 2 'base ''ActionDetails_v2
 
 -- See Note [acid-state]
 
@@ -784,28 +681,11 @@ data GlobalState = GlobalState {
   _dirty :: Bool }
   deriving (Show)
 
-deriveSafeCopySorted 6 'extension ''GlobalState
+deriveSafeCopySorted 7 'extension ''GlobalState
 makeLenses ''GlobalState
 
-data GlobalState_v5 = GlobalState_v5 {
-  _categories_v5 :: [Category],
-  _categoriesDeleted_v5 :: [Category],
-  _actions_v5 :: [(Action, ActionDetails)],
-  _pendingEdits_v5 :: [(Edit, EditDetails)],
-  _editIdCounter_v5 :: Int,
-  _dirty_v5 :: Bool }
-
-deriveSafeCopySimple 5 'base ''GlobalState_v5
-
-instance Migrate GlobalState where
-  type MigrateFrom GlobalState = GlobalState_v5
-  migrate GlobalState_v5{..} = GlobalState {
-    _categories = _categories_v5,
-    _categoriesDeleted = _categoriesDeleted_v5,
-    _actions = _actions_v5,
-    _pendingEdits = _pendingEdits_v5,
-    _editIdCounter = _editIdCounter_v5,
-    _dirty = _dirty_v5 }
+changelog ''GlobalState 7 []
+deriveSafeCopySorted 6 'base ''GlobalState_v6
 
 addGroupIfDoesNotExist :: Text -> Map Text Hue -> Map Text Hue
 addGroupIfDoesNotExist g gs