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Add --only-preserve-backend flag to lamdera check #93

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Add --only-preserve-backend flag to lamdera check #93

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1 change: 1 addition & 0 deletions extra/Lamdera/CLI.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -90,6 +90,7 @@ check =
flags Lamdera.CLI.Check.Flags
|-- onOff "destructive-migration" "Generate a migration that will drop all production data when deployed, instead of the usual automatic migration generation."
|-- onOff "force" "Force a production check for Evergreen, even if we're on a non main/master branch (i.e. a preview). You shouldn't be using this unless you know what you're doing."
|-- onOff "only-preserve-backend" "Generate migrations that only preserve the BackendModel; all other types use ModelUnchanged/MsgUnchanged."
in
Terminal.Command "check" (Common summary) details example noArgs checkFlags Lamdera.CLI.Check.run

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9 changes: 5 additions & 4 deletions extra/Lamdera/CLI/Check.hs
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Expand Up @@ -59,15 +59,16 @@ data Flags =
Flags
{ _destructiveMigration :: Bool
, _force :: Bool
, _onlyPreserveBackend :: Bool
}


run_ :: IO ()
run_ = run () (Lamdera.CLI.Check.Flags { _destructiveMigration = False, _force = False })
run_ = run () (Lamdera.CLI.Check.Flags { _destructiveMigration = False, _force = False, _onlyPreserveBackend = False })


run :: () -> Lamdera.CLI.Check.Flags -> IO ()
run () flags@(Lamdera.CLI.Check.Flags destructiveMigration force) = do
run () flags@(Lamdera.CLI.Check.Flags destructiveMigration force onlyPreserveBackend) = do
debug_ "Starting check..."

inProduction_ <- Lamdera.inProduction
Expand All @@ -93,7 +94,7 @@ run () flags@(Lamdera.CLI.Check.Flags destructiveMigration force) = do


runHelp :: () -> Lamdera.CLI.Check.Flags -> IO ()
runHelp () flags@(Lamdera.CLI.Check.Flags destructiveMigration force) = do
runHelp () flags@(Lamdera.CLI.Check.Flags destructiveMigration force onlyPreserveBackend) = do
Lamdera.setCheckMode True

-- appNameEnvM <- Env.lookupEnv "LAMDERA_APP_NAME"
Expand Down Expand Up @@ -320,7 +321,7 @@ onlineCheck root appName inDebug localTypes externalTypeWarnings isHoistRebuild
then
Lamdera.Evergreen.MigrationDestructive.generate lastLocalTypeChangeVersion nextVersion typeCompares
else
Lamdera.Evergreen.MigrationGenerator.betweenVersions typeCompares lastLocalTypeChangeVersion nextVersion root
Lamdera.Evergreen.MigrationGenerator.betweenVersions onlyPreserveBackend typeCompares lastLocalTypeChangeVersion nextVersion root
writeUtf8 nextMigrationPath defaultMigrations

Progress.throw $
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15 changes: 9 additions & 6 deletions extra/Lamdera/Evergreen/MigrationGenerator.hs
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Expand Up @@ -33,8 +33,8 @@ import qualified Lamdera.Wire3.Helpers
import Lamdera.Evergreen.MigrationGeneratorHelpers
import Lamdera.Evergreen.MigrationSpecialCases

betweenVersions :: CoreTypeDiffs -> Int -> Int -> String -> IO Text
betweenVersions coreTypeDiffs oldVersion newVersion root = do
betweenVersions :: Bool -> CoreTypeDiffs -> Int -> Int -> String -> IO Text
betweenVersions onlyPreserveBackend coreTypeDiffs oldVersion newVersion root = do
let
paths = NE.List ("src/Evergreen/V" <> show oldVersion <> "/Types.elm") ["src/Evergreen/V" <> show newVersion <> "/Types.elm"]
moduleNameString = "Evergreen.V" <> show newVersion <> ".Types"
Expand All @@ -46,26 +46,29 @@ betweenVersions coreTypeDiffs oldVersion newVersion root = do
case Map.lookup (N.fromChars moduleNameString) interfaces of
Just interface -> do
debug $ "starting generatefor"
generateFor coreTypeDiffs oldVersion newVersion interfaces (interfaces Sanity.! (N.fromChars $ "Evergreen.V" <> show newVersion <> ".Types"))
generateFor onlyPreserveBackend coreTypeDiffs oldVersion newVersion interfaces (interfaces Sanity.! (N.fromChars $ "Evergreen.V" <> show newVersion <> ".Types"))

Nothing ->
error $ "Fatal: could not find the module `" <> moduleNameString <> "`, please report this issue in Discord with your project code."

pure $ Ext.ElmFormat.formatOrPassthrough res

generateFor :: CoreTypeDiffs -> Int -> Int -> Interfaces -> Interface.Interface -> IO Text
generateFor coreTypeDiffs oldVersion newVersion interfaces iface_Types = do
generateFor :: Bool -> CoreTypeDiffs -> Int -> Int -> Interfaces -> Interface.Interface -> IO Text
generateFor onlyPreserveBackend coreTypeDiffs oldVersion newVersion interfaces iface_Types = do
let
moduleName :: ModuleName.Canonical
moduleName = ModuleName.Canonical (Pkg.Name "author" "project") (N.fromChars $ "Evergreen.V" <> show newVersion <> ".Types")

migrationModuleText = T.concat ["Evergreen.Migrate.V", show_ newVersion]

typeDidChange t oldHash newHash =
oldHash /= newHash && not (onlyPreserveBackend && t /= N.fromChars "BackendModel")

@miniBill miniBill Apr 1, 2026

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We probably also want BackendMsg here?

@CharlonTank CharlonTank Apr 8, 2026

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This is true only if we have scheduled things right?

@MavenRain MavenRain Apr 8, 2026

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There is no scheduling dependency here. coreTypeDiffs always contains all six core types (from Lamdera.Types.core), built via a zipWith3 in Check.hs. Every type flows through generateFor; the predicate is evaluated per-type. When onlyPreserveBackend is True, any type that isn't BackendModel evaluates to False regardless of the hashes, which causes coreTypeMigration to emit the Unchanged wrapper (line 194). The flag is purely a filter on which hash comparisons matter, not on what gets processed.

@CharlonTank CharlonTank Apr 9, 2026

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FYI, my question was to @miniBill comment "We probably also want BackendMsg here?"


coreMigrations :: [(N.Name, Migration)]
coreMigrations =
coreTypeDiffs
& fmap (\(t, oldHash, newHash) ->
(t, coreTypeMigration (oldHash /= newHash) oldVersion newVersion interfaces moduleName t iface_Types)
(t, coreTypeMigration (typeDidChange t oldHash newHash) oldVersion newVersion interfaces moduleName t iface_Types)
)
pure $ migrationsToFile migrationModuleText oldVersion newVersion coreMigrations moduleName

Expand Down