Setup new terminal command: outdated.

gren.cabal

@@ -59,6 +59,7 @@ Common gren-common
         Package
         Package.Install
         Package.Uninstall
+        Package.Outdated
         Package.Bump
         Package.Diff
         Package.Validate

terminal/src/Package.hs

@@ -9,6 +9,7 @@ import Data.List qualified as List
 import Package.Bump qualified as Bump
 import Package.Diff qualified as Diff
 import Package.Install qualified as Install
+import Package.Outdated qualified as Outdated
 import Package.Uninstall qualified as Uninstall
 import Package.Validate qualified as Validate
 import Terminal
@@ -25,6 +26,7 @@ run =
     P.empty
     [ install,
       uninstall,
+      outdated,
       bump,
       diff,
       validate
@@ -107,6 +109,35 @@ uninstall =
           |-- onOff "yes" "Assume yes for all interactive prompts."
    in Terminal.Command "uninstall" Uncommon details example uninstallArgs uninstallFlags Uninstall.run
 
+-- OUTDATED
+
+outdated :: Terminal.Command
+outdated =
+  let details =
+        "The `outdated` command lists the latest version of your dependencies, if you're\
+        \ not already using them:"
+
+      example =
+        stack
+          [ reflow
+              "For example, if you want to list any outdated dependencies in your project,\
+              \ you would say:",
+            P.indent 4 $
+              P.green $
+                P.vcat $
+                  [ "gren package outdated"
+                  ]
+          ]
+
+      outdatedArgs =
+        require0 Outdated.NoArgs
+
+      outdatedFlags =
+        flags Outdated.Flags
+          |-- onOff "yes" "Assume yes for all interactive prompts."
+          |-- onOff "unsafe" "Show latest version, even if it contains breaking changes."
+   in Terminal.Command "outdated" Uncommon details example outdatedArgs outdatedFlags Outdated.run
+
 -- VALIDATE
 
 validate :: Terminal.Command

terminal/src/Package/Outdated.hs

@@ -0,0 +1,49 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Package.Outdated
+  ( Args (..),
+    Flags (..),
+    run,
+  )
+where
+
+import Deps.Solver qualified as Solver
+import Directories qualified as Dirs
+import Gren.Outline qualified as Outline
+import Reporting qualified
+import Reporting.Exit qualified as Exit
+import Reporting.Task qualified as Task
+
+-- RUN
+
+data Args
+  = NoArgs
+
+data Flags = Flags
+  { _skipPrompts :: Bool
+  , _unsafe :: Bool
+  }
+
+run :: Args -> Flags -> IO ()
+run _ (Flags _skipPrompts _unafe) =
+  Reporting.attempt Exit.installToReport $
+    do
+      maybeRoot <- Dirs.findRoot
+      case maybeRoot of
+        Nothing ->
+          return (Left Exit.InstallNoOutline)
+        Just root ->
+          Task.run $
+            do
+              env <- Task.io Solver.initEnv
+              oldOutline <- Task.eio Exit.InstallBadOutline $ Outline.read root
+              installDependencies env oldOutline
+
+-- LIST OUTDATED
+
+type Task = Task.Task Exit.Install
+
+installDependencies :: Solver.Env -> Outline.Outline -> Task ()
+installDependencies (Solver.Env _) _ =
+    return ()
+