diff --git a/builder/src/Deps/Solver.hs b/builder/src/Deps/Solver.hs
index dacfd559..d98a6ffd 100644
--- a/builder/src/Deps/Solver.hs
+++ b/builder/src/Deps/Solver.hs
@@ -26,6 +26,7 @@ import File qualified
 import Gren.Constraint qualified as C
 import Gren.Outline qualified as Outline
 import Gren.Package qualified as Pkg
+import Gren.Platform qualified as Platform
 import Gren.Version qualified as V
 import Json.Decode qualified as D
 import Reporting.Exit qualified as Exit
@@ -68,7 +69,7 @@ data Details
 
 verify ::
   Dirs.PackageCache ->
-  Outline.Platform ->
+  Platform.Platform ->
   Map.Map Pkg.Name C.Constraint ->
   IO (Result (Map.Map Pkg.Name Details))
 verify cache rootPlatform constraints =
@@ -145,14 +146,14 @@ getTransitive constraints solution unvisited visited =
 
 -- TRY
 
-try :: Outline.Platform -> Map.Map Pkg.Name C.Constraint -> Solver (Map.Map Pkg.Name V.Version)
+try :: Platform.Platform -> Map.Map Pkg.Name C.Constraint -> Solver (Map.Map Pkg.Name V.Version)
 try rootPlatform constraints =
   exploreGoals (Goals (NE.List rootPlatform []) constraints Map.empty)
 
 -- EXPLORE GOALS
 
 data Goals = Goals
-  { _compatible_platforms :: NE.List Outline.Platform,
+  { _compatible_platforms :: NE.List Platform.Platform,
     _pending :: Map.Map Pkg.Name C.Constraint,
     _solved :: Map.Map Pkg.Name V.Version
   }
diff --git a/builder/src/Gren/Details.hs b/builder/src/Gren/Details.hs
index f0fd0468..2d8aa2d6 100644
--- a/builder/src/Gren/Details.hs
+++ b/builder/src/Gren/Details.hs
@@ -43,6 +43,7 @@ import Gren.Kernel qualified as Kernel
 import Gren.ModuleName qualified as ModuleName
 import Gren.Outline qualified as Outline
 import Gren.Package qualified as Pkg
+import Gren.Platform qualified as Platform
 import Gren.Version qualified as V
 import Json.Encode qualified as E
 import Parse.Module qualified as Parse
@@ -212,7 +213,7 @@ checkAppDeps (Outline.AppOutline _ _ _ direct indirect) =
 
 verifyConstraints ::
   Env ->
-  Outline.Platform ->
+  Platform.Platform ->
   Map.Map Pkg.Name Con.Constraint ->
   Task (Map.Map Pkg.Name Solver.Details)
 verifyConstraints (Env _ _ _ cache) rootPlatform constraints =
diff --git a/builder/src/Gren/Outline.hs b/builder/src/Gren/Outline.hs
index e1da2f14..603eda00 100644
--- a/builder/src/Gren/Outline.hs
+++ b/builder/src/Gren/Outline.hs
@@ -8,7 +8,6 @@ module Gren.Outline
     PkgOutline (..),
     Exposed (..),
     SrcDir (..),
-    Platform (..),
     read,
     write,
     encode,
@@ -34,6 +33,7 @@ import Gren.Constraint qualified as Con
 import Gren.Licenses qualified as Licenses
 import Gren.ModuleName qualified as ModuleName
 import Gren.Package qualified as Pkg
+import Gren.Platform qualified as Platform
 import Gren.Version qualified as V
 import Json.Decode qualified as D
 import Json.Encode ((==>))
@@ -54,7 +54,7 @@ data Outline
 
 data AppOutline = AppOutline
   { _app_gren_version :: V.Version,
-    _app_platform :: Platform,
+    _app_platform :: Platform.Platform,
     _app_source_dirs :: NE.List SrcDir,
     _app_deps_direct :: Map.Map Pkg.Name V.Version,
     _app_deps_indirect :: Map.Map Pkg.Name V.Version
@@ -68,7 +68,7 @@ data PkgOutline = PkgOutline
     _pkg_exposed :: Exposed,
     _pkg_deps :: Map.Map Pkg.Name Con.Constraint,
     _pkg_gren_version :: Con.Constraint,
-    _pkg_platform :: Platform
+    _pkg_platform :: Platform.Platform
   }
 
 data Exposed
@@ -79,11 +79,6 @@ data SrcDir
   = AbsoluteSrcDir FilePath
   | RelativeSrcDir FilePath
 
-data Platform
-  = Common
-  | Browser
-  | Node
-
 -- DEFAULTS
 
 defaultSummary :: Json.String
@@ -114,7 +109,7 @@ encode outline =
     App (AppOutline gren platform srcDirs depsDirect depsTrans) ->
       E.object
         [ "type" ==> E.chars "application",
-          "platform" ==> encodePlatform platform,
+          "platform" ==> Platform.encode platform,
           "source-directories" ==> E.list encodeSrcDir (NE.toList srcDirs),
           "gren-version" ==> V.encode gren,
           "dependencies"
@@ -126,7 +121,7 @@ encode outline =
     Pkg (PkgOutline name summary license version exposed deps gren platform) ->
       E.object
         [ "type" ==> E.string (Json.fromChars "package"),
-          "platform" ==> encodePlatform platform,
+          "platform" ==> Platform.encode platform,
           "name" ==> Pkg.encode name,
           "summary" ==> E.string summary,
           "license" ==> Licenses.encode license,
@@ -158,13 +153,6 @@ encodeSrcDir srcDir =
     AbsoluteSrcDir dir -> E.chars dir
     RelativeSrcDir dir -> E.chars dir
 
-encodePlatform :: Platform -> E.Value
-encodePlatform platform =
-  case platform of
-    Common -> E.chars "common"
-    Browser -> E.chars "browser"
-    Node -> E.chars "node"
-
 -- PARSE AND VERIFY
 
 read :: FilePath -> IO (Either Exit.Outline Outline)
@@ -272,7 +260,7 @@ appDecoder :: Decoder AppOutline
 appDecoder =
   AppOutline
     <$> D.field "gren-version" versionDecoder
-    <*> D.field "platform" platformDecoder
+    <*> D.field "platform" Platform.decoder
     <*> D.field "source-directories" dirsDecoder
     <*> D.field "dependencies" (D.field "direct" (depsDecoder versionDecoder))
     <*> D.field "dependencies" (D.field "indirect" (depsDecoder versionDecoder))
@@ -287,7 +275,7 @@ pkgDecoder =
     <*> D.field "exposed-modules" exposedDecoder
     <*> D.field "dependencies" (depsDecoder constraintDecoder)
     <*> D.field "gren-version" constraintDecoder
-    <*> D.field "platform" platformDecoder
+    <*> D.field "platform" Platform.decoder
 
 -- JSON DECODE HELPERS
 
@@ -309,19 +297,6 @@ constraintDecoder :: Decoder Con.Constraint
 constraintDecoder =
   D.mapError Exit.OP_BadConstraint Con.decoder
 
-platformDecoder :: Decoder Platform
-platformDecoder =
-  let common = Json.fromChars "common"
-      browser = Json.fromChars "browser"
-      node = Json.fromChars "node"
-   in do
-        platform <- D.string
-        if
-            | platform == common -> D.succeed Common
-            | platform == browser -> D.succeed Browser
-            | platform == node -> D.succeed Node
-            | otherwise -> D.failure Exit.OP_BadPlatform
-
 depsDecoder :: Decoder a -> Decoder (Map.Map Pkg.Name a)
 depsDecoder valueDecoder =
   D.dict (Pkg.keyDecoder Exit.OP_BadDependencyName) valueDecoder
diff --git a/builder/src/Gren/Platform.hs b/builder/src/Gren/Platform.hs
new file mode 100644
index 00000000..bcf533d0
--- /dev/null
+++ b/builder/src/Gren/Platform.hs
@@ -0,0 +1,52 @@
+module Gren.Platform
+  ( Platform (..),
+    --
+    compatible,
+    --
+    encode,
+    decoder,
+  )
+where
+
+import Json.Decode qualified as D
+import Json.Encode qualified as E
+import Json.String qualified as Json
+import Reporting.Exit qualified as Exit
+
+data Platform
+  = Common
+  | Browser
+  | Node
+  deriving (Eq)
+
+-- COMPATIBILITY
+
+compatible :: Platform -> Platform -> Bool
+compatible rootPlatform comparison =
+  rootPlatform == comparison || comparison == Common
+
+-- JSON
+
+encode :: Platform -> E.Value
+encode platform =
+  case platform of
+    Common -> E.chars "common"
+    Browser -> E.chars "browser"
+    Node -> E.chars "node"
+
+decoder :: D.Decoder Exit.OutlineProblem Platform
+decoder =
+  let common = Json.fromChars "common"
+      browser = Json.fromChars "browser"
+      node = Json.fromChars "node"
+   in do
+        platform <- D.string
+        if platform == common
+          then D.succeed Common
+          else
+            if platform == browser
+              then D.succeed Browser
+              else
+                if platform == node
+                  then D.succeed Node
+                  else D.failure Exit.OP_BadPlatform
diff --git a/gren.cabal b/gren.cabal
index 7512713d..bc47156f 100644
--- a/gren.cabal
+++ b/gren.cabal
@@ -85,6 +85,7 @@ Common gren-common
 
         -- Gren things
         Gren.Outline
+        Gren.Platform
         Gren.Details
         --
         Gren.Compiler.Imports
diff --git a/terminal/src/Init.hs b/terminal/src/Init.hs
index 423a860e..e716bf46 100644
--- a/terminal/src/Init.hs
+++ b/terminal/src/Init.hs
@@ -15,6 +15,7 @@ import Gren.Constraint qualified as Con
 import Gren.Licenses qualified as Licenses
 import Gren.Outline qualified as Outline
 import Gren.Package qualified as Pkg
+import Gren.Platform qualified as Platform
 import Gren.Version qualified as V
 import Json.String qualified as Json
 import Reporting qualified
@@ -85,7 +86,7 @@ init flags =
         return $ Left $ Exit.InitNoCompatibleDependencies $ Just gitError
       Right deps -> do
         -- TODO: Make root platform customizable
-        result <- Solver.verify cache Outline.Browser deps
+        result <- Solver.verify cache Platform.Browser deps
         case result of
           Solver.Err exit ->
             return (Left (Exit.InitSolverProblem exit))
@@ -115,7 +116,7 @@ pkgOutline deps =
       (Outline.ExposedList [])
       deps
       Con.defaultGren
-      Outline.Browser
+      Platform.Browser
 
 appOutlineFromSolverDetails :: (Map.Map Pkg.Name Solver.Details) -> Outline.Outline
 appOutlineFromSolverDetails details =
@@ -126,7 +127,7 @@ appOutlineFromSolverDetails details =
    in Outline.App $
         Outline.AppOutline
           V.compiler
-          Outline.Browser
+          Platform.Browser
           (NE.List (Outline.RelativeSrcDir "src") [])
           directs
           indirects
diff --git a/terminal/src/Repl.hs b/terminal/src/Repl.hs
index e89b0697..6fcc59e0 100644
--- a/terminal/src/Repl.hs
+++ b/terminal/src/Repl.hs
@@ -42,6 +42,7 @@ import Gren.Licenses qualified as Licenses
 import Gren.ModuleName qualified as ModuleName
 import Gren.Outline qualified as Outline
 import Gren.Package qualified as Pkg
+import Gren.Platform qualified as Platform
 import Gren.Version qualified as V
 import Parse.Declaration qualified as PD
 import Parse.Expression qualified as PE
@@ -523,7 +524,7 @@ getRoot =
                     (Outline.ExposedList [])
                     compatibleDeps
                     C.defaultGren
-                    Outline.Browser
+                    Platform.Browser
 
               return root