diff --git a/bin/index.js b/bin/index.js
index 0f84b6442..320b090c2 100755
--- a/bin/index.js
+++ b/bin/index.js
@@ -131,6 +131,9 @@ const io = {
       this.send({ index, value: ioRefs[id].value });
     }
   },
+  mVectorRead: function (index, i, array) {
+    this.send({ index, value: array[i] });
+  },
   dirCreateDirectoryIfMissing: function (index, createParents, filename) {
     fs.mkdir(filename, { recursive: createParents }, (err) => {
       this.send({ index, value: null });
diff --git a/src/Compiler/Type/Solve.elm b/src/Compiler/Type/Solve.elm
index 1503b6b19..b36493706 100644
--- a/src/Compiler/Type/Solve.elm
+++ b/src/Compiler/Type/Solve.elm
@@ -406,7 +406,7 @@ This sorts variables into the young and old pools accordingly.
 -}
 generalize : Mark -> Mark -> Int -> Pools -> IO ()
 generalize youngMark visitMark youngRank pools =
-    IO.mVectorRead (Decode.list UF.variableDecoder) pools youngRank
+    IO.mVectorRead (Decode.list UF.variableDecoder) (Encode.list UF.variableEncoder) pools youngRank
         |> IO.bind
             (\youngVars ->
                 poolToRankTable youngMark youngRank youngVars
diff --git a/src/Data/IO.elm b/src/Data/IO.elm
index 240954d28..7f7a5a303 100644
--- a/src/Data/IO.elm
+++ b/src/Data/IO.elm
@@ -90,6 +90,7 @@ type Effect
     = Exit String Int
     | NewIORef Encode.Value
     | ReadIORef Int
+    | MVectorRead Int Encode.Value
     | WriteIORef Int Encode.Value
     | GetLine
     | HPutStr Handle String
@@ -355,17 +356,25 @@ mVectorWrite decoder encoder ioRef i x =
         (Array.set i (Just x))
 
 
-mVectorRead : Decode.Decoder a -> IORef (Array (Maybe a)) -> Int -> IO a
-mVectorRead decoder ioRef i =
+mVectorRead : Decode.Decoder a -> (a -> Encode.Value) -> IORef (Array (Maybe a)) -> Int -> IO a
+mVectorRead decoder encoder ioRef i =
     readIORef (Decode.array (Decode.maybe decoder)) ioRef
-        |> fmap
+        |> bind
             (\vector ->
-                case Maybe.join (Array.get i vector) of
-                    Just a ->
-                        a
-
-                    Nothing ->
-                        todo "Failed to find index on vector"
+                make decoder
+                    (MVectorRead i
+                        (Encode.array
+                            (\maybeValue ->
+                                case maybeValue of
+                                    Just value ->
+                                        encoder value
+
+                                    Nothing ->
+                                        Encode.null
+                            )
+                            vector
+                        )
+                    )
             )
 
 
diff --git a/src/Terminal/Main.elm b/src/Terminal/Main.elm
index cae862e74..a3ef35ef6 100644
--- a/src/Terminal/Main.elm
+++ b/src/Terminal/Main.elm
@@ -126,6 +126,16 @@ effectToCmd index portOut effect =
                         ]
                 }
 
+        IO.MVectorRead i array ->
+            portOut
+                { index = index
+                , value =
+                    Encode.object
+                        [ ( "fn", Encode.string "mVectorRead" )
+                        , ( "args", Encode.list identity [ Encode.int i, array ] )
+                        ]
+                }
+
         IO.WriteIORef id value ->
             portOut
                 { index = index
diff --git a/src/Utils/Main.elm b/src/Utils/Main.elm
index 0c4c74ca3..e2896afd2 100644
--- a/src/Utils/Main.elm
+++ b/src/Utils/Main.elm
@@ -752,8 +752,12 @@ fpMakeRelative root path =
 
 
 fpAddTrailingPathSeparator : FilePath -> FilePath
-fpAddTrailingPathSeparator _ =
-    todo "fpAddTrailingPathSeparator"
+fpAddTrailingPathSeparator path =
+    if String.endsWith "/" path then
+        path
+
+    else
+        path ++ "/"
 
 
 fpPathSeparator : Char