add preferredColumn field to app type

fcore/app-type.sml

@@ -4,8 +4,13 @@ struct
     { buffer: LineGap.t
     , windowWidth: int
     , windowHeight: int
+    (* line to start drawing from *)
     , startLine: int
+    (* absolute index of movable cursor *)
     , cursorIdx: int
+    (* when moving cursor up or down a line, 
+     * move to this column if possible *)
+    , preferredColumn: int
     }
 
   fun init (buffer, windowWidth, windowHeight) : app_type =
@@ -14,5 +19,6 @@ struct
     , windowHeight = windowHeight
     , startLine = 0
     , cursorIdx = 0
+    , preferredColumn = 0
     }
 end

fcore/app-update.sml

@@ -8,7 +8,7 @@ struct
 
   fun resizeText (app: app_type, newWidth, newHeight) =
     let
-      val {buffer, windowWidth, windowHeight, startLine, cursorIdx} = app
+      val {buffer, windowWidth, windowHeight, startLine, cursorIdx, ...} = app
 
       val newBuffer = LineGap.goToLine (startLine, buffer)
       val drawMsg = TextBuilder.build
@@ -21,7 +21,7 @@ struct
 
   fun moveRight (app: app_type) =
     let
-      val {buffer, windowWidth, windowHeight, startLine, cursorIdx} = app
+      val {buffer, windowWidth, windowHeight, startLine, cursorIdx, ...} = app
 
       (* move LineGap to cursorIdx, which is necessary for finding newCursorIdx *)
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
@@ -39,7 +39,7 @@ struct
 
   fun moveLeft (app: app_type) =
     let
-      val {buffer, windowWidth, windowHeight, startLine, cursorIdx} = app
+      val {buffer, windowWidth, windowHeight, startLine, cursorIdx, ...} = app
 
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
       val cursorIdx = Cursor.viH (buffer, cursorIdx)

fcore/app-with.sml

@@ -4,27 +4,41 @@ struct
 
   fun bufferAndSize (app: app_type, newBuffer, newWidth, newHeight) =
     let
-      val {buffer = _, windowWidth = _, windowHeight = _, startLine, cursorIdx} =
-        app
+      val
+        { buffer = _
+        , windowWidth = _
+        , windowHeight = _
+        , startLine
+        , cursorIdx
+        , preferredColumn
+        } = app
     in
       { buffer = newBuffer
       , windowWidth = newWidth
       , windowHeight = newHeight
       , startLine = startLine
       , cursorIdx = cursorIdx
+      , preferredColumn = preferredColumn
       }
     end
 
   fun bufferAndCursorIdx (app: app_type, newBuffer, newCursorIdx) =
     let
-      val {buffer = _, cursorIdx = _, windowWidth, windowHeight, startLine} =
-        app
+      val
+        { buffer = _
+        , cursorIdx = _
+        , windowWidth
+        , windowHeight
+        , startLine
+        , preferredColumn
+        } = app
     in
       { buffer = newBuffer
       , cursorIdx = newCursorIdx
       , windowWidth = windowWidth
       , windowHeight = windowHeight
       , startLine = startLine
+      , preferredColumn = preferredColumn
       }
     end
 end