diff --git a/fcore/app-type.sml b/fcore/app-type.sml
new file mode 100644
index 0000000..2be927f
--- /dev/null
+++ b/fcore/app-type.sml
@@ -0,0 +1,4 @@
+structure AppType =
+struct
+
+end
diff --git a/fcore/buffer.sml b/fcore/text-builder.sml
similarity index 97%
rename from fcore/buffer.sml
rename to fcore/text-builder.sml
index 96f3bd0..fadb6ac 100644
--- a/fcore/buffer.sml
+++ b/fcore/text-builder.sml
@@ -1,4 +1,10 @@
-structure Buffer =
+signature TEXT_BUILDER =
+sig
+  val build: int * LineGap.t * int * int
+             -> Real32.real vector * LineGap.t
+end
+
+structure TextBuilder :> TEXT_BUILDER =
 struct
   val xSpace = 12
   val xSpace3 = xSpace * 3
@@ -169,7 +175,7 @@ buildTextString ( startIdx, rStrHd, [], 5, 5, 5
       Vector.concat acc
     end
 
-  fun startBuildTextLineGap
+  fun build
     (startLine, lineGap: LineGap.t, windowWidth, windowHeight) =
     let
       val lineGap = LineGap.goToLine (startLine, lineGap)
diff --git a/ffi/export.h b/ffi/export.h
index ec6f560..c210373 100644
--- a/ffi/export.h
+++ b/ffi/export.h
@@ -157,6 +157,7 @@ typedef Pointer Objptr;
 extern "C" {
 #endif
 
+MLLIB_PUBLIC(void mltonFramebufferSizeCallback (Int32 x0, Int32 x1);)
 
 #undef MLLIB_PRIVATE
 #undef MLLIB_PUBLIC
diff --git a/ffi/glfw-input.sml b/ffi/glfw-input.sml
index 1759120..bb8a843 100644
--- a/ffi/glfw-input.sml
+++ b/ffi/glfw-input.sml
@@ -14,4 +14,7 @@ struct
   val (RELEASE, _) =
     _symbol "RELEASE" public : ( unit -> int ) * ( int -> unit );
   val RELEASE = RELEASE ()
+
+  val exportFramebufferSizeCallback =
+    _export "mltonFramebufferSizeCallback" public : (int * int -> unit) -> unit;
 end
diff --git a/shell/shell.sml b/shell/shell.sml
index 2bc1e4e..9d6c94b 100644
--- a/shell/shell.sml
+++ b/shell/shell.sml
@@ -19,11 +19,11 @@ struct
       val _ = Gles3.loadGlad ()
 
       (* upload text vector *)
-      val io = TextIO.openIn "fcore/buffer.sml"
+      val io = TextIO.openIn "fcore/text-builder.sml"
       val lineGap = ioToLineGap (io, LineGap.empty)
       val _ = TextIO.closeIn io
 
-      val (textVec, _) = Buffer.startBuildTextLineGap (0, lineGap, 1920, 1080)
+      val (textVec, _) = TextBuilder.build (0, lineGap, 1920, 1080)
       val shellState = GlDraw.create window
       val shellState = GlDraw.uploadText (shellState, textVec)
 
diff --git a/shf b/shf
index a2f3311..325237c 100755
Binary files a/shf and b/shf differ
diff --git a/shf.mlb b/shf.mlb
index 4f8b375..f0ee053 100644
--- a/shf.mlb
+++ b/shf.mlb
@@ -5,7 +5,7 @@ lib/brolib-sml/src/line_gap.sml
 lib/cozette-sml/fonts/cozette-ascii.mlb
 
 (* FUNCTIONAL CORE *)
-fcore/buffer.sml
+fcore/text-builder.sml
 
 (* IMPERATIVE SHELL *)
 $(SML_LIB)/basis/mlton.mlb