can specify scope to tree compile

moonscript/compile.lua

@@ -450,7 +450,6 @@ Block = (function()
   _base_0.__class = _class_0
   return _class_0
 end)()
-local RootBlock
 RootBlock = (function()
   local _parent_0 = Block
   local _base_0 = {
@@ -515,8 +514,11 @@ value = function(value)
   end
   return out
 end
-tree = function(tree)
-  local scope = RootBlock()
+tree = function(tree, scope)
+  if scope == nil then
+    scope = RootBlock()
+  end
+  assert(tree, "missing tree")
   local runner = coroutine.create(function()
     local _list_0 = tree
     for _index_0 = 1, #_list_0 do

moonscript/compile.moon

@@ -17,7 +17,7 @@ import concat, insert from table
 import pos_to_line, get_closest_line, trim from util
 
 export tree, value, format_error
-export Block
+export Block, RootBlock
 
 -- buffer for building up a line
 class Line
@@ -300,8 +300,8 @@ value = (value) ->
     out = \render!
   out
 
-tree = (tree) ->
-  scope = RootBlock!
+tree = (tree, scope=RootBlock!) ->
+  assert tree, "missing tree"
 
   runner = coroutine.create ->
     scope\stm line for line in *tree