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diff --git a/core/result.moon b/core/result.moon
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+import base from require 'core.cycle'
+
+-- Result of evaluating an expression
+-- carries (all optional):
+-- - a Value
+-- - an Op (to update)
+-- - children (results of subexpressions that were evaluated)
+-- - cached list of all Dispatchers affecting all Ops in the subtree
+--
+-- Results form a tree that controls execution order and message passing
+-- between Ops.
+class Result
+  -- params: table with optional keys op, value, children
+  new: (params={}) =>
+    @value = params.value
+    @op = params.op
+    @children = params.children or {}
+
+    @side_inputs, is_child = {}, {}
+    for child in *@children
+      for s, d in pairs child.side_inputs
+        @side_inputs[s] = d
+      if child.value
+        is_child[child.value] = true
+
+    if @op
+      for input in @op\all_inputs!
+        if input.impure or not is_child[input.stream]
+          @side_inputs[input.stream] = input
+
+  is_const: => not next @side_inputs
+
+  -- asserts value-constness and returns the value
+  const: (msg) =>
+    assert not (next @side_inputs), msg or "eval-time const expected"
+    @value
+
+  -- asserts a value exists and returns its type
+  type: =>
+    assert @value, "Result with value expected"
+    @value.type
+
+  -- create a value-copy of this result that has the same impulses but without
+  -- affecting the original's update logic
+  make_ref: =>
+    with Result value: @value
+      .side_inputs = @side_inputs
+
+  -- tick all IO instances that are effecting this (sub) tree
+  -- should be called once per frame on the root, right before tick
+  tick_io: =>
+    for stream, input in pairs @side_inputs
+      if input.__class == base.IOInput
+        io = input!
+        io\tick!
+
+  -- in depth-first order, tick all Ops who have dirty Stream inputs or impulses
+  --
+  -- short-circuits if there are no dirty Streams in the entire subtree
+  tick: =>
+    any_dirty = false
+    for stream, input in pairs @side_inputs
+      if input\dirty!
+        any_dirty = true
+        break
+
+    -- early-out if no streams are dirty in this whole subtree
+    return unless any_dirty
+
+    for child in *@children
+      child\tick!
+
+    if @op
+      -- we have to check self_dirty here, because streams from child
+      -- expressions might have changed
+      self_dirty = false
+      for stream in @op\all_inputs!
+        if stream\dirty!
+          self_dirty = true
+          break
+
+      L\trace "#{@op} is #{if self_dirty then 'dirty' else 'clean'}"
+      return unless self_dirty
+
+      @op\tick!
+
+-- static
+  __tostring: =>
+    buf = "<result=#{@value}"
+    buf ..= " #{@op}" if @op
+    buf ..= " (#{#@children} children)" if #@children > 0
+    buf ..= ">"
+    buf
+
+{
+  :Result
+}