diff --git a/lua/u/buffer.lua b/lua/u/buffer.lua
index 06424f4..41cb645 100644
--- a/lua/u/buffer.lua
+++ b/lua/u/buffer.lua
@@ -1,16 +1,21 @@
 local Range = require 'u.range'
+local Renderer = require 'u.renderer'.Renderer
 
 ---@class Buffer
 ---@field buf number
+---@field private renderer Renderer
 local Buffer = {}
 
 ---@param buf? number
 ---@return Buffer
 function Buffer.from_nr(buf)
   if buf == nil or buf == 0 then buf = vim.api.nvim_get_current_buf() end
-  local b = { buf = buf }
-  setmetatable(b, { __index = Buffer })
-  return b
+
+  local renderer = Renderer.new(buf)
+  return setmetatable({
+    buf = buf,
+    renderer = renderer,
+  }, { __index = Buffer })
 end
 
 ---@return Buffer
@@ -69,4 +74,13 @@ function Buffer:text_object(txt_obj, opts)
   return Range.from_text_object(txt_obj, opts)
 end
 
+--- @param event string|string[]
+--- @param opts vim.api.keyset.create_autocmd
+function Buffer:autocmd(event, opts)
+  vim.api.nvim_create_autocmd(event, vim.tbl_extend('force', opts, { buffer = self.buf }))
+end
+
+--- @param tree Tree
+function Buffer:render(tree) return self.renderer:render(tree) end
+
 return Buffer
diff --git a/lua/u/range.lua b/lua/u/range.lua
index b7d4d34..4ac45d3 100644
--- a/lua/u/range.lua
+++ b/lua/u/range.lua
@@ -1,9 +1,9 @@
 local Pos = require 'u.pos'
 local State = require 'u.state'
 
-local orig_on_yank = vim.highlight.on_yank
+local orig_on_yank = (vim.hl or vim.highlight).on_yank
 local on_yank_enabled = true;
-(vim.highlight --[[@as any]]).on_yank = function(opts)
+((vim.hl or vim.highlight) --[[@as any]]).on_yank = function(opts)
   if not on_yank_enabled then return end
   return orig_on_yank(opts)
 end
@@ -493,7 +493,7 @@ function Range:highlight(group, opts)
   State.run(self.start.buf, function(s)
     if not in_macro then s:track_winview() end
 
-    vim.highlight.range(
+    (vim.hl or vim.highlight).range(
       self.start.buf,
       ns,
       group,
diff --git a/lua/u/renderer.lua b/lua/u/renderer.lua
new file mode 100644
index 0000000..f4030b0
--- /dev/null
+++ b/lua/u/renderer.lua
@@ -0,0 +1,432 @@
+local utils = require 'u.utils'
+
+local M = {}
+
+--- @alias Tag { kind: 'tag'; name: string, attributes: table<string, unknown>, children: Tree }
+--- @alias Node nil | boolean | string | Tag
+--- @alias Tree Node | Node[]
+local TagMetaTable = {}
+
+--- @param name string
+--- @param attributes? table<string, any>
+--- @param children? Node | Node[]
+--- @return Tag
+function M.h(name, attributes, children)
+  return setmetatable({
+    kind = 'tag',
+    name = name,
+    attributes = attributes or {},
+    children = children,
+  }, TagMetaTable)
+end
+
+--------------------------------------------------------------------------------
+-- Renderer class
+--------------------------------------------------------------------------------
+--- @alias RendererExtmark { id?: number; start: [number, number]; stop: [number, number]; opts: any; tag: any }
+
+--- @class Renderer
+--- @field bufnr number
+--- @field ns number
+--- @field changedtick number
+--- @field old { lines: string[]; extmarks: RendererExtmark[] }
+--- @field curr { lines: string[]; extmarks: RendererExtmark[] }
+local Renderer = {}
+Renderer.__index = Renderer
+M.Renderer = Renderer
+
+--- @param x any
+--- @return boolean
+function Renderer.is_tag(x) return type(x) == 'table' and getmetatable(x) == TagMetaTable end
+
+--- @param x any
+--- @return boolean
+function Renderer.is_tag_arr(x)
+  if type(x) ~= 'table' then return false end
+  return #x == 0 or not Renderer.is_tag(x)
+end
+
+--- @param bufnr number|nil
+function Renderer.new(bufnr)
+  if bufnr == nil then bufnr = vim.api.nvim_get_current_buf() end
+
+  if vim.b[bufnr]._renderer_ns == nil then
+    vim.b[bufnr]._renderer_ns = vim.api.nvim_create_namespace('my.renderer:' .. tostring(bufnr))
+  end
+
+  local self = setmetatable({
+    bufnr = bufnr,
+    ns = vim.b[bufnr]._renderer_ns,
+    changedtick = 0,
+    old = { lines = {}, extmarks = {} },
+    curr = { lines = {}, extmarks = {} },
+  }, Renderer)
+  return self
+end
+
+--- @param opts {
+---   tree: Tree;
+---   on_tag?: fun(tag: Tag, start0: [number, number], stop0: [number, number]): any;
+--- }
+function Renderer.markup_to_lines(opts)
+  --- @type string[]
+  local lines = {}
+
+  local curr_line1 = 1
+  local curr_col1 = 1 -- exclusive: sits one position **beyond** the last inserted text
+  --- @param s string
+  local function put(s)
+    lines[curr_line1] = (lines[curr_line1] or '') .. s
+    curr_col1 = #lines[curr_line1] + 1
+  end
+  local function put_line()
+    table.insert(lines, '')
+    curr_line1 = curr_line1 + 1
+    curr_col1 = 1
+  end
+
+  --- @param node Node
+  local function visit(node)
+    if node == nil or type(node) == 'boolean' then return end
+
+    if type(node) == 'string' then
+      local node_lines = vim.split(node, '\n')
+      for lnum, s in ipairs(node_lines) do
+        if lnum > 1 then put_line() end
+        put(s)
+      end
+    elseif Renderer.is_tag(node) then
+      local start0 = { curr_line1 - 1, curr_col1 - 1 }
+
+      -- visit the children:
+      if Renderer.is_tag_arr(node.children) then
+        for _, child in ipairs(node.children) do
+          -- newlines are not controlled by array entries, do NOT output a line here:
+          visit(child)
+        end
+      else
+        visit(node.children)
+      end
+
+      local stop0 = { curr_line1 - 1, curr_col1 - 1 }
+      if opts.on_tag then opts.on_tag(node, start0, stop0) end
+    elseif Renderer.is_tag_arr(node) then
+      for _, child in ipairs(node) do
+        -- newlines are not controlled by array entries, do NOT output a line here:
+        visit(child)
+      end
+    end
+  end
+  visit(opts.tree)
+
+  return lines
+end
+
+--- @param opts {
+---   tree: string;
+---   format_tag?: fun(tag: Tag): string;
+--- }
+function Renderer.markup_to_string(opts) return table.concat(Renderer.markup_to_lines(opts), '\n') end
+
+--- @param tree Tree
+function Renderer:render(tree)
+  local changedtick = vim.b[self.bufnr].changedtick
+  if changedtick ~= self.changedtick then
+    self.curr = { lines = vim.api.nvim_buf_get_lines(self.bufnr, 0, -1, false) }
+    self.changedtick = changedtick
+  end
+
+  --- @type RendererExtmark[]
+  local extmarks = {}
+
+  --- @type string[]
+  local lines = Renderer.markup_to_lines {
+    tree = tree,
+
+    on_tag = function(tag, start0, stop0)
+      if tag.name == 'text' then
+        local hl = tag.attributes.hl
+        if type(hl) == 'string' then
+          tag.attributes.extmark = tag.attributes.extmark or {}
+          tag.attributes.extmark.hl_group = tag.attributes.extmark.hl_group or hl
+        end
+
+        local extmark = tag.attributes.extmark
+
+        -- Force creating an extmark if there are key handlers. To accurately
+        -- sense the bounds of the text, we need an extmark:
+        if tag.attributes.on_key or tag.attributes.on_typed then extmark = extmark or {} end
+
+        if extmark then
+          table.insert(extmarks, {
+            start = start0,
+            stop = stop0,
+            opts = extmark,
+            tag = tag,
+          })
+        end
+      end
+    end,
+  }
+
+  self.old = self.curr
+  self.curr = { lines = lines, extmarks = extmarks }
+  self:_reconcile()
+end
+
+--- @private
+--- @param info string
+--- @param start integer
+--- @param end_ integer
+--- @param strict_indexing boolean
+--- @param replacement string[]
+function Renderer:_set_lines(info, start, end_, strict_indexing, replacement)
+  self:_log { 'set_lines', self.bufnr, start, end_, strict_indexing, replacement }
+  vim.api.nvim_buf_set_lines(self.bufnr, start, end_, strict_indexing, replacement)
+  self:_log { 'after(' .. info .. ')', vim.api.nvim_buf_get_lines(self.bufnr, 0, -1, false) }
+end
+
+--- @private
+--- @param info string
+--- @param start_row integer
+--- @param start_col integer
+--- @param end_row integer
+--- @param end_col integer
+--- @param replacement string[]
+function Renderer:_set_text(info, start_row, start_col, end_row, end_col, replacement)
+  self:_log { 'set_text', self.bufnr, start_row, start_col, end_row, end_col, replacement }
+  vim.api.nvim_buf_set_text(self.bufnr, start_row, start_col, end_row, end_col, replacement)
+  self:_log { 'after(' .. info .. ')', vim.api.nvim_buf_get_lines(self.bufnr, 0, -1, false) }
+end
+
+--- @private
+function Renderer:_log(...)
+  --
+  -- vim.print(...)
+end
+
+--- @private
+function Renderer:_reconcile()
+  local line_changes = utils.levenshtein(self.old.lines, self.curr.lines)
+  self.old = self.curr
+
+  --
+  -- Step 1: morph the text to the desired state:
+  --
+  self:_log { line_changes = line_changes }
+  for _, line_change in ipairs(line_changes) do
+    local lnum0 = line_change.index - 1
+
+    if line_change.kind == 'add' then
+      self:_set_lines('add-line', lnum0, lnum0, true, { line_change.item })
+    elseif line_change.kind == 'change' then
+      -- Compute inter-line diff, and apply:
+      self:_log '--------------------------------------------------------------------------------'
+      local col_changes = utils.levenshtein(vim.split(line_change.from, ''), vim.split(line_change.to, ''))
+
+      for _, col_change in ipairs(col_changes) do
+        local cnum0 = col_change.index - 1
+        self:_log { line_change = col_change, cnum = cnum0, lnum = lnum0 }
+        if col_change.kind == 'add' then
+          self:_set_text('add-char', lnum0, cnum0, lnum0, cnum0, { col_change.item })
+        elseif col_change.kind == 'change' then
+          self:_set_text('change-char', lnum0, cnum0, lnum0, cnum0 + 1, { col_change.to })
+        elseif col_change.kind == 'delete' then
+          self:_set_text('del-char', lnum0, cnum0, lnum0, cnum0 + 1, {})
+        else
+          -- No change
+        end
+      end
+    elseif line_change.kind == 'delete' then
+      self:_set_lines('del-line', lnum0, lnum0 + 1, true, {})
+    else
+      -- No change
+    end
+  end
+  self.changedtick = vim.b[self.bufnr].changedtick
+
+  --
+  -- Step 2: reconcile extmarks:
+  --
+  -- Clear current extmarks:
+  vim.api.nvim_buf_clear_namespace(self.bufnr, self.ns, 0, -1)
+  -- Set current extmarks:
+  for _, extmark in ipairs(self.curr.extmarks) do
+    extmark.id = vim.api.nvim_buf_set_extmark(
+      self.bufnr,
+      self.ns,
+      extmark.start[1],
+      extmark.start[2],
+      vim.tbl_extend('force', {
+        id = extmark.id,
+        end_row = extmark.stop[1],
+        end_col = extmark.stop[2],
+      }, extmark.opts)
+    )
+  end
+
+  --
+  -- Step 3: setup an updated on_key handler:
+  --
+
+  vim.on_key(nil, self.ns)
+  vim.on_key(function(key, typed)
+    -- Discard if not in the current buffer:
+    if
+      -- do not capture keys in the wrong buffer:
+      vim.api.nvim_get_current_buf() ~= self.bufnr
+      -- do not capture keys in COMMAND mode:
+      or vim.startswith(vim.api.nvim_get_mode().mode, 'c')
+    then
+      return
+    end
+
+    -- find the tag with the smallest intersection that contains the cursor:
+    local pos0 = vim.api.nvim_win_get_cursor(0)
+    pos0[1] = pos0[1] - 1 -- make it actually 0-based
+    local pos_infos = self:get_pos_infos(pos0)
+
+    -- Check the attributes for each matching_tag and fire events if they are
+    -- listening:
+    for _, pos_info in ipairs(pos_infos) do
+      local tag = pos_info.tag
+
+      -- is the tag listening?
+      if tag.attributes.on_key and type(tag.attributes.on_key[key]) == 'function' then
+        -- key:
+        local result = tag.attributes.on_key[key]()
+        if result == '' then return '' end
+      elseif tag.attributes.on_typed and type(tag.attributes.on_typed[typed]) == 'function' then
+        -- typed:
+        local result = tag.attributes.on_typed[typed]()
+        if result == '' then return '' end
+      end
+    end
+  end, self.ns)
+end
+
+--- Returns pairs of extmarks and tags associate with said extmarks. The
+--- returned tags/extmarks are sorted smallest (innermost) to largest
+--- (outermost).
+---
+--- @private (private for now)
+--- @param pos0 [number; number]
+--- @return { extmark: RendererExtmark; tag: Tag; }[]
+function Renderer:get_pos_infos(pos0)
+  local cursor_line0, cursor_col0 = pos0[1], pos0[2]
+
+  -- The cursor (block) occupies **two** extmark spaces: one for it's left
+  -- edge, and one for it's right. We need to do our own intersection test,
+  -- because the NeoVim API is over-inclusive in what it returns:
+  --- @type RendererExtmark[]
+  local intersecting_extmarks = vim
+    .iter(vim.api.nvim_buf_get_extmarks(self.bufnr, self.ns, pos0, pos0, { details = true, overlap = true }))
+    --- @return RendererExtmark
+    :map(function(ext)
+      --- @type number, number, number, { end_row?: number; end_col?: number }|nil
+      local id, line0, col0, details = unpack(ext)
+      local start = { line0, col0 }
+      local stop = { line0, col0 }
+      if details and details.end_row ~= nil and details.end_col ~= nil then
+        stop = { details.end_row, details.end_col }
+      end
+      return { id = id, start = start, stop = stop, opts = details }
+    end)
+    --- @param ext RendererExtmark
+    :filter(function(ext)
+      if ext.stop[1] ~= nil and ext.stop[2] ~= nil then
+        return cursor_line0 >= ext.start[1]
+          and cursor_col0 >= ext.start[2]
+          and cursor_line0 <= ext.stop[1]
+          and cursor_col0 < ext.stop[2]
+      else
+        return true
+      end
+    end)
+    :totable()
+
+  -- Sort the tags into smallest (inner) to largest (outer):
+  table.sort(
+    intersecting_extmarks,
+    --- @param x1 RendererExtmark
+    --- @param x2 RendererExtmark
+    function(x1, x2)
+      if
+        x1.start[1] == x2.start[1]
+        and x1.start[2] == x2.start[2]
+        and x1.stop[1] == x2.stop[1]
+        and x1.stop[2] == x2.stop[2]
+      then
+        return x1.id < x2.id
+      end
+
+      return x1.start[1] >= x2.start[1]
+        and x1.start[2] >= x2.start[2]
+        and x1.stop[1] <= x2.stop[1]
+        and x1.stop[2] <= x2.stop[2]
+    end
+  )
+
+  -- When we set the extmarks in the step above, we captured the IDs of the
+  -- created extmarks in self.curr.extmarks, which also has which tag each
+  -- extmark is associated with. Cross-reference with that list to get a list
+  -- of tags that we need to fire events for:
+  --- @type { extmark: RendererExtmark; tag: Tag }[]
+  local matching_tags = vim
+    .iter(intersecting_extmarks)
+    --- @param ext RendererExtmark
+    :map(function(ext)
+      for _, extmark_cache in ipairs(self.curr.extmarks) do
+        if extmark_cache.id == ext.id then return { extmark = ext, tag = extmark_cache.tag } end
+      end
+    end)
+    :totable()
+
+  return matching_tags
+end
+
+--------------------------------------------------------------------------------
+-- TreeBuilder class
+--------------------------------------------------------------------------------
+
+--- @class TreeBuilder
+--- @field private nodes Node[]
+local TreeBuilder = {}
+TreeBuilder.__index = TreeBuilder
+M.TreeBuilder = TreeBuilder
+
+function TreeBuilder.new()
+  local self = setmetatable({ nodes = {} }, TreeBuilder)
+  return self
+end
+
+--- @param nodes Tree
+--- @return TreeBuilder
+function TreeBuilder:put(nodes)
+  table.insert(self.nodes, nodes)
+  return self
+end
+
+--- @param name string
+--- @param attributes? table<string, any>
+--- @param children? Node | Node[]
+--- @return TreeBuilder
+function TreeBuilder:put_h(name, attributes, children)
+  local tag = M.h(name, attributes, children)
+  table.insert(self.nodes, tag)
+  return self
+end
+
+--- @param fn fun(TreeBuilder): any
+--- @return TreeBuilder
+function TreeBuilder:nest(fn)
+  local nested_writer = TreeBuilder.new()
+  fn(nested_writer)
+  table.insert(self.nodes, nested_writer.nodes)
+  return self
+end
+
+--- @return Tree
+function TreeBuilder:tree() return self.nodes end
+
+return M
diff --git a/lua/u/tracker.lua b/lua/u/tracker.lua
new file mode 100644
index 0000000..5bfe5f0
--- /dev/null
+++ b/lua/u/tracker.lua
@@ -0,0 +1,294 @@
+local M = {}
+
+M.debug = false
+
+--------------------------------------------------------------------------------
+-- class Signal
+--------------------------------------------------------------------------------
+
+--- @class Signal
+--- @field name? string
+--- @field private changing boolean
+--- @field private value any
+--- @field private subscribers table<function, boolean>
+--- @field private on_dispose_callbacks function[]
+local Signal = {}
+M.Signal = Signal
+Signal.__index = Signal
+
+--- @param value any
+--- @param name? string
+--- @return Signal
+function Signal:new(value, name)
+  local obj = setmetatable({
+    name = name,
+    changing = false,
+    value = value,
+    subscribers = {},
+    on_dispose_callbacks = {},
+  }, self)
+  return obj
+end
+
+--- @param value any
+function Signal:set(value)
+  self.value = value
+
+  -- We don't handle cyclic updates:
+  if self.changing then
+    if M.debug then
+      vim.notify('circular dependency detected' .. (self.name and (' in ' .. self.name) or ''), vim.log.levels.WARN)
+    end
+    return
+  end
+
+  local prev_changing = self.changing
+  self.changing = true
+  local ok = true
+  local err = nil
+  for _, cb in ipairs(self.subscribers) do
+    local ok2, err2 = pcall(cb, value)
+    if not ok2 then
+      ok = false
+      err = err or err2
+    end
+  end
+  self.changing = prev_changing
+
+  if not ok then
+    vim.notify(
+      'error notifying' .. (self.name and (' in ' .. self.name) or '') .. ': ' .. tostring(err),
+      vim.log.levels.WARN
+    )
+    error(err)
+  end
+end
+
+--- @return any
+function Signal:get()
+  local ctx = M.ExecutionContext.current()
+  if ctx then ctx:track(self) end
+  return self.value
+end
+
+--- @param fn function
+function Signal:update(fn) self:set(fn(self.value)) end
+
+--- @generic U
+--- @param fn fun(value: T): U
+--- @return Signal --<U>
+function Signal:map(fn)
+  local mapped_signal = M.create_memo(function()
+    local value = self:get()
+    return fn(value)
+  end, self.name and self.name .. ':mapped' or nil)
+  return mapped_signal
+end
+
+--- @return Signal
+function Signal:clone()
+  return self:map(function(x) return x end)
+end
+
+--- @param fn fun(value: T): boolean
+--- @return Signal -- <T>
+function Signal:filter(fn)
+  local filtered_signal = M.create_signal(nil, self.name and self.name .. ':filtered' or nil)
+  local unsubscribe_from_self = self:subscribe(function(value)
+    if fn(value) then filtered_signal:set(value) end
+  end)
+  filtered_signal:on_dispose(unsubscribe_from_self)
+  return filtered_signal
+end
+
+--- @param ms number
+--- @return Signal -- <T>
+function Signal:debounce(ms)
+  local function set_timeout(timeout, callback)
+    local timer = (vim.uv or vim.loop).new_timer()
+    timer:start(timeout, 0, function()
+      timer:stop()
+      timer:close()
+      callback()
+    end)
+    return timer
+  end
+
+  local filtered = M.create_signal(self.value, self.name and self.name .. ':debounced' or nil)
+
+  --- @type {
+  --   queued: { value: T, ts: number }[]
+  --   timer?: uv_timer_t
+  -- }
+  local state = { queued = {}, timer = nil }
+  local function clear_timeout()
+    if state.timer == nil then return end
+    pcall(function()
+      state.timer:stop()
+      state.timer:close()
+    end)
+    state.timer = nil
+  end
+
+  local unsubscribe_from_self = self:subscribe(function(value)
+    -- Stop any previously running timer:
+    if state.timer then clear_timeout() end
+    local now_ms = (vim.uv or vim.loop).hrtime() / 1e6
+
+    -- If there is anything older than `ms` in our queue, emit it:
+    local older_than_ms = vim.iter(state.queued):filter(function(item) return now_ms - item.ts > ms end):totable()
+    local last_older_than_ms = older_than_ms[#older_than_ms]
+    if last_older_than_ms then
+      filtered:set(last_older_than_ms.value)
+      state.queued = {}
+    end
+
+    -- overwrite anything young enough
+    table.insert(state.queued, { value = value, ts = now_ms })
+    state.timer = set_timeout(ms, function()
+      filtered:set(value)
+      -- If a timer was allowed to run to completion, that means that no other
+      -- item has been queued, since the timer is reset every time a new item
+      -- comes in. This means we can reset the queue
+      clear_timeout()
+      state.queued = {}
+    end)
+  end)
+  filtered:on_dispose(unsubscribe_from_self)
+
+  return filtered
+end
+
+--- @param callback function
+function Signal:subscribe(callback)
+  table.insert(self.subscribers, callback)
+  return function() self:unsubscribe(callback) end
+end
+
+--- @param callback function
+function Signal:on_dispose(callback) table.insert(self.on_dispose_callbacks, callback) end
+
+--- @param callback function
+function Signal:unsubscribe(callback)
+  for i, cb in ipairs(self.subscribers) do
+    if cb == callback then
+      table.remove(self.subscribers, i)
+      break
+    end
+  end
+end
+
+function Signal:dispose()
+  self.subscribers = {}
+  for _, callback in ipairs(self.on_dispose_callbacks) do
+    callback()
+  end
+end
+
+--------------------------------------------------------------------------------
+-- class ExecutionContext
+--------------------------------------------------------------------------------
+
+CURRENT_CONTEXT = nil
+
+--- @class ExecutionContext
+--- @field signals table<Signal, boolean>
+local ExecutionContext = {}
+M.ExecutionContext = ExecutionContext
+ExecutionContext.__index = ExecutionContext
+
+--- @return ExecutionContext
+function ExecutionContext:new()
+  return setmetatable({
+    signals = {},
+    subscribers = {},
+  }, ExecutionContext)
+end
+
+function ExecutionContext.current() return CURRENT_CONTEXT end
+
+--- @param fn function
+--- @param ctx ExecutionContext
+function ExecutionContext:run(fn, ctx)
+  local oldCtx = CURRENT_CONTEXT
+  CURRENT_CONTEXT = ctx
+  local result
+  local success, err = pcall(function() result = fn() end)
+
+  CURRENT_CONTEXT = oldCtx
+
+  if not success then error(err) end
+
+  return result
+end
+
+function ExecutionContext:track(signal) self.signals[signal] = true end
+
+--- @param callback function
+function ExecutionContext:subscribe(callback)
+  local wrapped_callback = function() callback() end
+  for signal in pairs(self.signals) do
+    signal:subscribe(wrapped_callback)
+  end
+
+  return function()
+    for signal in pairs(self.signals) do
+      signal:unsubscribe(wrapped_callback)
+    end
+  end
+end
+
+function ExecutionContext:dispose()
+  for signal, _ in pairs(self.signals) do
+    signal:dispose()
+  end
+  self.signals = {}
+end
+
+--------------------------------------------------------------------------------
+-- Helpers
+--------------------------------------------------------------------------------
+
+--- @param value any
+--- @param name? string
+--- @return Signal
+function M.create_signal(value, name) return Signal:new(value, name) end
+
+--- @param fn function
+--- @param name? string
+--- @return Signal
+function M.create_memo(fn, name)
+  --- @type Signal
+  local result
+  local unsubscribe = M.create_effect(function()
+    local value = fn()
+    if name and M.debug then vim.notify(name) end
+    if result then
+      result:set(value)
+    else
+      result = M.create_signal(value, name and ('m.s:' .. name) or nil)
+    end
+  end, name)
+  result:on_dispose(unsubscribe)
+  return result
+end
+
+--- @param fn function
+--- @param name? string
+function M.create_effect(fn, name)
+  local ctx = M.ExecutionContext:new()
+  M.ExecutionContext:run(fn, ctx)
+  return ctx:subscribe(function()
+    if name and M.debug then
+      local deps = vim
+        .iter(vim.tbl_keys(ctx.signals))
+        :map(function(s) return s.name end)
+        :filter(function(nm) return nm ~= nil end)
+        :join ','
+      vim.notify(name .. '(deps=' .. deps .. ')')
+    end
+    fn()
+  end)
+end
+
+return M
diff --git a/lua/u/utils.lua b/lua/u/utils.lua
index 228a9d2..ffe1d88 100644
--- a/lua/u/utils.lua
+++ b/lua/u/utils.lua
@@ -8,6 +8,18 @@ local M = {}
 ---@alias KeyMaps table<string, fun(): any | string> }
 ---@alias CmdArgs { args: string; bang: boolean; count: number; fargs: string[]; line1: number; line2: number; mods: string; name: string; range: 0|1|2; reg: string; smods: any; info: Range|nil }
 
+--- @generic T
+--- @param x `T`
+--- @param message? string
+--- @return T
+function M.dbg(x, message)
+  local t = {}
+  if message ~= nil then table.insert(t, message) end
+  table.insert(t, x)
+  vim.print(t)
+  return x
+end
+
 --- A utility for creating user commands that also pre-computes useful information
 --- and attaches it to the arguments.
 ---
@@ -107,28 +119,97 @@ function M.repeatablemap(mode, lhs, rhs, opts)
   end, vim.tbl_extend('force', opts or {}, { expr = true }))
 end
 
-function M.get_editor_dimensions()
-  local w = 0
-  local h = 0
-  local tabnr = vim.api.nvim_get_current_tabpage()
-  for _, winid in ipairs(vim.api.nvim_list_wins()) do
-    local tabpage = vim.api.nvim_win_get_tabpage(winid)
-    if tabpage == tabnr then
-      local pos = vim.api.nvim_win_get_position(winid)
-      local r, c = pos[1], pos[2]
-      local win_w = vim.api.nvim_win_get_width(winid)
-      local win_h = vim.api.nvim_win_get_height(winid)
-      local right = c + win_w
-      local bottom = r + win_h
-      if right > w then w = right end
-      if bottom > h then h = bottom end
+function M.get_editor_dimensions() return { width = vim.go.columns, height = vim.go.lines } end
+
+--- @alias LevenshteinChange<T> ({ kind: 'add'; item: T; index: number; } | { kind: 'delete'; item: T; index: number; } | { kind: 'change'; from: T; to: T; index: number; })
+--- @private
+--- @generic T
+--- @param x `T`[]
+--- @param y T[]
+--- @param cost? { of_delete?: fun(x: T): number; of_add?: fun(x: T): number; of_change?: fun(x: T, y: T): number; }
+--- @return LevenshteinChange<T>[]
+function M.levenshtein(x, y, cost)
+  cost = cost or {}
+  local cost_of_delete_f = cost.of_delete or function() return 1 end
+  local cost_of_add_f = cost.of_add or function() return 1 end
+  local cost_of_change_f = cost.of_change or function() return 1 end
+
+  local m, n = #x, #y
+  -- Initialize the distance matrix
+  local dp = {}
+  for i = 0, m do
+    dp[i] = {}
+    for j = 0, n do
+      dp[i][j] = 0
     end
   end
-  if w == 0 or h == 0 then
-    w = vim.api.nvim_win_get_width(0)
-    h = vim.api.nvim_win_get_height(0)
+
+  -- Fill the base cases
+  for i = 0, m do
+    dp[i][0] = i
   end
-  return { width = w, height = h }
+  for j = 0, n do
+    dp[0][j] = j
+  end
+
+  -- Compute the Levenshtein distance dynamically
+  for i = 1, m do
+    for j = 1, n do
+      if x[i] == y[j] then
+        dp[i][j] = dp[i - 1][j - 1] -- no cost if items are the same
+      else
+        local costDelete = dp[i - 1][j] + cost_of_delete_f(x[i])
+        local costAdd = dp[i][j - 1] + cost_of_add_f(y[j])
+        local costChange = dp[i - 1][j - 1] + cost_of_change_f(x[i], y[j])
+        dp[i][j] = math.min(costDelete, costAdd, costChange)
+      end
+    end
+  end
+
+  -- Backtrack to find the changes
+  local i = m
+  local j = n
+  --- @type LevenshteinChange[]
+  local changes = {}
+
+  while i > 0 or j > 0 do
+    local default_cost = dp[i][j]
+    local cost_of_change = (i > 0 and j > 0) and dp[i - 1][j - 1] or default_cost
+    local cost_of_add = j > 0 and dp[i][j - 1] or default_cost
+    local cost_of_delete = i > 0 and dp[i - 1][j] or default_cost
+
+    --- @param u number
+    --- @param v number
+    --- @param w number
+    local function is_first_min(u, v, w) return u <= v and u <= w end
+
+    if is_first_min(cost_of_change, cost_of_add, cost_of_delete) then
+      -- potential change
+      if x[i] ~= y[j] then
+        --- @type LevenshteinChange
+        local change = { kind = 'change', from = x[i], index = i, to = y[j] }
+        table.insert(changes, change)
+      end
+      i = i - 1
+      j = j - 1
+    elseif is_first_min(cost_of_add, cost_of_change, cost_of_delete) then
+      -- addition
+      --- @type LevenshteinChange
+      local change = { kind = 'add', item = y[j], index = i + 1 }
+      table.insert(changes, change)
+      j = j - 1
+    elseif is_first_min(cost_of_delete, cost_of_change, cost_of_add) then
+      -- deletion
+      --- @type LevenshteinChange
+      local change = { kind = 'delete', item = x[i], index = i }
+      table.insert(changes, change)
+      i = i - 1
+    else
+      error 'unreachable'
+    end
+  end
+
+  return changes
 end
 
 return M
diff --git a/spec/tracker_spec.lua b/spec/tracker_spec.lua
new file mode 100644
index 0000000..5da1a85
--- /dev/null
+++ b/spec/tracker_spec.lua
@@ -0,0 +1,206 @@
+local tracker = require 'u.tracker'
+local Signal = tracker.Signal
+local ExecutionContext = tracker.ExecutionContext
+
+describe('Signal', function()
+  local signal
+
+  before_each(function() signal = Signal:new(0, 'testSignal') end)
+
+  it('should initialize with correct parameters', function()
+    assert.is.equal(signal.value, 0)
+    assert.is.equal(signal.name, 'testSignal')
+    assert.is.not_nil(signal.subscribers)
+    assert.is.equal(#signal.subscribers, 0)
+    assert.is.equal(signal.changing, false)
+  end)
+
+  it('should set new value and notify subscribers', function()
+    local called = false
+    signal:subscribe(function(value)
+      called = true
+      assert.is.equal(value, 42)
+    end)
+
+    signal:set(42)
+    assert.is.equal(called, true)
+  end)
+
+  it('should not notify subscribers during circular dependency', function()
+    signal.changing = true
+    local notified = false
+
+    signal:subscribe(function() notified = true end)
+
+    signal:set(42)
+    assert.is.equal(notified, false) -- No notification should occur
+  end)
+
+  it('should get current value', function()
+    signal:set(100)
+    assert.is.equal(signal:get(), 100)
+  end)
+
+  it('should update value with function', function()
+    signal:set(10)
+    signal:update(function(value) return value * 2 end)
+    assert.is.equal(signal:get(), 20)
+  end)
+
+  it('should dispose subscribers', function()
+    local called = false
+    local unsubscribe = signal:subscribe(function() called = true end)
+
+    unsubscribe()
+    signal:set(10)
+    assert.is.equal(called, false) -- Should not be notified
+  end)
+
+  describe('Signal:map', function()
+    it('should transform the signal value', function()
+      local signal = Signal:new(5)
+      local mapped_signal = signal:map(function(value) return value * 2 end)
+
+      assert.is.equal(mapped_signal:get(), 10) -- Initial transformation
+      signal:set(10)
+      assert.is.equal(mapped_signal:get(), 20) -- Updated transformation
+    end)
+
+    it('should handle empty transformations', function()
+      local signal = Signal:new(nil)
+      local mapped_signal = signal:map(function(value) return value or 'default' end)
+
+      assert.is.equal(mapped_signal:get(), 'default') -- Return default
+      signal:set 'new value'
+      assert.is.equal(mapped_signal:get(), 'new value') -- Return new value
+    end)
+  end)
+
+  describe('Signal:filter', function()
+    it('should only emit values that pass the filter', function()
+      local signal = Signal:new(5)
+      local filtered_signal = signal:filter(function(value) return value > 10 end)
+
+      assert.is.equal(filtered_signal:get(), nil) -- Initial value should not pass
+      signal:set(15)
+      assert.is.equal(filtered_signal:get(), 15) -- Now filtered
+      signal:set(8)
+      assert.is.equal(filtered_signal:get(), 15) -- Does not pass the filter
+    end)
+
+    it('should handle empty initial values', function()
+      local signal = Signal:new(nil)
+      local filtered_signal = signal:filter(function(value) return value ~= nil end)
+
+      assert.is.equal(filtered_signal:get(), nil) -- Should be nil
+      signal:set(10)
+      assert.is.equal(filtered_signal:get(), 10) -- Should pass now
+    end)
+  end)
+
+  describe('create_memo', function()
+    it('should compute a derived value and update when dependencies change', function()
+      local signal = Signal:new(2)
+      local memoized_signal = tracker.create_memo(function() return signal:get() * 2 end)
+
+      assert.is.equal(memoized_signal:get(), 4) -- Initially compute 2 * 2
+
+      signal:set(3)
+      assert.is.equal(memoized_signal:get(), 6) -- Update to 3 * 2 = 6
+
+      signal:set(5)
+      assert.is.equal(memoized_signal:get(), 10) -- Update to 5 * 2 = 10
+    end)
+
+    it('should not recompute if the dependencies do not change', function()
+      local call_count = 0
+      local signal = Signal:new(10)
+      local memoized_signal = tracker.create_memo(function()
+        call_count = call_count + 1
+        return signal:get() + 1
+      end)
+
+      assert.is.equal(memoized_signal:get(), 11) -- Compute first value
+      assert.is.equal(call_count, 1) -- Should compute once
+
+      memoized_signal:get() -- Call again, should use memoized value
+      assert.is.equal(call_count, 1) -- Still should only be one call
+
+      signal:set(10) -- Set the same value
+      assert.is.equal(memoized_signal:get(), 11)
+      assert.is.equal(call_count, 2)
+
+      signal:set(20)
+      assert.is.equal(memoized_signal:get(), 21)
+      assert.is.equal(call_count, 3)
+    end)
+  end)
+
+  describe('create_effect', function()
+    it('should track changes and execute callback', function()
+      local signal = Signal:new(5)
+      local call_count = 0
+
+      tracker.create_effect(function()
+        signal:get() -- track as a dependency
+        call_count = call_count + 1
+      end)
+
+      assert.is.equal(call_count, 1)
+      signal:set(10)
+      assert.is.equal(call_count, 2)
+    end)
+
+    it('should clean up signals and not call after dispose', function()
+      local signal = Signal:new(5)
+      local call_count = 0
+
+      local unsubscribe = tracker.create_effect(function()
+        call_count = call_count + 1
+        return signal:get() * 2
+      end)
+
+      assert.is.equal(call_count, 1) -- Initially calls
+      unsubscribe() -- Unsubscribe the effect
+      signal:set(10) -- Update signal value
+      assert.is.equal(call_count, 1) -- Callback should not be called again
+    end)
+  end)
+end)
+
+describe('ExecutionContext', function()
+  local context
+
+  before_each(function() context = ExecutionContext:new() end)
+
+  it('should initialize a new context', function()
+    assert.is.table(context.signals)
+    assert.is.table(context.subscribers)
+  end)
+
+  it('should track signals', function()
+    local signal = Signal:new(0)
+    context:track(signal)
+
+    assert.is.equal(next(context.signals), signal) -- Check if signal is tracked
+  end)
+
+  it('should subscribe to signals', function()
+    local signal = Signal:new(0)
+    local callback_called = false
+
+    context:track(signal)
+    context:subscribe(function() callback_called = true end)
+
+    signal:set(100)
+    assert.is.equal(callback_called, true) -- Callback should be called
+  end)
+
+  it('should dispose tracked signals', function()
+    local signal = Signal:new(0)
+    context:track(signal)
+
+    context:dispose()
+    assert.is.falsy(next(context.signals)) -- Should not have any tracked signals
+  end)
+end)
diff --git a/spec/utils_spec.lua b/spec/utils_spec.lua
new file mode 100644
index 0000000..75bfdf9
--- /dev/null
+++ b/spec/utils_spec.lua
@@ -0,0 +1,70 @@
+local utils = require 'u.utils'
+
+--- @param s string
+local function split(s) return vim.split(s, '') end
+
+--- @param original string
+--- @param changes LevenshteinChange[]
+local function morph(original, changes)
+  local t = split(original)
+  for _, change in ipairs(changes) do
+    if change.kind == 'add' then
+      table.insert(t, change.index, change.item)
+    elseif change.kind == 'delete' then
+      table.remove(t, change.index)
+    elseif change.kind == 'change' then
+      t[change.index] = change.to
+    end
+  end
+  return vim.iter(t):join ''
+end
+
+describe('utils', function()
+  it('levenshtein', function()
+    local original = 'abc'
+    local result = 'absece'
+    local changes = utils.levenshtein(split(original), split(result))
+    assert.are.same(changes, {
+      {
+        item = 'e',
+        kind = 'add',
+        index = 4,
+      },
+      {
+        item = 'e',
+        kind = 'add',
+        index = 3,
+      },
+      {
+        item = 's',
+        kind = 'add',
+        index = 3,
+      },
+    })
+    assert.are.same(morph(original, changes), result)
+
+    original = 'jonathan'
+    result = 'ajoanthan'
+    changes = utils.levenshtein(split(original), split(result))
+    assert.are.same(changes, {
+      {
+        from = 'a',
+        index = 4,
+        kind = 'change',
+        to = 'n',
+      },
+      {
+        from = 'n',
+        index = 3,
+        kind = 'change',
+        to = 'a',
+      },
+      {
+        index = 1,
+        item = 'a',
+        kind = 'add',
+      },
+    })
+    assert.are.same(morph(original, changes), result)
+  end)
+end)