diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,23 @@
 Brick changelog
 ---------------
 
+0.41
+----
+
+New features:
+ * `Brick.Forms` got a new field constructor, `listField`, that provides
+   a form field using a `List`.
+ * `List`: added the `listMoveToElement` function for changing the list
+   selection to the specified element, if it exists.
+
+Package changes:
+ * Now depends on vty >= 5.24.
+
+Other changes:
+ * `viewport`: fixed failable patterns for forward compatibility with
+   GHC 8.6 (#183)
+ * Add `Generic`, `NFData`, and `Read` instances for some types
+
 0.40
 ----
 
diff --git a/brick.cabal b/brick.cabal
--- a/brick.cabal
+++ b/brick.cabal
@@ -1,5 +1,5 @@
 name:                brick
-version:             0.40
+version:             0.41
 synopsis:            A declarative terminal user interface library
 description:
   Write terminal applications painlessly with 'brick'! You write an
@@ -87,7 +87,7 @@
     Brick.Widgets.Internal
 
   build-depends:       base <= 4.11.1.0,
-                       vty >= 5.23.1,
+                       vty >= 5.24,
                        transformers,
                        data-clist >= 0.1,
                        dlist,
diff --git a/src/Brick/AttrMap.hs b/src/Brick/AttrMap.hs
--- a/src/Brick/AttrMap.hs
+++ b/src/Brick/AttrMap.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE DeriveGeneric #-}
 -- | This module provides types and functions for managing an attribute
 -- map which maps attribute names ('AttrName') to attributes ('Attr').
 -- This module is designed to be used with the 'OverloadedStrings'
@@ -54,6 +55,7 @@
 import Data.Maybe (catMaybes)
 import Data.List (inits)
 import Data.String (IsString(..))
+import GHC.Generics (Generic)
 
 import Graphics.Vty (Attr(..), MaybeDefault(..))
 
@@ -70,7 +72,7 @@
 -- "header" <> "clock" <> "seconds"
 -- @
 data AttrName = AttrName [String]
-              deriving (Show, Read, Eq, Ord)
+              deriving (Show, Read, Eq, Ord, Generic)
 
 instance Sem.Semigroup AttrName where
     (AttrName as) <> (AttrName bs) = AttrName $ as `mappend` bs
@@ -85,7 +87,7 @@
 -- | An attribute map which maps 'AttrName' values to 'Attr' values.
 data AttrMap = AttrMap Attr (M.Map AttrName Attr)
              | ForceAttr Attr
-             deriving Show
+             deriving (Show, Generic)
 
 -- | Create an attribute name from a string.
 attrName :: String -> AttrName
diff --git a/src/Brick/BorderMap.hs b/src/Brick/BorderMap.hs
--- a/src/Brick/BorderMap.hs
+++ b/src/Brick/BorderMap.hs
@@ -1,6 +1,8 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DeriveAnyClass #-}
 module Brick.BorderMap
     ( BorderMap
     , Edges(..)
@@ -18,6 +20,8 @@
 import Brick.Types.Common (Edges(..), Location(..), eTopL, eBottomL, eRightL, eLeftL, origin)
 import Control.Applicative (liftA2)
 import Data.IMap (IMap, Run(Run))
+import GHC.Generics
+import Control.DeepSeq
 import Prelude hiding (lookup)
 import qualified Data.IMap as IM
 
@@ -43,7 +47,7 @@
 data BorderMap a = BorderMap
     { _coordinates :: Edges Int
     , _values :: Edges (IMap a)
-    } deriving (Eq, Ord, Show, Functor)
+    } deriving (Eq, Ord, Show, Functor, Read, Generic, NFData)
 
 -- | Given a rectangle (specified as the coordinates of the top, left, bottom,
 -- and right sides), initialize an empty 'BorderMap'.
diff --git a/src/Brick/Forms.hs b/src/Brick/Forms.hs
--- a/src/Brick/Forms.hs
+++ b/src/Brick/Forms.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 -- | NOTE: This API is experimental and will probably change. Please try
 -- it out! Feedback is very much appreciated, and your patience in the
 -- face of breaking API changes is also appreciated!
@@ -65,6 +66,7 @@
   , editPasswordField
   , radioField
   , checkboxField
+  , listField
 
   -- * Advanced form field constructors
   , editField
@@ -82,10 +84,12 @@
 #endif
 import Data.Maybe (isJust, isNothing)
 import Data.List (elemIndex)
+import Data.Vector (Vector)
 
 import Brick
 import Brick.Focus
 import Brick.Widgets.Edit
+import Brick.Widgets.List
 import qualified Data.Text.Zipper as Z
 
 import qualified Data.Text as T
@@ -287,6 +291,50 @@
        addAttr $
        (str $ "[" <> (if val then "X" else " ") <> "] ") <+> txt label
 
+-- | A form field for selecting a single choice from a set of possible
+-- choices in a scrollable list. This uses a 'List' internally.
+--
+-- This field responds to the same input events that a 'List' does.
+listField :: forall s e n a . (Ord n, Show n, Eq a)
+          => (s -> Vector a)
+          -- ^ Possible choices.
+          -> Lens' s (Maybe a)
+          -- ^ The state lens for the initially/finally selected
+          -- element.
+          -> (Bool -> a -> Widget n)
+          -- ^ List item rendering function.
+          -> Int
+          -- ^ List item height in rows.
+          -> n
+          -- ^ The resource name for the input field.
+          -> s
+          -- ^ The initial form state.
+          -> FormFieldState s e n
+listField options stLens renderItem itemHeight name initialState =
+    let optionsVector = options initialState
+        initVal = initialState ^. customStLens
+
+        customStLens :: Lens' s (List n a)
+        customStLens = lens getList setList
+            where
+               getList s = let l = list name optionsVector itemHeight
+                           in case s ^. stLens of
+                               Nothing -> l
+                               Just e -> listMoveToElement e l
+               setList s l = s & stLens .~ (snd <$> listSelectedElement l)
+
+        handleEvent (VtyEvent e) s = handleListEvent e s
+        handleEvent _ s = return s
+
+    in FormFieldState { formFieldState        = initVal
+                      , formFields            = [ FormField name Just True
+                                                            (renderList renderItem)
+                                                            handleEvent
+                                                ]
+                      , formFieldLens         = customStLens
+                      , formFieldRenderHelper = id
+                      , formFieldConcat       = vBox
+                      }
 -- | A form field for selecting a single choice from a set of possible
 -- choices. Each choice has an associated value and text label.
 --
diff --git a/src/Brick/Main.hs b/src/Brick/Main.hs
--- a/src/Brick/Main.hs
+++ b/src/Brick/Main.hs
@@ -38,6 +38,11 @@
   -- * Rendering cache management
   , invalidateCacheEntry
   , invalidateCache
+
+  -- * Renderer internals (for benchmarking)
+  , renderFinal
+  , getRenderState
+  , resetRenderState
   )
 where
 
@@ -180,8 +185,7 @@
               Nothing -> readBChan brickChan
               Just uc -> readBrickEvent brickChan uc
             runInner rs st = do
-              (result, newRS) <- runVty vty readEvent app st (rs & observedNamesL .~ S.empty
-                                                                 & clickableNamesL .~ mempty)
+              (result, newRS) <- runVty vty readEvent app st (resetRenderState rs)
               case result of
                   SuspendAndResume act -> do
                       killThread pid
@@ -221,7 +225,8 @@
                     run newVty newRS newAppState brickChan
 
         emptyES = ES [] mempty
-        eventRO = EventRO M.empty initialVty mempty
+        emptyRS = RS M.empty mempty S.empty mempty mempty
+        eventRO = EventRO M.empty initialVty mempty emptyRS
     (st, eState) <- runStateT (runReaderT (runEventM (appStartEvent app initialAppState)) eventRO) emptyES
     let initialRS = RS M.empty (esScrollRequests eState) S.empty mempty []
     brickChan <- newBChan 20
@@ -303,7 +308,7 @@
         _ -> return (e, firstRS, exts)
 
     let emptyES = ES [] mempty
-        eventRO = EventRO (viewportMap nextRS) vty nextExts
+        eventRO = EventRO (viewportMap nextRS) vty nextExts nextRS
 
     (next, eState) <- runStateT (runReaderT (runEventM (appHandleEvent app appState e'))
                                 eventRO) emptyES
@@ -372,6 +377,14 @@
 withVty :: Vty -> (Vty -> IO a) -> IO a
 withVty vty useVty = do
     useVty vty `finally` shutdown vty
+
+getRenderState :: EventM n (RenderState n)
+getRenderState = EventM $ asks oldState
+
+resetRenderState :: RenderState n -> RenderState n
+resetRenderState s =
+    s & observedNamesL .~ S.empty
+      & clickableNamesL .~ mempty
 
 renderApp :: Vty -> App s e n -> s -> RenderState n -> IO (RenderState n, [Extent n])
 renderApp vty app appState rs = do
diff --git a/src/Brick/Types.hs b/src/Brick/Types.hs
--- a/src/Brick/Types.hs
+++ b/src/Brick/Types.hs
@@ -76,6 +76,8 @@
   , Padding(..)
   , Direction(..)
 
+  -- * Renderer internals (for benchmarking)
+  , RenderState
   )
 where
 
diff --git a/src/Brick/Types/Common.hs b/src/Brick/Types/Common.hs
--- a/src/Brick/Types/Common.hs
+++ b/src/Brick/Types/Common.hs
@@ -1,7 +1,8 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TemplateHaskell #-}
-
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DeriveAnyClass #-}
 module Brick.Types.Common
   ( Location(..)
   , locL
@@ -12,6 +13,8 @@
 
 import Brick.Types.TH (suffixLenses)
 import qualified Data.Semigroup as Sem
+import GHC.Generics
+import Control.DeepSeq
 import Lens.Micro (_1, _2)
 import Lens.Micro.Internal (Field1, Field2)
 
@@ -19,7 +22,7 @@
 data Location = Location { loc :: (Int, Int)
                          -- ^ (Column, Row)
                          }
-                deriving (Show, Eq, Ord)
+                deriving (Show, Eq, Ord, Read, Generic, NFData)
 
 suffixLenses ''Location
 
@@ -41,7 +44,7 @@
     mappend = (Sem.<>)
 
 data Edges a = Edges { eTop, eBottom, eLeft, eRight :: a }
-    deriving (Eq, Ord, Read, Show, Functor)
+    deriving (Eq, Ord, Read, Show, Functor, Generic, NFData)
 
 suffixLenses ''Edges
 
diff --git a/src/Brick/Types/Internal.hs b/src/Brick/Types/Internal.hs
--- a/src/Brick/Types/Internal.hs
+++ b/src/Brick/Types/Internal.hs
@@ -1,6 +1,8 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DeriveAnyClass #-}
 module Brick.Types.Internal
   ( ScrollRequest(..)
   , VisibilityRequest(..)
@@ -58,6 +60,8 @@
 import qualified Data.Set as S
 import qualified Data.Map as M
 import Graphics.Vty (Vty, Event, Button, Modifier, DisplayRegion, Image, Attr, emptyImage)
+import GHC.Generics
+import Control.DeepSeq (NFData)
 
 import Brick.BorderMap (BorderMap)
 import qualified Brick.BorderMap as BM
@@ -76,12 +80,13 @@
                    | VScrollToEnd
                    | SetTop Int
                    | SetLeft Int
+                   deriving (Read, Show, Generic, NFData)
 
 data VisibilityRequest =
     VR { vrPosition :: Location
        , vrSize :: DisplayRegion
        }
-       deriving (Show, Eq)
+       deriving (Show, Eq, Read, Generic, NFData)
 
 -- | Describes the state of a viewport as it appears as its most recent
 -- rendering.
@@ -93,7 +98,7 @@
        , _vpSize :: DisplayRegion
        -- ^ The size of the viewport.
        }
-       deriving Show
+       deriving (Show, Read, Generic, NFData)
 
 -- | The type of viewports that indicates the direction(s) in which a
 -- viewport is scrollable.
@@ -120,12 +125,7 @@
                        , extentSize      :: (Int, Int)
                        , extentOffset    :: Location
                        }
-              deriving (Show)
-
-data EventRO n = EventRO { eventViewportMap :: M.Map n Viewport
-                         , eventVtyHandle :: Vty
-                         , latestExtents :: [Extent n]
-                         }
+                       deriving (Show, Read, Generic, NFData)
 
 -- | The type of actions to take upon completion of an event handler.
 data Next a = Continue a
@@ -138,7 +138,7 @@
                -- ^ Up/left
                | Down
                -- ^ Down/right
-               deriving (Show, Eq)
+               deriving (Show, Eq, Read, Generic, NFData)
 
 -- | The class of types that behave like terminal locations.
 class TerminalLocation a where
@@ -163,7 +163,7 @@
                    , cursorLocationName :: !(Maybe n)
                    -- ^ The name of the widget associated with the location
                    }
-                   deriving Show
+                   deriving (Read, Show, Generic, NFData)
 
 -- | A border character has four segments, one extending in each direction
 -- (horizontally and vertically) from the center of the character.
@@ -175,7 +175,7 @@
     -- ^ Does this segment want to connect to its neighbor?
     , bsDraw :: Bool
     -- ^ Should this segment be represented visually?
-    } deriving (Eq, Ord, Read, Show)
+    } deriving (Eq, Ord, Read, Show, Generic, NFData)
 
 suffixLenses ''BorderSegment
 
@@ -191,7 +191,7 @@
     -- connections: only dynamic borders with equal 'Attr's will connect to
     -- each other.
     , dbSegments :: Edges BorderSegment
-    } deriving (Eq, Read, Show)
+    } deriving (Eq, Read, Show, Generic, NFData)
 
 suffixLenses ''DynBorder
 
@@ -224,7 +224,7 @@
            -- ^ Places where we may rewrite the edge of the image when
            -- placing this widget next to another one.
            }
-           deriving Show
+           deriving (Show, Read, Generic, NFData)
 
 suffixLenses ''Result
 
@@ -252,7 +252,13 @@
        , observedNames :: !(S.Set n)
        , renderCache :: M.Map n (Result n)
        , clickableNames :: [n]
-       }
+       } deriving (Read, Show, Generic, NFData)
+
+data EventRO n = EventRO { eventViewportMap :: M.Map n Viewport
+                         , eventVtyHandle :: Vty
+                         , latestExtents :: [Extent n]
+                         , oldState :: RenderState n
+                         }
 
 -- | The rendering context. This tells widgets how to render: how much
 -- space they have in which to render, which attribute they should use
diff --git a/src/Brick/Widgets/Border/Style.hs b/src/Brick/Widgets/Border/Style.hs
--- a/src/Brick/Widgets/Border/Style.hs
+++ b/src/Brick/Widgets/Border/Style.hs
@@ -1,4 +1,6 @@
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DeriveAnyClass #-}
 -- | This module provides styles for borders as used in terminal
 -- applications. Your mileage may vary on some of the fancier styles
 -- due to varying support for some border characters in the fonts your
@@ -21,6 +23,9 @@
   )
 where
 
+import GHC.Generics
+import Control.DeepSeq
+
 -- | A border style for use in any widget that needs to render borders
 -- in a consistent style.
 data BorderStyle =
@@ -47,7 +52,7 @@
                 , bsVertical :: Char
                 -- ^ Vertical border character
                 }
-                deriving (Show, Read, Eq)
+                deriving (Show, Read, Eq, Generic, NFData)
 
 defaultBorderStyle :: BorderStyle
 defaultBorderStyle = unicode
diff --git a/src/Brick/Widgets/Core.hs b/src/Brick/Widgets/Core.hs
--- a/src/Brick/Widgets/Core.hs
+++ b/src/Brick/Widgets/Core.hs
@@ -1041,33 +1041,42 @@
       reqs <- lift $ gets $ (^.rsScrollRequestsL)
       let relevantRequests = snd <$> filter (\(n, _) -> n == vpname) reqs
       when (not $ null relevantRequests) $ do
-          Just vp <- lift $ gets $ (^.viewportMapL.to (M.lookup vpname))
-          let updatedVp = applyRequests relevantRequests vp
-              applyRequests [] v = v
-              applyRequests (rq:rqs) v =
-                  case typ of
-                      Horizontal -> scrollTo typ rq (initialResult^.imageL) $ applyRequests rqs v
-                      Vertical -> scrollTo typ rq (initialResult^.imageL) $ applyRequests rqs v
-                      Both -> scrollTo Horizontal rq (initialResult^.imageL) $
-                              scrollTo Vertical rq (initialResult^.imageL) $
-                              applyRequests rqs v
-          lift $ modify (& viewportMapL %~ (M.insert vpname updatedVp))
-          return ()
+          mVp <- lift $ gets $ (^.viewportMapL.to (M.lookup vpname))
+          case mVp of
+              Nothing -> error $ "BUG: viewport: viewport name " <> show vpname <> " absent from viewport map"
+              Just vp -> do
+                  let updatedVp = applyRequests relevantRequests vp
+                      applyRequests [] v = v
+                      applyRequests (rq:rqs) v =
+                          case typ of
+                              Horizontal -> scrollTo typ rq (initialResult^.imageL) $ applyRequests rqs v
+                              Vertical -> scrollTo typ rq (initialResult^.imageL) $ applyRequests rqs v
+                              Both -> scrollTo Horizontal rq (initialResult^.imageL) $
+                                      scrollTo Vertical rq (initialResult^.imageL) $
+                                      applyRequests rqs v
+                  lift $ modify (& viewportMapL %~ (M.insert vpname updatedVp))
 
       -- If the sub-rendering requested visibility, update the scroll
       -- state accordingly
       when (not $ null $ initialResult^.visibilityRequestsL) $ do
-          Just vp <- lift $ gets $ (^.viewportMapL.to (M.lookup vpname))
-          let rqs = initialResult^.visibilityRequestsL
-              updateVp vp' rq = case typ of
-                  Both -> scrollToView Horizontal rq $ scrollToView Vertical rq vp'
-                  Horizontal -> scrollToView typ rq vp'
-                  Vertical -> scrollToView typ rq vp'
-          lift $ modify (& viewportMapL %~ (M.insert vpname $ foldl updateVp vp rqs))
+          mVp <- lift $ gets $ (^.viewportMapL.to (M.lookup vpname))
+          case mVp of
+              Nothing -> error $ "BUG: viewport: viewport name " <> show vpname <> " absent from viewport map"
+              Just vp -> do
+                  let rqs = initialResult^.visibilityRequestsL
+                      updateVp vp' rq = case typ of
+                          Both -> scrollToView Horizontal rq $ scrollToView Vertical rq vp'
+                          Horizontal -> scrollToView typ rq vp'
+                          Vertical -> scrollToView typ rq vp'
+                  lift $ modify (& viewportMapL %~ (M.insert vpname $ foldl updateVp vp rqs))
 
       -- If the size of the rendering changes enough to make the
       -- viewport offsets invalid, reset them
-      Just vp <- lift $ gets $ (^.viewportMapL.to (M.lookup vpname))
+      mVp <- lift $ gets $ (^.viewportMapL.to (M.lookup vpname))
+      vp <- case mVp of
+          Nothing -> error $ "BUG: viewport: viewport name " <> show vpname <> " absent from viewport map"
+          Just v -> return v
+
       let img = initialResult^.imageL
           fixTop v = if V.imageHeight img < v^.vpSize._2
                    then v & vpTop .~ 0
@@ -1082,7 +1091,10 @@
       lift $ modify (& viewportMapL %~ (M.insert vpname (updateVp vp)))
 
       -- Get the viewport state now that it has been updated.
-      Just vpFinal <- lift $ gets (M.lookup vpname . (^.viewportMapL))
+      mVpFinal <- lift $ gets (M.lookup vpname . (^.viewportMapL))
+      vpFinal <- case mVpFinal of
+          Nothing -> error $ "BUG: viewport: viewport name " <> show vpname <> " absent from viewport map"
+          Just v -> return v
 
       -- Then perform a translation of the sub-rendering to fit into the
       -- viewport
diff --git a/src/Brick/Widgets/List.hs b/src/Brick/Widgets/List.hs
--- a/src/Brick/Widgets/List.hs
+++ b/src/Brick/Widgets/List.hs
@@ -6,6 +6,7 @@
 {-# LANGUAGE DeriveTraversable #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE DeriveGeneric #-}
 -- | This module provides a scrollable list type and functions for
 -- manipulating and rendering it.
 module Brick.Widgets.List
@@ -38,6 +39,7 @@
   -- * Manipulating a list
   , listMoveBy
   , listMoveTo
+  , listMoveToElement
   , listMoveUp
   , listMoveDown
   , listMoveByPages
@@ -62,12 +64,14 @@
 import Data.Foldable (Foldable)
 import Data.Traversable (Traversable)
 #endif
+import Control.Applicative ((<|>))
 
 import Lens.Micro ((^.), (&), (.~), (%~), _2)
 import Data.Maybe (fromMaybe)
 import Data.Monoid ((<>))
 import Graphics.Vty (Event(..), Key(..), Modifier(..))
 import qualified Data.Vector as V
+import GHC.Generics (Generic)
 
 import Brick.Types
 import Brick.Main (lookupViewport)
@@ -92,7 +96,7 @@
          -- ^ The list's name.
          , listItemHeight :: Int
          -- ^ The height of the list items.
-         } deriving (Functor, Foldable, Traversable, Show)
+         } deriving (Functor, Foldable, Traversable, Show, Generic)
 
 suffixLenses ''List
 
@@ -348,6 +352,13 @@
     in l & listSelectedL .~ if len > 0
                             then Just newSel
                             else Nothing
+
+-- | Set the selected index for a list to the index of the specified
+-- element if it is in the list, or leave the list unmodified otherwise.
+listMoveToElement :: (Eq e) => e -> List n e -> List n e
+listMoveToElement e l =
+    let i = V.elemIndex e (l^.listElementsL)
+    in l & listSelectedL %~ (i <|>)
 
 -- | Return a list's selected element, if any.
 listSelectedElement :: List n e -> Maybe (Int, e)
diff --git a/src/Data/IMap.hs b/src/Data/IMap.hs
--- a/src/Data/IMap.hs
+++ b/src/Data/IMap.hs
@@ -1,4 +1,6 @@
 {-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DeriveAnyClass #-}
 module Data.IMap
     ( IMap
     , Run(..)
@@ -22,13 +24,15 @@
 import Data.List (foldl')
 import Data.Monoid
 import Data.IntMap.Strict (IntMap)
+import GHC.Generics
+import Control.DeepSeq
 import Prelude hiding (lookup)
 import qualified Data.IntMap.Strict as IM
 
 -- | Semantically, 'IMap' and 'IntMap' are identical; but 'IMap' is more
 -- efficient when large sequences of contiguous keys are mapped to the same
 -- value.
-newtype IMap a = IMap { _runs :: IntMap (Run a) } deriving (Show, Functor)
+newtype IMap a = IMap { _runs :: IntMap (Run a) } deriving (Show, Functor, Read, Generic, NFData)
 
 {-# INLINE unsafeRuns #-}
 -- | This function is unsafe because 'IMap's that compare equal may split their
@@ -72,7 +76,7 @@
 data Run a = Run
     { len :: !Int
     , val :: !a
-    } deriving (Eq, Ord, Read, Show, Functor)
+    } deriving (Eq, Ord, Read, Show, Functor, Generic, NFData)
 
 instance Foldable    Run where foldMap f r = f (val r)
 instance Traversable Run where sequenceA (Run n v) = Run n <$> v