diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -6,8 +6,7 @@
 Sample from a posterior using Markov chain Monte Carlo methods.
 
 At the moment, the library is tailored to the Metropolis-Hastings algorithm
-since it covers most use cases. However, implementation of more algorithms is
-planned in the future.
+since it covers most use cases. More algorithms will be implemented soon.
 
 
 ## Documentation
diff --git a/bench/Normal.hs b/bench/Normal.hs
--- a/bench/Normal.hs
+++ b/bench/Normal.hs
@@ -37,7 +37,7 @@
 proposals :: Cycle Double
 proposals =
   fromList
-    [slideSymmetric "medium" 1 1.0 True]
+    [slideSymmetric 1.0 "medium" 1 True]
 
 mons :: [MonitorParameter Double]
 mons = [monitorDouble "mu"]
@@ -68,7 +68,7 @@
 proposalsBactrian :: Cycle Double
 proposalsBactrian =
   fromList
-    [slideBactrian "bactrian" 1 0.5 1.0 True]
+    [slideBactrian 0.5 1.0 "bactrian" 1 True]
 
 normalBactrianBench :: GenIO -> IO ()
 normalBactrianBench g = do
diff --git a/bench/Poisson.hs b/bench/Poisson.hs
--- a/bench/Poisson.hs
+++ b/bench/Poisson.hs
@@ -48,10 +48,10 @@
   product [f ft yr x | (ft, yr) <- zip fatalities normalizedYears]
 
 proposalAlpha :: Proposal I
-proposalAlpha = _1 @~ slideSymmetric "alpha" 2 0.2 False
+proposalAlpha = _1 @~ slideSymmetric 0.2 "alpha" 2 False
 
 proposalBeta :: Proposal I
-proposalBeta = _2 @~ slideSymmetric "beta" 1 0.2 False
+proposalBeta = _2 @~ slideSymmetric 0.2 "beta" 1 False
 
 proposals :: Cycle I
 proposals = fromList [proposalAlpha, proposalBeta]
@@ -60,10 +60,10 @@
 initial = (0, 0)
 
 monAlpha :: MonitorParameter I
-monAlpha = fst @. monitorDouble "alpha"
+monAlpha = fst >$< monitorDouble "alpha"
 
 monBeta :: MonitorParameter I
-monBeta = snd @. monitorDouble "beta"
+monBeta = snd >$< monitorDouble "beta"
 
 monStd :: MonitorStdOut I
 monStd = monitorStdOut [monAlpha, monBeta] 150
diff --git a/mcmc.cabal b/mcmc.cabal
--- a/mcmc.cabal
+++ b/mcmc.cabal
@@ -1,6 +1,6 @@
 cabal-version:  2.2
 name:           mcmc
-version:        0.2.3
+version:        0.2.4
 synopsis:       Sample from a posterior using Markov chain Monte Carlo
 description:    Please see the README on GitHub at <https://github.com/dschrempf/mcmc#readme>
 category:       Math, Statistics
diff --git a/src/Mcmc.hs b/src/Mcmc.hs
--- a/src/Mcmc.hs
+++ b/src/Mcmc.hs
@@ -86,10 +86,13 @@
     (@~),
     scale,
     scaleUnbiased,
+    scaleContrarily,
+    scaleBactrian,
     slide,
-    slideBactrian,
     slideSymmetric,
     slideUniform,
+    slideContrarily,
+    slideBactrian,
     Cycle,
     fromList,
     Order (..),
diff --git a/src/Mcmc/Item.hs b/src/Mcmc/Item.hs
--- a/src/Mcmc/Item.hs
+++ b/src/Mcmc/Item.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE OverloadedStrings #-}
-{-# OPTIONS_GHC -fno-warn-orphans #-}
 
 -- |
 -- Module      :  Mcmc.Item
@@ -18,15 +17,9 @@
 where
 
 import Data.Aeson
+import Data.Aeson.Types
 import Numeric.Log
 
-instance ToJSON a => ToJSON (Log a) where
-  toJSON (Exp x) = toJSON x
-  toEncoding (Exp x) = toEncoding x
-
-instance FromJSON a => FromJSON (Log a) where
-  parseJSON v = Exp <$> parseJSON v
-
 -- | An 'Item', or link of the Markov chain. For reasons of computational
 -- efficiency, each state is associated with the corresponding prior and
 -- likelihood.
@@ -41,13 +34,15 @@
   deriving (Eq, Ord, Show, Read)
 
 instance ToJSON a => ToJSON (Item a) where
-  toJSON (Item x p l) = object ["s" .= x, "p" .= p, "l" .= l]
-  toEncoding (Item x p l) = pairs ("s" .= x <> "p" .= p <> "l" .= l)
+  toJSON (Item x (Exp p) (Exp l)) = object ["s" .= x, "p" .= p, "l" .= l]
+  toEncoding (Item x (Exp p) (Exp l)) = pairs ("s" .= x <> "p" .= p <> "l" .= l)
 
+item :: FromJSON a => Object -> Parser (Item a)
+item v = do
+  s <- v .: "s"
+  p <- v .: "p"
+  l <- v .: "l"
+  return $ Item s (Exp p) (Exp l)
+
 instance FromJSON a => FromJSON (Item a) where
-  parseJSON = withObject "Item" $
-    \v ->
-      Item
-        <$> v .: "s"
-        <*> v .: "p"
-        <*> v .: "l"
+  parseJSON = withObject "Item" item
diff --git a/src/Mcmc/Mcmc.hs b/src/Mcmc/Mcmc.hs
--- a/src/Mcmc/Mcmc.hs
+++ b/src/Mcmc/Mcmc.hs
@@ -11,6 +11,8 @@
 -- Portability :  portable
 --
 -- Creation date: Fri May 29 10:19:45 2020.
+--
+-- Functions to work with the 'Mcmc' state transformer.
 module Mcmc.Mcmc
   ( Mcmc,
     mcmcOutT,
diff --git a/src/Mcmc/Metropolis.hs b/src/Mcmc/Metropolis.hs
--- a/src/Mcmc/Metropolis.hs
+++ b/src/Mcmc/Metropolis.hs
@@ -12,6 +12,8 @@
 -- Portability :  portable
 --
 -- Creation date: Tue May  5 20:11:30 2020.
+--
+-- Metropolis-Hastings algorithm.
 module Mcmc.Metropolis
   ( mh,
     mhContinue,
@@ -51,7 +53,7 @@
 
 mhPropose :: Proposal a -> Mcmc a ()
 mhPropose m = do
-  let p = pSample $ pSimple m
+  let p = pSimple m
   s <- get
   let (Item x pX lX) = item s
       pF = priorF s
diff --git a/src/Mcmc/Monitor.hs b/src/Mcmc/Monitor.hs
--- a/src/Mcmc/Monitor.hs
+++ b/src/Mcmc/Monitor.hs
@@ -149,10 +149,6 @@
     mfPeriod :: Int
   }
 
--- XXX: The file monitor also includes iteration, prior, likelihood, and
--- posterior. What if I want to log trees; or other complex objects? In this
--- case, we need a simpler monitor to a file.
-
 -- | Monitor parameters to a file.
 monitorFile ::
   -- | Name; used as part of the file name.
@@ -246,10 +242,6 @@
     mbParams :: [MonitorParameterBatch a],
     mbSize :: Int
   }
-
--- XXX: The batch monitor also includes iteration, prior, likelihood, and
--- posterior. What if I want to log trees; or other complex objects? In this
--- case, we need a simpler monitor to a file.
 
 -- | Monitor parameters to a file, see 'MonitorBatch'.
 monitorBatch ::
diff --git a/src/Mcmc/Monitor/Parameter.hs b/src/Mcmc/Monitor/Parameter.hs
--- a/src/Mcmc/Monitor/Parameter.hs
+++ b/src/Mcmc/Monitor/Parameter.hs
@@ -14,6 +14,7 @@
 module Mcmc.Monitor.Parameter
   ( -- * Parameter monitors
     MonitorParameter (..),
+    (>$<),
     (@.),
     monitorInt,
     monitorDouble,
@@ -24,8 +25,17 @@
 
 import qualified Data.ByteString.Builder as BB
 import qualified Data.Double.Conversion.ByteString as BC
+import Data.Functor.Contravariant
 
 -- | Instruction about a parameter to monitor.
+--
+-- Convert a parameter monitor from one data type to another with '(>$<)'.
+--
+-- For example, monitor a 'Double' value being the first entry of a tuple:
+--
+-- @
+-- mon = fst >$< monitorDouble
+-- @
 data MonitorParameter a = MonitorParameter
   { -- | Name of parameter.
     mpName :: String,
@@ -33,15 +43,21 @@
     mpFunc :: a -> BB.Builder
   }
 
+instance Contravariant (MonitorParameter) where
+  contramap f (MonitorParameter n m) = MonitorParameter n (m . f)
+
 -- | Convert a parameter monitor from one data type to another.
 --
+-- DEPRECATED.
+--
 -- For example, to monitor a 'Double' value being the first entry of a tuple:
 --
 -- @
 -- mon = fst @. monitorDouble
 -- @
 (@.) :: (b -> a) -> MonitorParameter a -> MonitorParameter b
-(@.) f (MonitorParameter n m) = MonitorParameter n (m . f)
+(@.) = contramap
+{-# DEPRECATED (@.) "Superseded by the contravariant instance, use '(>$<)'." #-}
 
 -- | Monitor 'Int'.
 monitorInt ::
diff --git a/src/Mcmc/Monitor/ParameterBatch.hs b/src/Mcmc/Monitor/ParameterBatch.hs
--- a/src/Mcmc/Monitor/ParameterBatch.hs
+++ b/src/Mcmc/Monitor/ParameterBatch.hs
@@ -16,6 +16,7 @@
 module Mcmc.Monitor.ParameterBatch
   ( -- * Batch parameter monitors
     MonitorParameterBatch (..),
+    (>$<),
     (@#),
     monitorBatchMean,
     monitorBatchMeanF,
@@ -26,12 +27,21 @@
 
 import qualified Data.ByteString.Builder as BB
 import qualified Data.Double.Conversion.ByteString as BC
+import Data.Functor.Contravariant
 
 -- | Instruction about a parameter to monitor via batch means. Usually, the
 -- monitored parameter is average over the batch size. However, arbitrary
 -- functions performing more complicated analyses on the states in the batch can
 -- be provided.
 --
+-- Convert a batch monitor from one data type to another with '(>$<)'.
+--
+-- For example, batch monitor the mean of the first entry of a tuple:
+--
+-- @
+-- mon = fst >$< monitorBatchMean
+-- @
+--
 -- XXX: Batch monitors are slow at the moment because the monitored parameter
 -- has to be extracted from the state for each iteration.
 data MonitorParameterBatch a = MonitorParameterBatch
@@ -41,6 +51,9 @@
     mbpFunc :: [a] -> BB.Builder
   }
 
+instance Contravariant (MonitorParameterBatch) where
+  contramap f (MonitorParameterBatch n m) = MonitorParameterBatch n (m . map f)
+
 -- | Convert a batch parameter monitor from one data type to another.
 --
 -- For example, to batch monitor the mean of the first entry of a tuple:
@@ -50,6 +63,7 @@
 -- @
 (@#) :: (b -> a) -> MonitorParameterBatch a -> MonitorParameterBatch b
 (@#) f (MonitorParameterBatch n m) = MonitorParameterBatch n (m . map f)
+{-# DEPRECATED (@#) "Superseded by the contravariant instance, use '(>$<)'." #-}
 
 mean :: Real a => [a] -> Double
 mean xs = realToFrac (sum xs) / fromIntegral (length xs)
diff --git a/src/Mcmc/Prior.hs b/src/Mcmc/Prior.hs
--- a/src/Mcmc/Prior.hs
+++ b/src/Mcmc/Prior.hs
@@ -19,9 +19,7 @@
     normal,
     exponential,
     gamma,
-
-    -- * Discrete priors
-
+    -- -- * Discrete priors
     -- No discrete priors are available yet.
 
     -- * Auxiliary functions
@@ -98,8 +96,8 @@
 
 -- | Intelligent product that stops when encountering a zero.
 --
--- Use with care because the elements have to be checked for positiveness, and
--- this can take some time if the list is long and does not contain any zeroes.
+-- Use with care because the elements are checked for positiveness, and this can
+-- take some time if the list is long and does not contain any zeroes.
 product' :: [Log Double] -> Log Double
 product' = fromMaybe 0 . prodM
 
diff --git a/src/Mcmc/Proposal.hs b/src/Mcmc/Proposal.hs
--- a/src/Mcmc/Proposal.hs
+++ b/src/Mcmc/Proposal.hs
@@ -19,7 +19,7 @@
   ( -- * Proposal
     Proposal (..),
     (@~),
-    ProposalSimple (..),
+    ProposalSimple,
     Tuner (tParam, tFunc),
     createProposal,
     tune,
@@ -45,6 +45,7 @@
 where
 
 import Data.Aeson
+import Data.Bifunctor
 import qualified Data.ByteString.Builder as BB
 import qualified Data.ByteString.Lazy.Char8 as BL
 import Data.Default
@@ -92,7 +93,7 @@
 -- For example:
 --
 -- @
--- scaleFirstEntryOfTuple = scale >>> _1
+-- scaleFirstEntryOfTuple = _1 @~ scale
 -- @
 (@~) :: Lens' b a -> Proposal a -> Proposal b
 (@~) l (Proposal n w s t) = Proposal n w (convertS l s) (convertT l <$> t)
@@ -104,13 +105,12 @@
 --
 -- In order to calculate the Metropolis-Hastings ratio, we need to know the
 -- ratio of the backward to forward kernels (i.e., the probability masses or
--- probability densities). For unbiased proposals, this ratio is 1.0.
-newtype ProposalSimple a = ProposalSimple
-  { pSample :: a -> GenIO -> IO (a, Log Double)
-  }
+-- probability densities). For unbiased proposals, this ratio is 1.0. For biased
+-- proposals, the ratio is ?.
+type ProposalSimple a = a -> GenIO -> IO (a, Log Double)
 
 convertS :: Lens' b a -> ProposalSimple a -> ProposalSimple b
-convertS l (ProposalSimple s) = ProposalSimple s'
+convertS l s = s'
   where
     s' v g = do
       (x', r) <- s (v ^. l) g
@@ -129,19 +129,19 @@
 
 -- | Create a possibly tuneable proposal.
 createProposal ::
-  -- | Name.
-  String ->
-  -- | Weight.
-  Int ->
   -- | Function creating a simple proposal for a given tuning parameter. The
   -- larger the tuning parameter, the larger the proposal (and the lower the
   -- expected acceptance ratio), and vice versa.
   (Double -> ProposalSimple a) ->
+  -- | Name.
+  String ->
+  -- | Weight.
+  Int ->
   -- | Activate tuning?
   Bool ->
   Proposal a
-createProposal n w f True = Proposal n w (f 1.0) (Just $ Tuner 1.0 f)
-createProposal n w f False = Proposal n w (f 1.0) Nothing
+createProposal f n w True = Proposal n w (f 1.0) (Just $ Tuner 1.0 f)
+createProposal f n w False = Proposal n w (f 1.0) Nothing
 
 -- Minimal tuning parameter; subject to change.
 tuningParamMin :: Double
@@ -345,8 +345,8 @@
 
 -- | For key @k@, prepend an accepted (True) or rejected (False) proposal.
 pushA :: (Ord k, Show k) => k -> Bool -> Acceptance k -> Acceptance k
-pushA k True = Acceptance . M.adjust (\(a, r) -> (succ a, r)) k . fromAcceptance
-pushA k False = Acceptance . M.adjust (\(a, r) -> (a, succ r)) k . fromAcceptance
+pushA k True = Acceptance . M.adjust (first succ) k . fromAcceptance
+pushA k False = Acceptance . M.adjust (second succ) k . fromAcceptance
 {-# INLINEABLE pushA #-}
 
 -- | Reset acceptance storage.
diff --git a/src/Mcmc/Proposal/Bactrian.hs b/src/Mcmc/Proposal/Bactrian.hs
--- a/src/Mcmc/Proposal/Bactrian.hs
+++ b/src/Mcmc/Proposal/Bactrian.hs
@@ -67,30 +67,32 @@
   | m < 0 = error "bactrianAdditiveSimple: Spike parameter negative."
   | m >= 1 = error "bactrianAdditiveSimple: Spike parameter 1.0 or larger."
   | s <= 0 = error "bactrianAdditiveSimple: Standard deviation 0.0 or smaller."
-  | otherwise = ProposalSimple $ bactrianAdditive m (t * s)
+  | otherwise = bactrianAdditive m (t * s)
 
 -- | Additive symmetric proposal with kernel similar to the silhouette of a
--- Bactrian camel. The [Bactrian
--- kernel](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3845170/figure/fig01)
--- is a mixture of two symmetrically arranged normal distributions. The spike
+-- Bactrian camel.
+--
+-- The [Bactrian
+-- kernel](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3845170/figure/fig01) is
+-- a mixture of two symmetrically arranged normal distributions. The spike
 -- parameter loosely determines the standard deviations of the individual humps
--- while the other parameter refers to the standard deviation of the complete
+-- while the second parameter refers to the standard deviation of the complete
 -- Bactrian kernel.
 --
 -- See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3845170/.
 slideBactrian ::
-  -- | Name.
-  String ->
-  -- | Weight.
-  Int ->
   -- | Spike parameter.
   Double ->
   -- | Standard deviation.
   Double ->
+  -- | Name.
+  String ->
+  -- | Weight.
+  Int ->
   -- | Enable tuning.
   Bool ->
   Proposal Double
-slideBactrian n w m s = createProposal n w (bactrianAdditiveSimple m s)
+slideBactrian m s = createProposal (bactrianAdditiveSimple m s)
 
 -- We have:
 -- x  (1+dx ) = x'
@@ -118,20 +120,20 @@
   | m < 0 = error "bactrianMultSimple: Spike parameter negative."
   | m >= 1 = error "bactrianMultSimple: Spike parameter 1.0 or larger."
   | s <= 0 = error "bactrianMultSimple: Standard deviation 0.0 or smaller."
-  | otherwise = ProposalSimple $ bactrianMult m (t * s)
+  | otherwise = bactrianMult m (t * s)
 
 -- | Multiplicative proposal with kernel similar to the silhouette of a Bactrian
 -- camel. See 'slideBactrian'.
 scaleBactrian ::
-  -- | Name.
-  String ->
-  -- | Weight.
-  Int ->
   -- | Spike parameter.
   Double ->
   -- | Standard deviation.
   Double ->
+  -- | Name.
+  String ->
+  -- | Weight.
+  Int ->
   -- | Enable tuning.
   Bool ->
   Proposal Double
-scaleBactrian n w m s = createProposal n w (bactrianMultSimple m s)
+scaleBactrian m s = createProposal (bactrianMultSimple m s)
diff --git a/src/Mcmc/Proposal/Generic.hs b/src/Mcmc/Proposal/Generic.hs
--- a/src/Mcmc/Proposal/Generic.hs
+++ b/src/Mcmc/Proposal/Generic.hs
@@ -50,7 +50,7 @@
   -- required for biased proposals.
   Maybe (Double -> Double) ->
   ProposalSimple a
-genericContinuous d f fInv = ProposalSimple $ sampleCont d f fInv
+genericContinuous d f fInv = sampleCont d f fInv
 
 sampleDiscrete ::
   (DiscreteDistr d, DiscreteGen d) =>
@@ -82,4 +82,4 @@
   -- required for biased proposals.
   Maybe (Int -> Int) ->
   ProposalSimple a
-genericDiscrete fd f fInv = ProposalSimple $ sampleDiscrete fd f fInv
+genericDiscrete fd f fInv = sampleDiscrete fd f fInv
diff --git a/src/Mcmc/Proposal/Scale.hs b/src/Mcmc/Proposal/Scale.hs
--- a/src/Mcmc/Proposal/Scale.hs
+++ b/src/Mcmc/Proposal/Scale.hs
@@ -29,37 +29,37 @@
 
 -- | Multiplicative proposal with Gamma distributed kernel.
 scale ::
-  -- | Name.
-  String ->
-  -- | Weight.
-  Int ->
   -- | Shape.
   Double ->
   -- | Scale.
   Double ->
+  -- | Name.
+  String ->
+  -- | Weight.
+  Int ->
   -- | Enable tuning.
   Bool ->
   Proposal Double
-scale n w k th = createProposal n w (scaleSimple k th)
+scale k th = createProposal (scaleSimple k th)
 
 -- | Multiplicative proposal with Gamma distributed kernel.
 --
 -- The scale of the Gamma distributions is set to (shape)^{-1}, so that the mean
 -- of the Gamma distribution is 1.0.
 scaleUnbiased ::
+  -- | Shape.
+  Double ->
   -- | Name.
   String ->
   -- | Weight.
   Int ->
-  -- | Shape.
-  Double ->
   -- | Enable tuning.
   Bool ->
   Proposal Double
-scaleUnbiased n w k = createProposal n w (scaleSimple k (1 / k))
+scaleUnbiased k = createProposal (scaleSimple k (1 / k))
 
 contra :: (Double, Double) -> Double -> (Double, Double)
-contra (x, y) z = (x*z, y/z)
+contra (x, y) z = (x * z, y / z)
 
 scaleContrarilySimple :: Double -> Double -> Double -> ProposalSimple (Double, Double)
 scaleContrarilySimple k th t = genericContinuous (gammaDistr (k / t) (th * t)) contra (Just recip)
@@ -69,15 +69,15 @@
 -- The two values are scaled contrarily so that their product stays constant.
 -- Contrary proposals are useful when parameters are confounded.
 scaleContrarily ::
-  -- | Name.
-  String ->
-  -- | Weight.
-  Int ->
   -- | Shape.
   Double ->
   -- | Scale.
   Double ->
+  -- | Name.
+  String ->
+  -- | Weight.
+  Int ->
   -- | Enable tuning.
   Bool ->
   Proposal (Double, Double)
-scaleContrarily n w k th = createProposal n w (scaleContrarilySimple k th)
+scaleContrarily k th = createProposal (scaleContrarilySimple k th)
diff --git a/src/Mcmc/Proposal/Slide.hs b/src/Mcmc/Proposal/Slide.hs
--- a/src/Mcmc/Proposal/Slide.hs
+++ b/src/Mcmc/Proposal/Slide.hs
@@ -30,18 +30,18 @@
 
 -- | Additive proposal with normally distributed kernel.
 slide ::
-  -- | Name.
-  String ->
-  -- | Weight.
-  Int ->
   -- | Mean.
   Double ->
   -- | Standard deviation.
   Double ->
+  -- | Name.
+  String ->
+  -- | Weight.
+  Int ->
   -- | Enable tuning.
   Bool ->
   Proposal Double
-slide n w m s = createProposal n w (slideSimple m s)
+slide m s = createProposal (slideSimple m s)
 
 -- The actual proposal with tuning parameter.
 slideSymmetricSimple :: Double -> Double -> ProposalSimple Double
@@ -51,16 +51,16 @@
 -- proposal is very fast, because the Metropolis-Hastings ratio does not include
 -- calculation of the forwards and backwards kernels.
 slideSymmetric ::
+  -- | Standard deviation.
+  Double ->
   -- | Name.
   String ->
   -- | Weight.
   Int ->
-  -- | Standard deviation.
-  Double ->
   -- | Enable tuning.
   Bool ->
   Proposal Double
-slideSymmetric n w s = createProposal n w (slideSymmetricSimple s)
+slideSymmetric s = createProposal (slideSymmetricSimple s)
 
 -- The actual proposal with tuning parameter.
 slideUniformSimple :: Double -> Double -> ProposalSimple Double
@@ -71,16 +71,16 @@
 -- because the Metropolis-Hastings ratio does not include calculation of the
 -- forwards and backwards kernels.
 slideUniform ::
+  -- | Delta.
+  Double ->
   -- | Name.
   String ->
   -- | Weight.
   Int ->
-  -- | Delta.
-  Double ->
   -- | Enable tuning.
   Bool ->
   Proposal Double
-slideUniform n w d = createProposal n w (slideUniformSimple d)
+slideUniform d = createProposal (slideUniformSimple d)
 
 contra :: (Double, Double) -> Double -> (Double, Double)
 contra (x, y) d = (x + d, y - d)
@@ -93,15 +93,15 @@
 -- The two values are slid contrarily so that their sum stays constant. Contrary
 -- proposals are useful when parameters are confounded.
 slideContrarily ::
-  -- | Name.
-  String ->
-  -- | Weight.
-  Int ->
   -- | Mean.
   Double ->
   -- | Standard deviation.
   Double ->
+  -- | Name.
+  String ->
+  -- | Weight.
+  Int ->
   -- | Enable tuning.
   Bool ->
   Proposal (Double, Double)
-slideContrarily n w m s = createProposal n w (slideContrarilySimple m s)
+slideContrarily m s = createProposal (slideContrarilySimple m s)
diff --git a/src/Mcmc/Save.hs b/src/Mcmc/Save.hs
--- a/src/Mcmc/Save.hs
+++ b/src/Mcmc/Save.hs
@@ -93,19 +93,8 @@
 
 -- | Save a 'Status' to file.
 --
--- Saved information:
--- - state
--- - iteration
--- - trace
--- - acceptance ratios
--- - generator
---
--- Important information that cannot be saved and has to be provided again when
--- a chain is restored:
--- - prior function
--- - likelihood function
--- - cycle
--- - monitor
+-- Some important values have to be provided upon restoring the status. See
+-- 'loadStatus'.
 saveStatus :: ToJSON a => FilePath -> Status a -> IO ()
 saveStatus fn s = BL.writeFile fn $ compress $ encode (toSave s)
 
@@ -175,13 +164,9 @@
   -- already a good indicator.
   when
     (p x /= svp)
-    ( error
-        "loadStatus: Provided prior function does not match the saved prior."
-    )
+    (error "loadStatus: Provided prior function does not match the saved prior.")
   when
     (l x /= svl)
-    ( error
-        "loadStatus: Provided likelihood function does not match the saved likelihood."
-    )
+    (error "loadStatus: Provided likelihood function does not match the saved likelihood.")
   removeFile fn
   return s
diff --git a/src/Mcmc/Status.hs b/src/Mcmc/Status.hs
--- a/src/Mcmc/Status.hs
+++ b/src/Mcmc/Status.hs
@@ -1,18 +1,13 @@
--- XXX: Add possibility to store supplementary information about the chain.
---
--- Maybe something like Trace b; and give a function a -> b to extract
--- supplementary info.
+-- Note: It is not necessary to add another type @b@ to store supplementary
+-- information about the chain. The information can just be stored in @a@
+-- equally well.
 
--- XXX: Status tuned exclusively to the Metropolis-Hastings algorithm. We
--- should abstract the algorithm from the chain. For example,
+-- XXX: Status tuned exclusively to the Metropolis-Hastings algorithm. We should
+-- abstract the algorithm from the chain. Maybe something like:
 --
 -- @
--- data Status a b = Status { Chain a; Algorithm a b}
+-- data Status a = Status { Chain a; Algorithm a}
 -- @
---
--- where a described the state space and b the auxiliary information of the
--- algorithm. This would also solve the above problem, for example in terms of
--- the Hamiltonian algorithm
 
 -- |
 -- Module      :  Mcmc.Status
@@ -49,6 +44,9 @@
 
 -- | The 'Status' contains all information to run an MCMC chain. It is
 -- constructed using the function 'status'.
+--
+-- The polymorphic type @a@ stores the state of the chain. It can also be used
+-- to store auxiliary information.
 data Status a = Status
   { -- MCMC related variables; saved.
 
@@ -136,7 +134,7 @@
   -- that auto tuning only happens during burn in.
   Int ->
   -- | A source of randomness. For reproducible runs, make
-  -- sure to use a generator with the same seed.
+  -- sure to use generators with the same, fixed seed.
   GenIO ->
   Status a
 status n p l c m x mB mT nI g
diff --git a/test/Mcmc/ProposalSpec.hs b/test/Mcmc/ProposalSpec.hs
--- a/test/Mcmc/ProposalSpec.hs
+++ b/test/Mcmc/ProposalSpec.hs
@@ -20,10 +20,10 @@
 import Test.Hspec
 
 p1 :: Proposal Double
-p1 = slideSymmetric "test1" 1 1.0 True
+p1 = slideSymmetric 1.0 "test1" 1 True
 
 p2 :: Proposal Double
-p2 = slideSymmetric "test2" 3 1.0 True
+p2 = slideSymmetric 1.0 "test2" 3 True
 
 c :: Cycle Double
 c = fromList [p1, p2]
diff --git a/test/Mcmc/SaveSpec.hs b/test/Mcmc/SaveSpec.hs
--- a/test/Mcmc/SaveSpec.hs
+++ b/test/Mcmc/SaveSpec.hs
@@ -38,10 +38,10 @@
 proposals :: Cycle Double
 proposals =
   fromList
-    [ slideSymmetric "small" 5 0.1 True,
-      slideSymmetric "medium" 2 1.0 True,
-      slideSymmetric "large" 2 5.0 True,
-      slide "skewed" 1 1.0 4.0 True
+    [ slideSymmetric 0.1 "small" 5 True,
+      slideSymmetric 1.0 "medium" 2 True,
+      slideSymmetric 5.0 "large" 2 True,
+      slide 1.0 4.0 "skewed" 1 True
     ]
 
 monStd :: MonitorStdOut Double