Introduce Val class from #1815

This commit only adds the class and an instance for `Coin`. It differs
from #1815 in the following ways:

- Class methods are now intended for qualified import (as per Michael's
comment in #1803)
- Added some additional documentation. Some functions still need
significantly more documentation.
- Use the `Semigroup`, `Monoid` and `Group` superclasses. This removes
the need for various functions defined in this class.
- Removed the instance for `Integer` and define the instance for `Coin`
directly.

shelley/chain-and-ledger/executable-spec/shelley-spec-ledger.cabal

@@ -75,6 +75,7 @@ library
  Shelley.Spec.Ledger.Tx
  Shelley.Spec.Ledger.TxData
  Shelley.Spec.Ledger.UTxO
+ Shelley.Spec.Ledger.Val
  other-modules: Shelley.Spec.Ledger.API.Mempool
  Shelley.Spec.Ledger.API.Wallet
  Shelley.Spec.Ledger.API.Types
@@ -105,6 +106,7 @@ library
  cborg,
  cborg-json,
  containers,
+ groups,
  iproute,
  mtl,
  network,

shelley/chain-and-ledger/executable-spec/src/Shelley/Spec/Ledger/Coin.hs

@@ -15,6 +15,8 @@ where
 import Cardano.Binary (DecoderError (..), FromCBOR (..), ToCBOR (..))
 import Cardano.Prelude (NFData, NoUnexpectedThunks (..), cborError)
 import Data.Aeson (FromJSON, ToJSON)
+import Data.Group (Abelian, Group (..))
+import Data.Monoid (Sum (..))
 import Data.Text (pack)
 import Data.Word (Word64)
 import GHC.Generics (Generic)
@@ -34,6 +36,7 @@ newtype Coin = Coin {unCoin :: Integer}
  NFData
  )
  deriving (Show) via Quiet Coin
+ deriving (Semigroup, Monoid, Group, Abelian) via Sum Integer
 
 word64ToCoin :: Word64 -> Coin
 word64ToCoin w = Coin $ fromIntegral w

shelley/chain-and-ledger/executable-spec/src/Shelley/Spec/Ledger/Val.hs

@@ -0,0 +1,63 @@
+{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE EmptyDataDecls #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeFamilies #-}
+
+-- | This module defines a generalised notion of a "value" - that is, something
+-- with which we may quantify a transaction output.
+module Shelley.Spec.Ledger.Val where
+
+import Cardano.Prelude (NFData (), NoUnexpectedThunks (..))
+import Data.Group (Abelian)
+import Data.Typeable (Typeable)
+import Shelley.Spec.Ledger.Coin (Coin (..))
+
+data Comparison = Gt | Lt | Gteq | Lteq | Neq | Equal
+
+class
+ ( Abelian t,
+ Eq t,
+ -- Do we really need these?
+ Show t,
+ Typeable t,
+ NFData t,
+ NoUnexpectedThunks t
+ ) =>
+ Val t
+ where
+ -- | TODO This needs documenting. what is it?
+ scalev :: Integer -> t -> t
+
+ -- | Compare two values. Note that we only have a partial ordering; two values
+ -- may not necessarily be comparible, in which case `False` will be returned
+ -- for all comparisons.
+ compare :: Comparison -> t -> t -> Bool
+
+ -- | Is the argument zero?
+ isZero :: t -> Bool
+ isZero t = t == mempty
+
+ coin :: t -> Coin -- get the Coin amount
+ inject :: Coin -> t -- inject Coin into the Val instance
+ size :: t -> Integer -- compute size of Val instance
+ -- TODO add PACK/UNPACK stuff to this class
+
+instance Val Coin where
+ scalev n (Coin x) = Coin $ n * x
+ compare Gt x y = x > y
+ compare Lt x y = x < y
+ compare Gteq x y = x >= y
+ compare Lteq x y = x <= y
+ compare Neq x y = not (x == y)
+ compare Equal x y = x == y
+ coin = id
+ inject = id
+ size _ = 1