never executed always true always false

    1 -- Strict counterparts to common data structures,
    2 -- e.g. tuples, lists, maybes, etc.
    3 --
    4 -- Import this module qualified as Strict.
    5 
    6 {-# LANGUAGE DeriveDataTypeable #-}
    7 {-# LANGUAGE DeriveTraversable #-}
    8 
    9 module GHC.Data.Strict (
   10     Maybe(Nothing, Just),
   11     fromMaybe,
   12     Pair(And),
   13 
   14     -- Not used at the moment:
   15     --
   16     -- Either(Left, Right),
   17     -- List(Nil, Cons),
   18   ) where
   19 
   20 import GHC.Prelude hiding (Maybe(..), Either(..))
   21 import Control.Applicative
   22 import Data.Semigroup
   23 import Data.Data
   24 
   25 data Maybe a = Nothing | Just !a
   26   deriving (Eq, Ord, Show, Functor, Foldable, Traversable, Data)
   27 
   28 fromMaybe :: a -> Maybe a -> a
   29 fromMaybe d Nothing = d
   30 fromMaybe _ (Just x) = x
   31 
   32 apMaybe :: Maybe (a -> b) -> Maybe a -> Maybe b
   33 apMaybe (Just f) (Just x) = Just (f x)
   34 apMaybe _ _ = Nothing
   35 
   36 altMaybe :: Maybe a -> Maybe a -> Maybe a
   37 altMaybe Nothing r = r
   38 altMaybe l _ = l
   39 
   40 instance Semigroup a => Semigroup (Maybe a) where
   41   Nothing <> b       = b
   42   a       <> Nothing = a
   43   Just a  <> Just b  = Just (a <> b)
   44 
   45 instance Semigroup a => Monoid (Maybe a) where
   46   mempty = Nothing
   47 
   48 instance Applicative Maybe where
   49   pure = Just
   50   (<*>) = apMaybe
   51 
   52 instance Alternative Maybe where
   53   empty = Nothing
   54   (<|>) = altMaybe
   55 
   56 data Pair a b = !a `And` !b
   57   deriving (Eq, Ord, Show, Functor, Foldable, Traversable, Data)
   58 
   59 -- The definitions below are commented out because they are
   60 -- not used anywhere in the compiler, but are useful to showcase
   61 -- the intent behind this module (i.e. how it may evolve).
   62 --
   63 -- data Either a b = Left !a | Right !b
   64 --   deriving (Eq, Ord, Show, Functor, Foldable, Traversable, Data)
   65 --
   66 -- data List a = Nil | !a `Cons` !(List a)
   67 --   deriving (Eq, Ord, Show, Functor, Foldable, Traversable, Data)