GHC/Base.hs

    {-

    NOTA BENE: Do NOT use ($) anywhere in this module! The type of ($) is
    slightly magical (it can return unlifted types), and it is wired in.
    But, it is also *defined* in this module, with a non-magical type.
    GHC gets terribly confused (and *hangs*) if you try to use ($) in this
    module, because it has different types in different scenarios.

    This is not a problem in general, because the type ($), being wired in, is not
    written out to the interface file, so importing files don't get confused.
    The problem is only if ($) is used here. So don't!

    ---------------------------------------------

    The overall structure of the GHC Prelude is a bit tricky.

      a) We want to avoid "orphan modules", i.e. ones with instance
            decls that don't belong either to a tycon or a class
            defined in the same module

      b) We want to avoid giant modules

    So the rough structure is as follows, in (linearised) dependency order

    GHC.Prim        Has no implementation.  It defines built-in things, and
                    by importing it you bring them into scope.
                    The source file is GHC.Prim.hi-boot, which is just
                    copied to make GHC.Prim.hi

    GHC.Base        Classes: Eq, Ord, Functor, Monad
                    Types:   list, (), Int, Bool, Ordering, Char, String

    Data.Tuple      Types: tuples, plus instances for GHC.Base classes

    GHC.Show        Class: Show, plus instances for GHC.Base/GHC.Tup types

    GHC.Enum        Class: Enum,  plus instances for GHC.Base/GHC.Tup types

    Data.Maybe      Type: Maybe, plus instances for GHC.Base classes

    GHC.List        List functions

    GHC.Num         Class: Num, plus instances for Int
                    Type:  Integer, plus instances for all classes so far (Eq, Ord, Num, Show)

                    Integer is needed here because it is mentioned in the signature
                    of 'fromInteger' in class Num

    GHC.Real        Classes: Real, Integral, Fractional, RealFrac
                             plus instances for Int, Integer
                    Types:  Ratio, Rational
                            plus instances for classes so far

                    Rational is needed here because it is mentioned in the signature
                    of 'toRational' in class Real

    GHC.ST  The ST monad, instances and a few helper functions

    Ix              Classes: Ix, plus instances for Int, Bool, Char, Integer, Ordering, tuples

    GHC.Arr         Types: Array, MutableArray, MutableVar

                    Arrays are used by a function in GHC.Float

    GHC.Float       Classes: Floating, RealFloat
                    Types:   Float, Double, plus instances of all classes so far

                    This module contains everything to do with floating point.
                    It is a big module (900 lines)
                    With a bit of luck, many modules can be compiled without ever reading GHC.Float.hi

    Other Prelude modules are much easier with fewer complex dependencies.
    -}

    {-# LANGUAGE Unsafe #-}
    {-# LANGUAGE CPP
               , NoImplicitPrelude
               , BangPatterns
               , ExplicitForAll
               , MagicHash
               , UnboxedTuples
               , ExistentialQuantification
               , RankNTypes
               , KindSignatures
               , PolyKinds
               , DataKinds
      #-}
    -- -Wno-orphans is needed for things like:
    -- Orphan rule: "x# -# x#" ALWAYS forall x# :: Int# -# x# x# = 0
    {-# OPTIONS_GHC -Wno-orphans #-}
    {-# OPTIONS_HADDOCK hide #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Base
    -- Copyright   :  (c) The University of Glasgow, 1992-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC extensions)
    --
    -- Basic data types and classes.
    --
    -----------------------------------------------------------------------------

    #include "MachDeps.h"

    module GHC.Base
            (
            module GHC.Base,
            module GHC.Classes,
            module GHC.CString,
            module GHC.Magic,
            module GHC.Types,
            module GHC.Prim,        -- Re-export GHC.Prim and [boot] GHC.Err,
                                    -- to avoid lots of people having to
            module GHC.Err,         -- import it explicitly
            module GHC.Maybe
      )
            where

    import GHC.Types
    import GHC.Classes
    import GHC.CString
    import GHC.Magic
    import GHC.Prim
    import GHC.Err
    import GHC.Maybe
    import {-# SOURCE #-} GHC.IO (failIO,mplusIO)

    import GHC.Tuple ()              -- Note [Depend on GHC.Tuple]
    import GHC.Integer ()            -- Note [Depend on GHC.Integer]
    import GHC.Natural ()            -- Note [Depend on GHC.Natural]

    -- for 'class Semigroup'
    import {-# SOURCE #-} GHC.Real (Integral)
    import {-# SOURCE #-} Data.Semigroup.Internal ( stimesDefault
                                                  , stimesMaybe
                                                  , stimesList
                                                  , stimesIdempotentMonoid
                                                  )

    infixr 9  .
    infixr 5  ++
    infixl 4  <$
    infixl 1  >>, >>=
    infixr 1  =<<
    infixr 0  $, $!

    infixl 4 <*>, <*, *>, <**>

    default ()              -- Double isn't available yet

    {-
    Note [Depend on GHC.Integer]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The Integer type is special because TidyPgm uses
    GHC.Integer.Type.mkInteger to construct Integer literal values
    Currently it reads the interface file whether or not the current
    module *has* any Integer literals, so it's important that
    GHC.Integer.Type (in package integer-gmp or integer-simple) is
    compiled before any other module.  (There's a hack in GHC to disable
    this for packages ghc-prim, integer-gmp, integer-simple, which aren't
    allowed to contain any Integer literals.)

    Likewise we implicitly need Integer when deriving things like Eq
    instances.

    The danger is that if the build system doesn't know about the dependency
    on Integer, it'll compile some base module before GHC.Integer.Type,
    resulting in:
      Failed to load interface for ‘GHC.Integer.Type’
        There are files missing in the ‘integer-gmp’ package,

    Bottom line: we make GHC.Base depend on GHC.Integer; and everything
    else either depends on GHC.Base, or does not have NoImplicitPrelude
    (and hence depends on Prelude).

    Note [Depend on GHC.Tuple]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Similarly, tuple syntax (or ()) creates an implicit dependency on
    GHC.Tuple, so we use the same rule as for Integer --- see Note [Depend on
    GHC.Integer] --- to explain this to the build system.  We make GHC.Base
    depend on GHC.Tuple, and everything else depends on GHC.Base or Prelude.

    Note [Depend on GHC.Natural]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Similar to GHC.Integer.
    -}

    #if 0
    -- for use when compiling GHC.Base itself doesn't work
    data  Bool  =  False | True
    data Ordering = LT | EQ | GT
    data Char = C# Char#
    type  String = [Char]
    data Int = I# Int#
    data  ()  =  ()
    data [] a = MkNil

    not True = False
    (&&) True True = True
    otherwise = True

    build = errorWithoutStackTrace "urk"
    foldr = errorWithoutStackTrace "urk"
    #endif

    infixr 6 <>

    -- | The class of semigroups (types with an associative binary operation).
    --
    -- Instances should satisfy the associativity law:
    --
    --  * @x '<>' (y '<>' z) = (x '<>' y) '<>' z@
    --
    -- @since 4.9.0.0
    class Semigroup a where
            -- | An associative operation.
            (<>) :: a -> a -> a

            -- | Reduce a non-empty list with @\<\>@
            --
            -- The default definition should be sufficient, but this can be
            -- overridden for efficiency.
            --
            sconcat :: NonEmpty a -> a
            sconcat (a :| as) = go a as where
              go b (c:cs) = b <> go c cs
              go b []     = b

            -- | Repeat a value @n@ times.
            --
            -- Given that this works on a 'Semigroup' it is allowed to fail if
            -- you request 0 or fewer repetitions, and the default definition
            -- will do so.
            --
            -- By making this a member of the class, idempotent semigroups
            -- and monoids can upgrade this to execute in /O(1)/ by
            -- picking @stimes = 'stimesIdempotent'@ or @stimes =
            -- 'stimesIdempotentMonoid'@ respectively.
            stimes :: Integral b => b -> a -> a
            stimes = stimesDefault

    -- | The class of monoids (types with an associative binary operation that
    -- has an identity).  Instances should satisfy the following laws:
    --
    --  * @x '<>' 'mempty' = x@
    --
    --  * @'mempty' '<>' x = x@
    --
    --  * @x '<>' (y '<>' z) = (x '<>' y) '<>' z@ ('Semigroup' law)
    --
    --  * @'mconcat' = 'foldr' '(<>)' 'mempty'@
    --
    -- The method names refer to the monoid of lists under concatenation,
    -- but there are many other instances.
    --
    -- Some types can be viewed as a monoid in more than one way,
    -- e.g. both addition and multiplication on numbers.
    -- In such cases we often define @newtype@s and make those instances
    -- of 'Monoid', e.g. 'Sum' and 'Product'.
    --
    -- __NOTE__: 'Semigroup' is a superclass of 'Monoid' since /base-4.11.0.0/.
    class Semigroup a => Monoid a where
            -- | Identity of 'mappend'
            mempty  :: a

            -- | An associative operation
            --
            -- __NOTE__: This method is redundant and has the default
            -- implementation @'mappend' = '(<>)'@ since /base-4.11.0.0/.
            mappend :: a -> a -> a
            mappend = (<>)
            {-# INLINE mappend #-}

            -- | Fold a list using the monoid.
            --
            -- For most types, the default definition for 'mconcat' will be
            -- used, but the function is included in the class definition so
            -- that an optimized version can be provided for specific types.
            mconcat :: [a] -> a
            mconcat = foldr mappend mempty

    -- | @since 4.9.0.0
    instance Semigroup [a] where
            (<>) = (++)
            {-# INLINE (<>) #-}

            stimes = stimesList

    -- | @since 2.01
    instance Monoid [a] where
            {-# INLINE mempty #-}
            mempty  = []
            {-# INLINE mconcat #-}
            mconcat xss = [x | xs <- xss, x <- xs]
    -- See Note: [List comprehensions and inlining]

    {-
    Note: [List comprehensions and inlining]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The list monad operations are traditionally described in terms of concatMap:

    xs >>= f = concatMap f xs

    Similarly, mconcat for lists is just concat. Here in Base, however, we don't
    have concatMap, and we'll refrain from adding it here so it won't have to be
    hidden in imports. Instead, we use GHC's list comprehension desugaring
    mechanism to define mconcat and the Applicative and Monad instances for lists.
    We mark them INLINE because the inliner is not generally too keen to inline
    build forms such as the ones these desugar to without our insistence.  Defining
    these using list comprehensions instead of foldr has an additional potential
    benefit, as described in compiler/deSugar/DsListComp.hs: if optimizations
    needed to make foldr/build forms efficient are turned off, we'll get reasonably
    efficient translations anyway.
    -}

    -- | @since 4.9.0.0
    instance Semigroup (NonEmpty a) where
            (a :| as) <> ~(b :| bs) = a :| (as ++ b : bs)

    -- | @since 4.9.0.0
    instance Semigroup b => Semigroup (a -> b) where
            f <> g = \x -> f x <> g x
            stimes n f e = stimes n (f e)

    -- | @since 2.01
    instance Monoid b => Monoid (a -> b) where
            mempty _ = mempty

    -- | @since 4.9.0.0
    instance Semigroup () where
            _ <> _      = ()
            sconcat _   = ()
            stimes  _ _ = ()

    -- | @since 2.01
    instance Monoid () where
            -- Should it be strict?
            mempty        = ()
            mconcat _     = ()

    -- | @since 4.9.0.0
    instance (Semigroup a, Semigroup b) => Semigroup (a, b) where
            (a,b) <> (a',b') = (a<>a',b<>b')
            stimes n (a,b) = (stimes n a, stimes n b)

    -- | @since 2.01
    instance (Monoid a, Monoid b) => Monoid (a,b) where
            mempty = (mempty, mempty)

    -- | @since 4.9.0.0
    instance (Semigroup a, Semigroup b, Semigroup c) => Semigroup (a, b, c) where
            (a,b,c) <> (a',b',c') = (a<>a',b<>b',c<>c')
            stimes n (a,b,c) = (stimes n a, stimes n b, stimes n c)

    -- | @since 2.01
    instance (Monoid a, Monoid b, Monoid c) => Monoid (a,b,c) where
            mempty = (mempty, mempty, mempty)

    -- | @since 4.9.0.0
    instance (Semigroup a, Semigroup b, Semigroup c, Semigroup d)
             => Semigroup (a, b, c, d) where
            (a,b,c,d) <> (a',b',c',d') = (a<>a',b<>b',c<>c',d<>d')
            stimes n (a,b,c,d) = (stimes n a, stimes n b, stimes n c, stimes n d)

    -- | @since 2.01
    instance (Monoid a, Monoid b, Monoid c, Monoid d) => Monoid (a,b,c,d) where
            mempty = (mempty, mempty, mempty, mempty)

    -- | @since 4.9.0.0
    instance (Semigroup a, Semigroup b, Semigroup c, Semigroup d, Semigroup e)
             => Semigroup (a, b, c, d, e) where
            (a,b,c,d,e) <> (a',b',c',d',e') = (a<>a',b<>b',c<>c',d<>d',e<>e')
            stimes n (a,b,c,d,e) =
                (stimes n a, stimes n b, stimes n c, stimes n d, stimes n e)

    -- | @since 2.01
    instance (Monoid a, Monoid b, Monoid c, Monoid d, Monoid e) =>
                    Monoid (a,b,c,d,e) where
            mempty = (mempty, mempty, mempty, mempty, mempty)

    -- | @since 4.9.0.0
    instance Semigroup Ordering where
        LT <> _ = LT
        EQ <> y = y
        GT <> _ = GT

        stimes = stimesIdempotentMonoid

    -- lexicographical ordering
    -- | @since 2.01
    instance Monoid Ordering where
        mempty             = EQ

    -- | @since 4.9.0.0
    instance Semigroup a => Semigroup (Maybe a) where
        Nothing <> b       = b
        a       <> Nothing = a
        Just a  <> Just b  = Just (a <> b)

        stimes = stimesMaybe

    -- | Lift a semigroup into 'Maybe' forming a 'Monoid' according to
    -- <http://en.wikipedia.org/wiki/Monoid>: \"Any semigroup @S@ may be
    -- turned into a monoid simply by adjoining an element @e@ not in @S@
    -- and defining @e*e = e@ and @e*s = s = s*e@ for all @s ∈ S@.\"
    --
    -- /Since 4.11.0/: constraint on inner @a@ value generalised from
    -- 'Monoid' to 'Semigroup'.
    --
    -- @since 2.01
    instance Semigroup a => Monoid (Maybe a) where
        mempty = Nothing

    -- | For tuples, the 'Monoid' constraint on @a@ determines
    -- how the first values merge.
    -- For example, 'String's concatenate:
    --
    -- > ("hello ", (+15)) <*> ("world!", 2002)
    -- > ("hello world!",2017)
    --
    -- @since 2.01
    instance Monoid a => Applicative ((,) a) where
        pure x = (mempty, x)
        (u, f) <*> (v, x) = (u <> v, f x)
        liftA2 f (u, x) (v, y) = (u <> v, f x y)

    -- | @since 4.9.0.0
    instance Monoid a => Monad ((,) a) where
        (u, a) >>= k = case k a of (v, b) -> (u <> v, b)

    -- | @since 4.10.0.0
    instance Semigroup a => Semigroup (IO a) where
        (<>) = liftA2 (<>)

    -- | @since 4.9.0.0
    instance Monoid a => Monoid (IO a) where
        mempty = pure mempty

    {- | The 'Functor' class is used for types that can be mapped over.
    Instances of 'Functor' should satisfy the following laws:

    > fmap id  ==  id
    > fmap (f . g)  ==  fmap f . fmap g

    The instances of 'Functor' for lists, 'Data.Maybe.Maybe' and 'System.IO.IO'
    satisfy these laws.
    -}

    class  Functor f  where
        fmap        :: (a -> b) -> f a -> f b

        -- | Replace all locations in the input with the same value.
        -- The default definition is @'fmap' . 'const'@, but this may be
        -- overridden with a more efficient version.
        (<$)        :: a -> f b -> f a
        (<$)        =  fmap . const

    -- | A functor with application, providing operations to
    --
    -- * embed pure expressions ('pure'), and
    --
    -- * sequence computations and combine their results ('<*>' and 'liftA2').
    --
    -- A minimal complete definition must include implementations of 'pure'
    -- and of either '<*>' or 'liftA2'. If it defines both, then they must behave
    -- the same as their default definitions:
    --
    --      @('<*>') = 'liftA2' 'id'@
    --
    --      @'liftA2' f x y = f '<$>' x '<*>' y@
    --
    -- Further, any definition must satisfy the following:
    --
    -- [/identity/]
    --
    --      @'pure' 'id' '<*>' v = v@
    --
    -- [/composition/]
    --
    --      @'pure' (.) '<*>' u '<*>' v '<*>' w = u '<*>' (v '<*>' w)@
    --
    -- [/homomorphism/]
    --
    --      @'pure' f '<*>' 'pure' x = 'pure' (f x)@
    --
    -- [/interchange/]
    --
    --      @u '<*>' 'pure' y = 'pure' ('$' y) '<*>' u@
    --
    --
    -- The other methods have the following default definitions, which may
    -- be overridden with equivalent specialized implementations:
    --
    --   * @u '*>' v = ('id' '<$' u) '<*>' v@
    --
    --   * @u '<*' v = 'liftA2' 'const' u v@
    --
    -- As a consequence of these laws, the 'Functor' instance for @f@ will satisfy
    --
    --   * @'fmap' f x = 'pure' f '<*>' x@
    --
    --
    -- It may be useful to note that supposing
    --
    --      @forall x y. p (q x y) = f x . g y@
    --
    -- it follows from the above that
    --
    --      @'liftA2' p ('liftA2' q u v) = 'liftA2' f u . 'liftA2' g v@
    --
    --
    -- If @f@ is also a 'Monad', it should satisfy
    --
    --   * @'pure' = 'return'@
    --
    --   * @('<*>') = 'ap'@
    --
    --   * @('*>') = ('>>')@
    --
    -- (which implies that 'pure' and '<*>' satisfy the applicative functor laws).

    class Functor f => Applicative f where
        {-# MINIMAL pure, ((<*>) | liftA2) #-}
        -- | Lift a value.
        pure :: a -> f a

        -- | Sequential application.
        --
        -- A few functors support an implementation of '<*>' that is more
        -- efficient than the default one.
        (<*>) :: f (a -> b) -> f a -> f b
        (<*>) = liftA2 id

        -- | Lift a binary function to actions.
        --
        -- Some functors support an implementation of 'liftA2' that is more
        -- efficient than the default one. In particular, if 'fmap' is an
        -- expensive operation, it is likely better to use 'liftA2' than to
        -- 'fmap' over the structure and then use '<*>'.
        liftA2 :: (a -> b -> c) -> f a -> f b -> f c
        liftA2 f x = (<*>) (fmap f x)

        -- | Sequence actions, discarding the value of the first argument.
        (*>) :: f a -> f b -> f b
        a1 *> a2 = (id <$ a1) <*> a2
        -- This is essentially the same as liftA2 (flip const), but if the
        -- Functor instance has an optimized (<$), it may be better to use
        -- that instead. Before liftA2 became a method, this definition
        -- was strictly better, but now it depends on the functor. For a
        -- functor supporting a sharing-enhancing (<$), this definition
        -- may reduce allocation by preventing a1 from ever being fully
        -- realized. In an implementation with a boring (<$) but an optimizing
        -- liftA2, it would likely be better to define (*>) using liftA2.

        -- | Sequence actions, discarding the value of the second argument.
        (<*) :: f a -> f b -> f a
        (<*) = liftA2 const

    -- | A variant of '<*>' with the arguments reversed.
    (<**>) :: Applicative f => f a -> f (a -> b) -> f b
    (<**>) = liftA2 (\a f -> f a)
    -- Don't use $ here, see the note at the top of the page

    -- | Lift a function to actions.
    -- This function may be used as a value for `fmap` in a `Functor` instance.
    liftA :: Applicative f => (a -> b) -> f a -> f b
    liftA f a = pure f <*> a
    -- Caution: since this may be used for `fmap`, we can't use the obvious
    -- definition of liftA = fmap.

    -- | Lift a ternary function to actions.
    liftA3 :: Applicative f => (a -> b -> c -> d) -> f a -> f b -> f c -> f d
    liftA3 f a b c = liftA2 f a b <*> c

    {-# INLINABLE liftA #-}
    {-# SPECIALISE liftA :: (a1->r) -> IO a1 -> IO r #-}
    {-# SPECIALISE liftA :: (a1->r) -> Maybe a1 -> Maybe r #-}
    {-# INLINABLE liftA3 #-}
    {-# SPECIALISE liftA3 :: (a1->a2->a3->r) -> IO a1 -> IO a2 -> IO a3 -> IO r #-}
    {-# SPECIALISE liftA3 :: (a1->a2->a3->r) ->
                                    Maybe a1 -> Maybe a2 -> Maybe a3 -> Maybe r #-}

    -- | The 'join' function is the conventional monad join operator. It
    -- is used to remove one level of monadic structure, projecting its
    -- bound argument into the outer level.
    --
    -- ==== __Examples__
    --
    -- A common use of 'join' is to run an 'IO' computation returned from
    -- an 'GHC.Conc.STM' transaction, since 'GHC.Conc.STM' transactions
    -- can't perform 'IO' directly. Recall that
    --
    -- @
    -- 'GHC.Conc.atomically' :: STM a -> IO a
    -- @
    --
    -- is used to run 'GHC.Conc.STM' transactions atomically. So, by
    -- specializing the types of 'GHC.Conc.atomically' and 'join' to
    --
    -- @
    -- 'GHC.Conc.atomically' :: STM (IO b) -> IO (IO b)
    -- 'join'       :: IO (IO b)  -> IO b
    -- @
    --
    -- we can compose them as
    --
    -- @
    -- 'join' . 'GHC.Conc.atomically' :: STM (IO b) -> IO b
    -- @
    --
    -- to run an 'GHC.Conc.STM' transaction and the 'IO' action it
    -- returns.
    join              :: (Monad m) => m (m a) -> m a
    join x            =  x >>= id

    {- | The 'Monad' class defines the basic operations over a /monad/,
    a concept from a branch of mathematics known as /category theory/.
    From the perspective of a Haskell programmer, however, it is best to
    think of a monad as an /abstract datatype/ of actions.
    Haskell's @do@ expressions provide a convenient syntax for writing
    monadic expressions.

    Instances of 'Monad' should satisfy the following laws:

    * @'return' a '>>=' k  =  k a@
    * @m '>>=' 'return'  =  m@
    * @m '>>=' (\\x -> k x '>>=' h)  =  (m '>>=' k) '>>=' h@

    Furthermore, the 'Monad' and 'Applicative' operations should relate as follows:

    * @'pure' = 'return'@
    * @('<*>') = 'ap'@

    The above laws imply:

    * @'fmap' f xs  =  xs '>>=' 'return' . f@
    * @('>>') = ('*>')@

    and that 'pure' and ('<*>') satisfy the applicative functor laws.

    The instances of 'Monad' for lists, 'Data.Maybe.Maybe' and 'System.IO.IO'
    defined in the "Prelude" satisfy these laws.
    -}
    class Applicative m => Monad m where
        -- | Sequentially compose two actions, passing any value produced
        -- by the first as an argument to the second.
        (>>=)       :: forall a b. m a -> (a -> m b) -> m b

        -- | Sequentially compose two actions, discarding any value produced
        -- by the first, like sequencing operators (such as the semicolon)
        -- in imperative languages.
        (>>)        :: forall a b. m a -> m b -> m b
        m >> k = m >>= \_ -> k -- See Note [Recursive bindings for Applicative/Monad]
        {-# INLINE (>>) #-}

        -- | Inject a value into the monadic type.
        return      :: a -> m a
        return      = pure

        -- | Fail with a message.  This operation is not part of the
        -- mathematical definition of a monad, but is invoked on pattern-match
        -- failure in a @do@ expression.
        --
        -- As part of the MonadFail proposal (MFP), this function is moved
        -- to its own class 'MonadFail' (see "Control.Monad.Fail" for more
        -- details). The definition here will be removed in a future
        -- release.
        fail        :: String -> m a
        fail s      = errorWithoutStackTrace s

    {- Note [Recursive bindings for Applicative/Monad]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    The original Applicative/Monad proposal stated that after
    implementation, the designated implementation of (>>) would become

      (>>) :: forall a b. m a -> m b -> m b
      (>>) = (*>)

    by default. You might be inclined to change this to reflect the stated
    proposal, but you really shouldn't! Why? Because people tend to define
    such instances the /other/ way around: in particular, it is perfectly
    legitimate to define an instance of Applicative (*>) in terms of (>>),
    which would lead to an infinite loop for the default implementation of
    Monad! And people do this in the wild.

    This turned into a nasty bug that was tricky to track down, and rather
    than eliminate it everywhere upstream, it's easier to just retain the
    original default.

    -}

    -- | Same as '>>=', but with the arguments interchanged.
    {-# SPECIALISE (=<<) :: (a -> [b]) -> [a] -> [b] #-}
    (=<<)           :: Monad m => (a -> m b) -> m a -> m b
    f =<< x         = x >>= f

    -- | Conditional execution of 'Applicative' expressions. For example,
    --
    -- > when debug (putStrLn "Debugging")
    --
    -- will output the string @Debugging@ if the Boolean value @debug@
    -- is 'True', and otherwise do nothing.
    when      :: (Applicative f) => Bool -> f () -> f ()
    {-# INLINABLE when #-}
    {-# SPECIALISE when :: Bool -> IO () -> IO () #-}
    {-# SPECIALISE when :: Bool -> Maybe () -> Maybe () #-}
    when p s  = if p then s else pure ()

    -- | Evaluate each action in the sequence from left to right,
    -- and collect the results.
    sequence :: Monad m => [m a] -> m [a]
    {-# INLINE sequence #-}
    sequence = mapM id
    -- Note: [sequence and mapM]

    -- | @'mapM' f@ is equivalent to @'sequence' . 'map' f@.
    mapM :: Monad m => (a -> m b) -> [a] -> m [b]
    {-# INLINE mapM #-}
    mapM f as = foldr k (return []) as
                where
                  k a r = do { x <- f a; xs <- r; return (x:xs) }

    {-
    Note: [sequence and mapM]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Originally, we defined

    mapM f = sequence . map f

    This relied on list fusion to produce efficient code for mapM, and led to
    excessive allocation in cryptarithm2. Defining

    sequence = mapM id

    relies only on inlining a tiny function (id) and beta reduction, which tends to
    be a more reliable aspect of simplification. Indeed, this does not lead to
    similar problems in nofib.
    -}

    -- | Promote a function to a monad.
    liftM   :: (Monad m) => (a1 -> r) -> m a1 -> m r
    liftM f m1              = do { x1 <- m1; return (f x1) }

    -- | Promote a function to a monad, scanning the monadic arguments from
    -- left to right.  For example,
    --
    -- > liftM2 (+) [0,1] [0,2] = [0,2,1,3]
    -- > liftM2 (+) (Just 1) Nothing = Nothing
    --
    liftM2  :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
    liftM2 f m1 m2          = do { x1 <- m1; x2 <- m2; return (f x1 x2) }
    -- Caution: since this may be used for `liftA2`, we can't use the obvious
    -- definition of liftM2 = liftA2.

    -- | Promote a function to a monad, scanning the monadic arguments from
    -- left to right (cf. 'liftM2').
    liftM3  :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
    liftM3 f m1 m2 m3       = do { x1 <- m1; x2 <- m2; x3 <- m3; return (f x1 x2 x3) }

    -- | Promote a function to a monad, scanning the monadic arguments from
    -- left to right (cf. 'liftM2').
    liftM4  :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
    liftM4 f m1 m2 m3 m4    = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; return (f x1 x2 x3 x4) }

    -- | Promote a function to a monad, scanning the monadic arguments from
    -- left to right (cf. 'liftM2').
    liftM5  :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
    liftM5 f m1 m2 m3 m4 m5 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; x5 <- m5; return (f x1 x2 x3 x4 x5) }

    {-# INLINABLE liftM #-}
    {-# SPECIALISE liftM :: (a1->r) -> IO a1 -> IO r #-}
    {-# SPECIALISE liftM :: (a1->r) -> Maybe a1 -> Maybe r #-}
    {-# INLINABLE liftM2 #-}
    {-# SPECIALISE liftM2 :: (a1->a2->r) -> IO a1 -> IO a2 -> IO r #-}
    {-# SPECIALISE liftM2 :: (a1->a2->r) -> Maybe a1 -> Maybe a2 -> Maybe r #-}
    {-# INLINABLE liftM3 #-}
    {-# SPECIALISE liftM3 :: (a1->a2->a3->r) -> IO a1 -> IO a2 -> IO a3 -> IO r #-}
    {-# SPECIALISE liftM3 :: (a1->a2->a3->r) -> Maybe a1 -> Maybe a2 -> Maybe a3 -> Maybe r #-}
    {-# INLINABLE liftM4 #-}
    {-# SPECIALISE liftM4 :: (a1->a2->a3->a4->r) -> IO a1 -> IO a2 -> IO a3 -> IO a4 -> IO r #-}
    {-# SPECIALISE liftM4 :: (a1->a2->a3->a4->r) -> Maybe a1 -> Maybe a2 -> Maybe a3 -> Maybe a4 -> Maybe r #-}
    {-# INLINABLE liftM5 #-}
    {-# SPECIALISE liftM5 :: (a1->a2->a3->a4->a5->r) -> IO a1 -> IO a2 -> IO a3 -> IO a4 -> IO a5 -> IO r #-}
    {-# SPECIALISE liftM5 :: (a1->a2->a3->a4->a5->r) -> Maybe a1 -> Maybe a2 -> Maybe a3 -> Maybe a4 -> Maybe a5 -> Maybe r #-}

    {- | In many situations, the 'liftM' operations can be replaced by uses of
    'ap', which promotes function application.

    > return f `ap` x1 `ap` ... `ap` xn

    is equivalent to

    > liftMn f x1 x2 ... xn

    -}

    ap                :: (Monad m) => m (a -> b) -> m a -> m b
    ap m1 m2          = do { x1 <- m1; x2 <- m2; return (x1 x2) }
    -- Since many Applicative instances define (<*>) = ap, we
    -- cannot define ap = (<*>)
    {-# INLINABLE ap #-}
    {-# SPECIALISE ap :: IO (a -> b) -> IO a -> IO b #-}
    {-# SPECIALISE ap :: Maybe (a -> b) -> Maybe a -> Maybe b #-}

    -- instances for Prelude types

    -- | @since 2.01
    instance Functor ((->) r) where
        fmap = (.)

    -- | @since 2.01
    instance Applicative ((->) a) where
        pure = const
        (<*>) f g x = f x (g x)
        liftA2 q f g x = q (f x) (g x)

    -- | @since 2.01
    instance Monad ((->) r) where
        f >>= k = \ r -> k (f r) r

    -- | @since 2.01
    instance Functor ((,) a) where
        fmap f (x,y) = (x, f y)

    -- | @since 2.01
    instance  Functor Maybe  where
        fmap _ Nothing       = Nothing
        fmap f (Just a)      = Just (f a)

    -- | @since 2.01
    instance Applicative Maybe where
        pure = Just

        Just f  <*> m       = fmap f m
        Nothing <*> _m      = Nothing

        liftA2 f (Just x) (Just y) = Just (f x y)
        liftA2 _ _ _ = Nothing

        Just _m1 *> m2      = m2
        Nothing  *> _m2     = Nothing

    -- | @since 2.01
    instance  Monad Maybe  where
        (Just x) >>= k      = k x
        Nothing  >>= _      = Nothing

        (>>) = (*>)

        fail _              = Nothing

    -- -----------------------------------------------------------------------------
    -- The Alternative class definition

    infixl 3 <|>

    -- | A monoid on applicative functors.
    --
    -- If defined, 'some' and 'many' should be the least solutions
    -- of the equations:
    --
    -- * @'some' v = (:) '<$>' v '<*>' 'many' v@
    --
    -- * @'many' v = 'some' v '<|>' 'pure' []@
    class Applicative f => Alternative f where
        -- | The identity of '<|>'
        empty :: f a
        -- | An associative binary operation
        (<|>) :: f a -> f a -> f a

        -- | One or more.
        some :: f a -> f [a]
        some v = some_v
          where
            many_v = some_v <|> pure []
            some_v = liftA2 (:) v many_v

        -- | Zero or more.
        many :: f a -> f [a]
        many v = many_v
          where
            many_v = some_v <|> pure []
            some_v = liftA2 (:) v many_v

    -- | @since 2.01
    instance Alternative Maybe where
        empty = Nothing
        Nothing <|> r = r
        l       <|> _ = l

    -- -----------------------------------------------------------------------------
    -- The MonadPlus class definition

    -- | Monads that also support choice and failure.
    class (Alternative m, Monad m) => MonadPlus m where
       -- | The identity of 'mplus'.  It should also satisfy the equations
       --
       -- > mzero >>= f  =  mzero
       -- > v >> mzero   =  mzero
       --
       -- The default definition is
       --
       -- @
       -- mzero = 'empty'
       -- @
       mzero :: m a
       mzero = empty

       -- | An associative operation. The default definition is
       --
       -- @
       -- mplus = ('<|>')
       -- @
       mplus :: m a -> m a -> m a
       mplus = (<|>)

    -- | @since 2.01
    instance MonadPlus Maybe

    ---------------------------------------------
    -- The non-empty list type

    infixr 5 :|

    -- | Non-empty (and non-strict) list type.
    --
    -- @since 4.9.0.0
    data NonEmpty a = a :| [a]
      deriving ( Eq  -- ^ @since 4.9.0.0
               , Ord -- ^ @since 4.9.0.0
               )

    -- | @since 4.9.0.0
    instance Functor NonEmpty where
      fmap f ~(a :| as) = f a :| fmap f as
      b <$ ~(_ :| as)   = b   :| (b <$ as)

    -- | @since 4.9.0.0
    instance Applicative NonEmpty where
      pure a = a :| []
      (<*>) = ap
      liftA2 = liftM2

    -- | @since 4.9.0.0
    instance Monad NonEmpty where
      ~(a :| as) >>= f = b :| (bs ++ bs')
        where b :| bs = f a
              bs' = as >>= toList . f
              toList ~(c :| cs) = c : cs

    ----------------------------------------------
    -- The list type

    -- | @since 2.01
    instance Functor [] where
        {-# INLINE fmap #-}
        fmap = map

    -- See Note: [List comprehensions and inlining]
    -- | @since 2.01
    instance Applicative [] where
        {-# INLINE pure #-}
        pure x    = [x]
        {-# INLINE (<*>) #-}
        fs <*> xs = [f x | f <- fs, x <- xs]
        {-# INLINE liftA2 #-}
        liftA2 f xs ys = [f x y | x <- xs, y <- ys]
        {-# INLINE (*>) #-}
        xs *> ys  = [y | _ <- xs, y <- ys]

    -- See Note: [List comprehensions and inlining]
    -- | @since 2.01
    instance Monad []  where
        {-# INLINE (>>=) #-}
        xs >>= f             = [y | x <- xs, y <- f x]
        {-# INLINE (>>) #-}
        (>>) = (*>)
        {-# INLINE fail #-}
        fail _              = []

    -- | @since 2.01
    instance Alternative [] where
        empty = []
        (<|>) = (++)

    -- | @since 2.01
    instance MonadPlus []

    {-
    A few list functions that appear here because they are used here.
    The rest of the prelude list functions are in GHC.List.
    -}

    ----------------------------------------------
    --      foldr/build/augment
    ----------------------------------------------

    -- | 'foldr', applied to a binary operator, a starting value (typically
    -- the right-identity of the operator), and a list, reduces the list
    -- using the binary operator, from right to left:
    --
    -- > foldr f z [x1, x2, ..., xn] == x1 `f` (x2 `f` ... (xn `f` z)...)

    foldr            :: (a -> b -> b) -> b -> [a] -> b
    -- foldr _ z []     =  z
    -- foldr f z (x:xs) =  f x (foldr f z xs)
    {-# INLINE [0] foldr #-}
    -- Inline only in the final stage, after the foldr/cons rule has had a chance
    -- Also note that we inline it when it has *two* parameters, which are the
    -- ones we are keen about specialising!
    foldr k z = go
              where
                go []     = z
                go (y:ys) = y `k` go ys

    -- | A list producer that can be fused with 'foldr'.
    -- This function is merely
    --
    -- >    build g = g (:) []
    --
    -- but GHC's simplifier will transform an expression of the form
    -- @'foldr' k z ('build' g)@, which may arise after inlining, to @g k z@,
    -- which avoids producing an intermediate list.

    build   :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
    {-# INLINE [1] build #-}
            -- The INLINE is important, even though build is tiny,
            -- because it prevents [] getting inlined in the version that
            -- appears in the interface file.  If [] *is* inlined, it
            -- won't match with [] appearing in rules in an importing module.
            --
            -- The "1" says to inline in phase 1

    build g = g (:) []

    -- | A list producer that can be fused with 'foldr'.
    -- This function is merely
    --
    -- >    augment g xs = g (:) xs
    --
    -- but GHC's simplifier will transform an expression of the form
    -- @'foldr' k z ('augment' g xs)@, which may arise after inlining, to
    -- @g k ('foldr' k z xs)@, which avoids producing an intermediate list.

    augment :: forall a. (forall b. (a->b->b) -> b -> b) -> [a] -> [a]
    {-# INLINE [1] augment #-}
    augment g xs = g (:) xs

    {-# RULES
    "fold/build"    forall k z (g::forall b. (a->b->b) -> b -> b) .
                    foldr k z (build g) = g k z

    "foldr/augment" forall k z xs (g::forall b. (a->b->b) -> b -> b) .
                    foldr k z (augment g xs) = g k (foldr k z xs)

    "foldr/id"                        foldr (:) [] = \x  -> x
    "foldr/app"     [1] forall ys. foldr (:) ys = \xs -> xs ++ ys
            -- Only activate this from phase 1, because that's
            -- when we disable the rule that expands (++) into foldr

    -- The foldr/cons rule looks nice, but it can give disastrously
    -- bloated code when commpiling
    --      array (a,b) [(1,2), (2,2), (3,2), ...very long list... ]
    -- i.e. when there are very very long literal lists
    -- So I've disabled it for now. We could have special cases
    -- for short lists, I suppose.
    -- "foldr/cons" forall k z x xs. foldr k z (x:xs) = k x (foldr k z xs)

    "foldr/single"  forall k z x. foldr k z [x] = k x z
    "foldr/nil"     forall k z.   foldr k z []  = z

    "foldr/cons/build" forall k z x (g::forall b. (a->b->b) -> b -> b) .
                               foldr k z (x:build g) = k x (g k z)

    "augment/build" forall (g::forall b. (a->b->b) -> b -> b)
                           (h::forall b. (a->b->b) -> b -> b) .
                           augment g (build h) = build (\c n -> g c (h c n))
    "augment/nil"   forall (g::forall b. (a->b->b) -> b -> b) .
                            augment g [] = build g
     #-}

    -- This rule is true, but not (I think) useful:
    --      augment g (augment h t) = augment (\cn -> g c (h c n)) t

    ----------------------------------------------
    --              map
    ----------------------------------------------

    -- | 'map' @f xs@ is the list obtained by applying @f@ to each element
    -- of @xs@, i.e.,
    --
    -- > map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn]
    -- > map f [x1, x2, ...] == [f x1, f x2, ...]

    map :: (a -> b) -> [a] -> [b]
    {-# NOINLINE [0] map #-}
      -- We want the RULEs "map" and "map/coerce" to fire first.
      -- map is recursive, so won't inline anyway,
      -- but saying so is more explicit, and silences warnings
    map _ []     = []
    map f (x:xs) = f x : map f xs

    -- Note eta expanded
    mapFB ::  (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst
    {-# INLINE [0] mapFB #-} -- See Note [Inline FB functions] in GHC.List
    mapFB c f = \x ys -> c (f x) ys

    {- Note [The rules for map]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The rules for map work like this.

    * Up to (but not including) phase 1, we use the "map" rule to
      rewrite all saturated applications of map with its build/fold
      form, hoping for fusion to happen.

      In phase 1 and 0, we switch off that rule, inline build, and
      switch on the "mapList" rule, which rewrites the foldr/mapFB
      thing back into plain map.

      It's important that these two rules aren't both active at once
      (along with build's unfolding) else we'd get an infinite loop
      in the rules.  Hence the activation control below.

    * This same pattern is followed by many other functions:
      e.g. append, filter, iterate, repeat, etc. in GHC.List

      See also Note [Inline FB functions] in GHC.List

    * The "mapFB" rule optimises compositions of map

    * The "mapFB/id" rule gets rid of 'map id' calls.
      You might think that (mapFB c id) will turn into c simply
      when mapFB is inlined; but before that happens the "mapList"
      rule turns
         (foldr (mapFB (:) id) [] a
      back into
         map id
      Which is not very clever.

    * Any similarity to the Functor laws for [] is expected.
    -}

    {-# RULES
    "map"       [~1] forall f xs.   map f xs                = build (\c n -> foldr (mapFB c f) n xs)
    "mapList"   [1]  forall f.      foldr (mapFB (:) f) []  = map f
    "mapFB"     forall c f g.       mapFB (mapFB c f) g     = mapFB c (f.g)
    "mapFB/id"  forall c.           mapFB c (\x -> x)       = c
      #-}

    -- See Breitner, Eisenberg, Peyton Jones, and Weirich, "Safe Zero-cost
    -- Coercions for Haskell", section 6.5:
    --   http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/coercible.pdf

    {-# RULES "map/coerce" [1] map coerce = coerce #-}

    ----------------------------------------------
    --              append
    ----------------------------------------------

    -- | Append two lists, i.e.,
    --
    -- > [x1, ..., xm] ++ [y1, ..., yn] == [x1, ..., xm, y1, ..., yn]
    -- > [x1, ..., xm] ++ [y1, ...] == [x1, ..., xm, y1, ...]
    --
    -- If the first list is not finite, the result is the first list.

    (++) :: [a] -> [a] -> [a]
    {-# NOINLINE [1] (++) #-}    -- We want the RULE to fire first.
                                 -- It's recursive, so won't inline anyway,
                                 -- but saying so is more explicit
    (++) []     ys = ys
    (++) (x:xs) ys = x : xs ++ ys

    {-# RULES
    "++"    [~1] forall xs ys. xs ++ ys = augment (\c n -> foldr c n xs) ys
      #-}

    -- |'otherwise' is defined as the value 'True'.  It helps to make
    -- guards more readable.  eg.
    --
    -- >  f x | x < 0     = ...
    -- >      | otherwise = ...
    otherwise               :: Bool
    otherwise               =  True

    ----------------------------------------------
    -- Type Char and String
    ----------------------------------------------

    -- | A 'String' is a list of characters.  String constants in Haskell are values
    -- of type 'String'.
    --
    type String = [Char]

    unsafeChr :: Int -> Char
    unsafeChr (I# i#) = C# (chr# i#)

    -- | The 'Prelude.fromEnum' method restricted to the type 'Data.Char.Char'.
    ord :: Char -> Int
    ord (C# c#) = I# (ord# c#)

    -- | This 'String' equality predicate is used when desugaring
    -- pattern-matches against strings.
    eqString :: String -> String -> Bool
    eqString []       []       = True
    eqString (c1:cs1) (c2:cs2) = c1 == c2 && cs1 `eqString` cs2
    eqString _        _        = False

    {-# RULES "eqString" (==) = eqString #-}
    -- eqString also has a BuiltInRule in PrelRules.hs:
    --      eqString (unpackCString# (Lit s1)) (unpackCString# (Lit s2)) = s1==s2

    ----------------------------------------------
    -- 'Int' related definitions
    ----------------------------------------------

    maxInt, minInt :: Int

    {- Seems clumsy. Should perhaps put minInt and MaxInt directly into MachDeps.h -}
    #if WORD_SIZE_IN_BITS == 31
    minInt  = I# (-0x40000000#)
    maxInt  = I# 0x3FFFFFFF#
    #elif WORD_SIZE_IN_BITS == 32
    minInt  = I# (-0x80000000#)
    maxInt  = I# 0x7FFFFFFF#
    #else
    minInt  = I# (-0x8000000000000000#)
    maxInt  = I# 0x7FFFFFFFFFFFFFFF#
    #endif

    ----------------------------------------------
    -- The function type
    ----------------------------------------------

    -- | Identity function.
    --
    -- > id x = x
    id                      :: a -> a
    id x                    =  x

    -- Assertion function.  This simply ignores its boolean argument.
    -- The compiler may rewrite it to @('assertError' line)@.

    -- | If the first argument evaluates to 'True', then the result is the
    -- second argument.  Otherwise an 'AssertionFailed' exception is raised,
    -- containing a 'String' with the source file and line number of the
    -- call to 'assert'.
    --
    -- Assertions can normally be turned on or off with a compiler flag
    -- (for GHC, assertions are normally on unless optimisation is turned on
    -- with @-O@ or the @-fignore-asserts@
    -- option is given).  When assertions are turned off, the first
    -- argument to 'assert' is ignored, and the second argument is
    -- returned as the result.

    --      SLPJ: in 5.04 etc 'assert' is in GHC.Prim,
    --      but from Template Haskell onwards it's simply
    --      defined here in Base.hs
    assert :: Bool -> a -> a
    assert _pred r = r

    breakpoint :: a -> a
    breakpoint r = r

    breakpointCond :: Bool -> a -> a
    breakpointCond _ r = r

    data Opaque = forall a. O a
    -- | @const x@ is a unary function which evaluates to @x@ for all inputs.
    --
    -- >>> const 42 "hello"
    -- 42
    --
    -- >>> map (const 42) [0..3]
    -- [42,42,42,42]
    const                   :: a -> b -> a
    const x _               =  x

    -- | Function composition.
    {-# INLINE (.) #-}
    -- Make sure it has TWO args only on the left, so that it inlines
    -- when applied to two functions, even if there is no final argument
    (.)    :: (b -> c) -> (a -> b) -> a -> c
    (.) f g = \x -> f (g x)

    -- | @'flip' f@ takes its (first) two arguments in the reverse order of @f@.
    --
    -- >>> flip (++) "hello" "world"
    -- "worldhello"
    flip                    :: (a -> b -> c) -> b -> a -> c
    flip f x y              =  f y x

    -- | Application operator.  This operator is redundant, since ordinary
    -- application @(f x)@ means the same as @(f '$' x)@. However, '$' has
    -- low, right-associative binding precedence, so it sometimes allows
    -- parentheses to be omitted; for example:
    --
    -- > f $ g $ h x  =  f (g (h x))
    --
    -- It is also useful in higher-order situations, such as @'map' ('$' 0) xs@,
    -- or @'Data.List.zipWith' ('$') fs xs@.
    --
    -- Note that @($)@ is levity-polymorphic in its result type, so that
    --     foo $ True    where  foo :: Bool -> Int#
    -- is well-typed
    {-# INLINE ($) #-}
    ($) :: forall r a (b :: TYPE r). (a -> b) -> a -> b
    f $ x =  f x

    -- | Strict (call-by-value) application operator. It takes a function and an
    -- argument, evaluates the argument to weak head normal form (WHNF), then calls
    -- the function with that value.

    ($!) :: forall r a (b :: TYPE r). (a -> b) -> a -> b
    f $! x = let !vx = x in f vx  -- see #2273

    -- | @'until' p f@ yields the result of applying @f@ until @p@ holds.
    until                   :: (a -> Bool) -> (a -> a) -> a -> a
    until p f = go
      where
        go x | p x          = x
             | otherwise    = go (f x)

    -- | 'asTypeOf' is a type-restricted version of 'const'.  It is usually
    -- used as an infix operator, and its typing forces its first argument
    -- (which is usually overloaded) to have the same type as the second.
    asTypeOf                :: a -> a -> a
    asTypeOf                =  const

    ----------------------------------------------
    -- Functor/Applicative/Monad instances for IO
    ----------------------------------------------

    -- | @since 2.01
    instance  Functor IO where
       fmap f x = x >>= (pure . f)

    -- | @since 2.01
    instance Applicative IO where
        {-# INLINE pure #-}
        {-# INLINE (*>) #-}
        {-# INLINE liftA2 #-}
        pure  = returnIO
        (*>)  = thenIO
        (<*>) = ap
        liftA2 = liftM2

    -- | @since 2.01
    instance  Monad IO  where
        {-# INLINE (>>)   #-}
        {-# INLINE (>>=)  #-}
        (>>)      = (*>)
        (>>=)     = bindIO
        fail s    = failIO s

    -- | @since 4.9.0.0
    instance Alternative IO where
        empty = failIO "mzero"
        (<|>) = mplusIO

    -- | @since 4.9.0.0
    instance MonadPlus IO

    returnIO :: a -> IO a
    returnIO x = IO (\ s -> (# s, x #))

    bindIO :: IO a -> (a -> IO b) -> IO b
    bindIO (IO m) k = IO (\ s -> case m s of (# new_s, a #) -> unIO (k a) new_s)

    thenIO :: IO a -> IO b -> IO b
    thenIO (IO m) k = IO (\ s -> case m s of (# new_s, _ #) -> unIO k new_s)

    unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
    unIO (IO a) = a

    {- |
    Returns the 'tag' of a constructor application; this function is used
    by the deriving code for Eq, Ord and Enum.

    The primitive dataToTag# requires an evaluated constructor application
    as its argument, so we provide getTag as a wrapper that performs the
    evaluation before calling dataToTag#.  We could have dataToTag#
    evaluate its argument, but we prefer to do it this way because (a)
    dataToTag# can be an inline primop if it doesn't need to do any
    evaluation, and (b) we want to expose the evaluation to the
    simplifier, because it might be possible to eliminate the evaluation
    in the case when the argument is already known to be evaluated.
    -}
    {-# INLINE getTag #-}
    getTag :: a -> Int#
    getTag !x = dataToTag# x

    ----------------------------------------------
    -- Numeric primops
    ----------------------------------------------

    -- Definitions of the boxed PrimOps; these will be
    -- used in the case of partial applications, etc.

    {-# INLINE quotInt #-}
    {-# INLINE remInt #-}

    quotInt, remInt, divInt, modInt :: Int -> Int -> Int
    (I# x) `quotInt`  (I# y) = I# (x `quotInt#` y)
    (I# x) `remInt`   (I# y) = I# (x `remInt#`  y)
    (I# x) `divInt`   (I# y) = I# (x `divInt#`  y)
    (I# x) `modInt`   (I# y) = I# (x `modInt#`  y)

    quotRemInt :: Int -> Int -> (Int, Int)
    (I# x) `quotRemInt` (I# y) = case x `quotRemInt#` y of
                                 (# q, r #) ->
                                     (I# q, I# r)

    divModInt :: Int -> Int -> (Int, Int)
    (I# x) `divModInt` (I# y) = case x `divModInt#` y of
                                (# q, r #) -> (I# q, I# r)

    divModInt# :: Int# -> Int# -> (# Int#, Int# #)
    x# `divModInt#` y#
     | isTrue# (x# ># 0#) && isTrue# (y# <# 0#) =
                                        case (x# -# 1#) `quotRemInt#` y# of
                                          (# q, r #) -> (# q -# 1#, r +# y# +# 1# #)
     | isTrue# (x# <# 0#) && isTrue# (y# ># 0#) =
                                        case (x# +# 1#) `quotRemInt#` y# of
                                          (# q, r #) -> (# q -# 1#, r +# y# -# 1# #)
     | otherwise                                =
                                        x# `quotRemInt#` y#

    -- Wrappers for the shift operations.  The uncheckedShift# family are
    -- undefined when the amount being shifted by is greater than the size
    -- in bits of Int#, so these wrappers perform a check and return
    -- either zero or -1 appropriately.
    --
    -- Note that these wrappers still produce undefined results when the
    -- second argument (the shift amount) is negative.

    -- | Shift the argument left by the specified number of bits
    -- (which must be non-negative).
    shiftL# :: Word# -> Int# -> Word#
    a `shiftL#` b   | isTrue# (b >=# WORD_SIZE_IN_BITS#) = 0##
                    | otherwise                          = a `uncheckedShiftL#` b

    -- | Shift the argument right by the specified number of bits
    -- (which must be non-negative).
    -- The "RL" means "right, logical" (as opposed to RA for arithmetic)
    -- (although an arithmetic right shift wouldn't make sense for Word#)
    shiftRL# :: Word# -> Int# -> Word#
    a `shiftRL#` b  | isTrue# (b >=# WORD_SIZE_IN_BITS#) = 0##
                    | otherwise                          = a `uncheckedShiftRL#` b

    -- | Shift the argument left by the specified number of bits
    -- (which must be non-negative).
    iShiftL# :: Int# -> Int# -> Int#
    a `iShiftL#` b  | isTrue# (b >=# WORD_SIZE_IN_BITS#) = 0#
                    | otherwise                          = a `uncheckedIShiftL#` b

    -- | Shift the argument right (signed) by the specified number of bits
    -- (which must be non-negative).
    -- The "RA" means "right, arithmetic" (as opposed to RL for logical)
    iShiftRA# :: Int# -> Int# -> Int#
    a `iShiftRA#` b | isTrue# (b >=# WORD_SIZE_IN_BITS#) = if isTrue# (a <# 0#)
                                                              then (-1#)
                                                              else 0#
                    | otherwise                          = a `uncheckedIShiftRA#` b

    -- | Shift the argument right (unsigned) by the specified number of bits
    -- (which must be non-negative).
    -- The "RL" means "right, logical" (as opposed to RA for arithmetic)
    iShiftRL# :: Int# -> Int# -> Int#
    a `iShiftRL#` b | isTrue# (b >=# WORD_SIZE_IN_BITS#) = 0#
                    | otherwise                          = a `uncheckedIShiftRL#` b

    -- Rules for C strings (the functions themselves are now in GHC.CString)
    {-# RULES
    "unpack"       [~1] forall a   . unpackCString# a             = build (unpackFoldrCString# a)
    "unpack-list"  [1]  forall a   . unpackFoldrCString# a (:) [] = unpackCString# a
    "unpack-append"     forall a n . unpackFoldrCString# a (:) n  = unpackAppendCString# a n

    -- There's a built-in rule (in PrelRules.hs) for
    --      unpackFoldr "foo" c (unpackFoldr "baz" c n)  =  unpackFoldr "foobaz" c n

      #-}