Base.lhs

\section[GHC.Base]{Module @GHC.Base@}

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 intances 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.

\begin{code}
{-# LANGUAGE Unsafe #-}
{-# LANGUAGE CPP
           , NoImplicitPrelude
           , BangPatterns
           , ExplicitForAll
           , MagicHash
           , UnboxedTuples
           , ExistentialQuantification
           , RankNTypes
  #-}
-- -fno-warn-orphans is needed for things like:
-- Orphan rule: "x# -# x#" ALWAYS forall x# :: Int# -# x# x# = 0
{-# OPTIONS_GHC -fno-warn-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
  )
        where

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

-- This is not strictly speaking required by this module, but is an
-- implicit dependency whenever () or tuples are mentioned, so adding it
-- as an import here helps to get the dependencies right in the new
-- build system.
import GHC.Tuple ()
-- Likewise we need Integer when deriving things like Eq instances, and
-- this is a convenient place to force it to be built
import GHC.Integer ()

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

default ()              -- Double isn't available yet
\end{code}


%*********************************************************
%*                                                      *
\subsection{DEBUGGING STUFF}
%*  (for use when compiling GHC.Base itself doesn't work)
%*                                                      *
%*********************************************************

\begin{code}
{-
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 = error "urk"
foldr = error "urk"
-}
\end{code}


%*********************************************************
%*                                                      *
\subsection{Monadic classes @Functor@, @Monad@ }
%*                                                      *
%*********************************************************

\begin{code}
{- | 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

{- | 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.

Minimal complete definition: '>>=' and 'return'.

Instances of 'Monad' should satisfy the following laws:

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

Instances of both 'Monad' and 'Functor' should additionally satisfy the law:

> fmap f xs  ==  xs >>= return . f

The instances of 'Monad' for lists, 'Data.Maybe.Maybe' and 'System.IO.IO'
defined in the "Prelude" satisfy these laws.
-}

class  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
        -- Explicit for-alls so that we know what order to
        -- give type arguments when desugaring

    -- | Inject a value into the monadic type.
    return      :: a -> m a
    -- | 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.
    fail        :: String -> m a

    {-# INLINE (>>) #-}
    m >> k      = m >>= \_ -> k
    fail s      = error s

instance Functor ((->) r) where
    fmap = (.)

instance Monad ((->) r) where
    return = const
    f >>= k = \ r -> k (f r) r

instance Functor ((,) a) where
    fmap f (x,y) = (x, f y)
\end{code}


%*********************************************************
%*                                                      *
\subsection{The list type}
%*                                                      *
%*********************************************************

\begin{code}
instance Functor [] where
    fmap = map

instance  Monad []  where
    m >>= k             = foldr ((++) . k) [] m
    m >> k              = foldr ((++) . (\ _ -> k)) [] m
    return x            = [x]
    fail _              = []
\end{code}

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

\begin{code}
-- | '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

"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
\end{code}


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

\begin{code}
-- | '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 [1] map #-}    -- We want the RULE to fire first.
                            -- It's recursive, so won't inline anyway,
                            -- but saying so is more explicit
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 #-}
mapFB c f = \x ys -> c (f x) ys

-- 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.
--
-- The "mapFB" rule optimises compositions of map.
--
-- This same pattern is followed by many other functions:
-- e.g. append, filter, iterate, repeat, etc.

{-# 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)
  #-}

-- There's also a rule for Map and Data.Coerce. See "Safe Coercions",
-- section 6.4:
--
--   http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/coercible.pdf

{-# RULES "map/coerce" map coerce = coerce #-}

\end{code}


----------------------------------------------
--              append
----------------------------------------------
\begin{code}
-- | 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
  #-}

\end{code}


%*********************************************************
%*                                                      *
\subsection{Type @Bool@}
%*                                                      *
%*********************************************************

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

%*********************************************************
%*                                                      *
\subsection{Type @Char@ and @String@}
%*                                                      *
%*********************************************************

\begin{code}
-- | 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#)
\end{code}

String equality is used when desugaring pattern-matches against strings.

\begin{code}
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.lhs:
--      eqString (unpackCString# (Lit s1)) (unpackCString# (Lit s2) = s1==s2
\end{code}


%*********************************************************
%*                                                      *
\subsection{Type @Int@}
%*                                                      *
%*********************************************************

\begin{code}
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
\end{code}


%*********************************************************
%*                                                      *
\subsection{The function type}
%*                                                      *
%*********************************************************

\begin{code}
-- | Identity function.
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.lhs
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

-- | Constant function.
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                    :: (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@.
{-# INLINE ($) #-}
($)                     :: (a -> b) -> a -> b
f $ x                   =  f x

-- | @'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
\end{code}

%*********************************************************
%*                                                      *
\subsection{@Functor@ and @Monad@ instances for @IO@}
%*                                                      *
%*********************************************************

\begin{code}
instance  Functor IO where
   fmap f x = x >>= (return . f)

instance  Monad IO  where
    {-# INLINE return #-}
    {-# INLINE (>>)   #-}
    {-# INLINE (>>=)  #-}
    m >> k    = m >>= \ _ -> k
    return    = returnIO
    (>>=)     = bindIO
    fail s    = failIO s

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
\end{code}

%*********************************************************
%*                                                      *
\subsection{@getTag@}
%*                                                      *
%*********************************************************

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.

\begin{code}
{-# INLINE getTag #-}
getTag :: a -> Int#
getTag x = x `seq` dataToTag# x
\end{code}

%*********************************************************
%*                                                      *
\subsection{Numeric primops}
%*                                                      *
%*********************************************************

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

\begin{code}
{-# 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.lhs) for
--      unpackFoldr "foo" c (unpackFoldr "baz" c n)  =  unpackFoldr "foobaz" c n

  #-}
\end{code}


#ifdef __HADDOCK__
\begin{code}
-- | A special argument for the 'Control.Monad.ST.ST' type constructor,
-- indexing a state embedded in the 'Prelude.IO' monad by
-- 'Control.Monad.ST.stToIO'.
data RealWorld
\end{code}
#endif