[ghc-hetmet.git] / ghc / docs / users_guide / libraries.lit

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\begin{document}
\title{The GHC Prelude and Libraries}
\author{Simon L Peyton Jones \and Simon Marlow \and Will Partain}

\maketitle
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\tableofcontents
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\section[ghc-prelude]{The GHC prelude and libraries}

This document describes GHC's prelude and libraries.  The basic story is that of
the Haskell 1.4 Report and Libraries document (which we do not reproduce here),
but this document describes in addition:
\begin{itemize}
\item   GHC's additional non-standard libraries and types, such as
        state transformers, packed strings, foreign objects, stable
        pointers, and so on.

\item   GHC's primitive types and operations.  The standard Haskell
        functions are implemented on top of these, and it is sometimes
        useful to use them directly.

\item   The organisation of these libraries into directories.
\item   Short description of programmer interface to the non-standard,
        `built-in' libraries provided in addition to the standard
        prelude and libraries.
\end{itemize}

In addition to the GHC prelude libraries, GHC comes with a number of
system libraries, which are presented in Section \ref{syslibs}.

\subsection{Prelude library organisation}

{\em Probably only of interest to implementors...}

The prelude libraries are organised into the following three groups,
each of which is kept in a separate sub-directory of GHC's source
@lib/@ directory: 
\begin{description}
\item[@lib/required/@]  These are the libraries {\em required} by the
Haskell definition.  All are defined by the Haskell Report, or by the
Haskell Libraries Report. 
They currently comprise:
\begin{itemize}
\item @Array@: monolithic arrays.
\item @Char@: more functions on characters.
\item @Complex@: interface defining complex number type and functions
over it.
\item @CPUTime@: get the CPU time used by the program.
\item @Directory@: basic functions for accessing the file system.
\item @Ix@: the @Ix@ class of indexing operations.
\item @IO@: additional input/output functions.
\item @List@: more functions on lists.
\item @Locale@: localisation functions.
\item @Maybe@: more functions on @Maybe@ types.
\item @Monad@: functions on monads.
\item @Numeric@: operations for reading and showing number values.
\item @Prelude@: standard prelude interface.
\item @Random@: pseudo-random number generator.
\item @Ratio@: functions on rational numbers.
\item @System@: basic operating-system interface functions.
\item @Time@: operations on time.
\end{itemize}

\item[@lib/glaExts@]  Extension libraries, currently comprising:
\begin{itemize}
\item @Addr@: primitive pointer type.
\item @Bits@: a class of bitwise operations.
\item @ByteArray@: operations over immutable chunks of (heap allocated) bytes.
\item @CCall@: classes @CCallable@ and @CReturnable@ for calling C.
\item @Foreign@: types and operations for GHC's foreign-language
interface.
\item @GlaExts@: interface for extensions that are only implemented in
GHC: namely unboxed types and primitive operations.
\item @IOExts@: extensions to the @IO@ library.
\item @Int@: 8, 16, 32 and 64-bit integers with bit operations.
\item @LazyST@: a lazy version of the @ST@ monad.
\item @MutableArray@: operations over mutable arrays.
\item @ST@: the state transformer monad, @STRef@s and @STArray@s.
\item @Word@: 8, 16, 32 and 64-bit naturals with bit operations.
\end{itemize}

\item[@lib/concurrent@] GHC extension libraries to support Concurrent Haskell, currently comprising:
\begin{itemize}
\item @Concurrent@: main library.
\item @Parallel@: stuff for multi-processor parallelism.
\item @Channel@
\item @ChannelVar@
\item @Merge@
\item @SampleVar@
\item @Semaphore@
\end{itemize}

\item[@lib/ghc@] These libraries are the pieces on which all the
others are built.  They aren't typically imported by Joe Programmer,
but there's nothing to stop you doing so if you want.  In general, the
modules prefixed by @Prel@ are pieces that go towards building
@Prelude@.

\begin{itemize}
\item @GHC@: this ``library'' brings into scope all the primitive
types and operations, such as @Int#@, @+#@, @encodeFloat#@, etc etc.
It is unique in that there is no Haskell source code for it.  Details
in Section \ref{sect:ghc}.

\item @PrelBase@: defines the basic types and classes without which
very few Haskell programs can work.  The classes are: @Eq@, @Ord@,
@Enum@, @Bounded@, @Num@, @Show@, @Eval@, @Monad@, @MonadZero@,
@MonadPlus@.  The types are: list, @Bool@, @Char@, @Ordering@,
@String@, @Int@, @Integer@.

\item @PrelMaybe@: defines the @Maybe@ type.

\item @PrelEither@: defines the @Either@ type.

\item @PrelTup@: defines tuples and their instances.

\item @PrelList@: defines most of the list operations required by
@Prelude@.  (A few are in @PrelBase@, to avoid gratuitous mutual
recursion between modules.)

\item @PrelNum@ defines: the numeric classes beyond @Num@, namely
@Real@, @Integral@, @Fractional@, @Floating@, @RealFrac@, @RealFloat@;
instances for appropriate classes for @Int@ and @Integer@; the types
@Float@, @Double@, and @Ratio@ and their instances.

\item @PrelRead@: the @Read@ class and all its instances.  It's kept
separate because many programs don't use @Read@ at all, so we don't
even want to link in its code. (If the prelude libraries are built by
splitting the object files, this is all a non-issue)

\item @ConcBase@: substrate stuff for Concurrent Haskell.

\item @IOBase@: substrate stuff for the main I/O libraries.
\item @IOHandle@: large blob of code for doing I/O on handles.
\item @PrelIO@: the remaining small pieces to produce the I/O stuff
needed by @Prelude@.

\item @STBase@: substrate stuff for @ST@.
\item @ArrBase@: substrate stuff for @Array@.

\item @GHCerr@: error reporting code, called from code that the
compiler plants in compiled programs.

\item @GHCmain@: the definition of @mainIO@, which is what {\em
        really} gets called by the runtime system.  @mainIO@ in turn
        calls @main@.

\item @PackBase@: low-level packing/unpacking operations.

\item @Error@: the definition of @error@, placed in its own module
with a hand-written @.hi-boot@ file in order to break recursive
dependencies in the libraries (everything needs @error@, but the
definition of @error@ itself needs a few things...).
\end{itemize}
\end{description}

The @...Base@ modules generally export representation information that
is hidden from the public interface.  For example the module @STBase@
exports the type @ST@ including its representation, whereas the module
@ST@ exports @ST@ abstractly.

None of these modules are involved in any mutual recursion, with the
sole exception that many modules import @Error.error@.

\subsection[ghc-libs-ghc]{The module @GHC@: really primitive stuff}
\label{sect:ghc}

This section defines all the types which are primitive in Glasgow
Haskell, and the operations provided for them.

A primitive type is one which cannot be defined in Haskell, and which
is therefore built into the language and compiler.  Primitive types
are always unboxed; that is, a value of primitive type cannot be
bottom.

Primitive values are often represented by a simple bit-pattern, such
as @Int#@, @Float#@, @Double#@.  But this is not necessarily the case:
a primitive value might be represented by a pointer to a
heap-allocated object.  Examples include @Array#@, the type of
primitive arrays.  You might think this odd: doesn't being
heap-allocated mean that it has a box?  No, it does not.  A primitive
array is heap-allocated because it is too big a value to fit in a
register, and would be too expensive to copy around; in a sense, it is
accidental that it is represented by a pointer.  If a pointer
represents a primitive value, then it really does point to that value:
no unevaluated thunks, no indirections...nothing can be at the other
end of the pointer than the primitive value.

This section also describes a few non-primitive types, which are needed 
to express the result types of some primitive operations.

\subsubsection{Character and numeric types}

There are the following obvious primitive types:
\begin{verbatim}
type Char#
type Int#       -- see also Word# and Addr#, later
type Float#
type Double#
\end{verbatim}
If you really want to know their exact equivalents in C, see
@ghc/includes/StgTypes.lh@ in the GHC source tree.

Literals for these types may be written as follows:
\begin{verbatim}
1#              an Int#
1.2#            a Float#
1.34##          a Double#
'a'#            a Char#; for weird characters, use '\o<octal>'#
"a"#            an Addr# (a `char *')
\end{verbatim}

\subsubsubsection{Comparison operations}
\begin{verbatim}
{>,>=,==,/=,<,<=}# :: Int# -> Int# -> Bool

{gt,ge,eq,ne,lt,le}Char# :: Char# -> Char# -> Bool
    -- ditto for Word#, Float#, Double#, and Addr#
\end{verbatim}

\subsubsubsection{Unboxed-character operations}
\begin{verbatim}
ord# :: Char# -> Int#
chr# :: Int# -> Char#
\end{verbatim}


\subsubsubsection{Unboxed-@Int@ operations}
\begin{verbatim}
{+,-,*,quotInt,remInt}# :: Int# -> Int# -> Int#
negateInt# :: Int# -> Int#

iShiftL#, iShiftRA#, iShiftRL# :: Int# -> Int# -> Int#
        -- shift left, right arithmetic, right logical
\end{verbatim}

{\bf Note:} No error/overflow checking!

\subsubsubsection{Unboxed-@Double@ and @Float@ operations}
\begin{verbatim}
{plus,minus,times,divide}Double# :: Double# -> Double# -> Double#
negateDouble# :: Double# -> Double#

float2Int#      :: Double# -> Int#   -- just a cast, no checking!
int2Double#     :: Int# -> Double#

expDouble#      :: Double# -> Double#
logDouble#      :: Double# -> Double#
sqrtDouble#     :: Double# -> Double#
sinDouble#      :: Double# -> Double#
cosDouble#      :: Double# -> Double#
tanDouble#      :: Double# -> Double#
asinDouble#     :: Double# -> Double#
acosDouble#     :: Double# -> Double#
atanDouble#     :: Double# -> Double#
sinhDouble#     :: Double# -> Double#
coshDouble#     :: Double# -> Double#
tanhDouble#     :: Double# -> Double#
powerDouble#    :: Double# -> Double# -> Double#
\end{verbatim}

There's an exactly-matching set of unboxed-@Float@ ops; replace
@Double#@ with @Float#@ in the list above.  There are two
coercion functions for @Float#@/@Double#@:
\begin{verbatim}
float2Double#   :: Float# -> Double#
double2Float#   :: Double# -> Float#
\end{verbatim}

The primitive versions of @encodeDouble@/@decodeDouble@:
\begin{verbatim}
encodeDouble#   :: Int# -> Int# -> ByteArray#   -- Integer mantissa
                -> Int#                         -- Int exponent
                -> Double#

decodeDouble#   :: Double# -> PrelNum.ReturnIntAndGMP
\end{verbatim}

(And the same for @Float#@s.)

\subsubsection{Operations on/for @Integers@ (interface to GMP)}
\label{sect:horrid-Integer-pairing-types}

We implement @Integers@ (arbitrary-precision integers) using the GNU
multiple-precision (GMP) package (version 1.3.2).

{\bf Note:} some of this might change when we upgrade to using GMP~2.x.

The data type for @Integer@ must mirror that for @MP_INT@ in @gmp.h@
(see @gmp.info@ in \tr{ghc/includes/runtime/gmp}).  It comes out as:
\begin{verbatim}
data Integer = J# Int# Int# ByteArray#
\end{verbatim}

So, @Integer@ is really just a ``pairing'' type for a particular
collection of primitive types.

The operations in the GMP return other combinations of
GMP-plus-something, so we need ``pairing'' types for those, too:
\begin{verbatim}
data Return2GMPs     = Return2GMPs Int# Int# ByteArray# Int# Int# ByteArray#
data ReturnIntAndGMP = ReturnIntAndGMP Int# Int# Int# ByteArray#

-- ????? something to return a string of bytes (in the heap?)
\end{verbatim}

The primitive ops to support @Integers@ use the ``pieces'' of the
representation, and are as follows:
\begin{verbatim}
negateInteger#  :: Int# -> Int# -> ByteArray# -> Integer

{plus,minus,times}Integer# :: Int# -> Int# -> ByteArray#
                           -> Int# -> Int# -> ByteArray#
                           -> Integer

cmpInteger# :: Int# -> Int# -> ByteArray#
            -> Int# -> Int# -> ByteArray#
            -> Int# -- -1 for <; 0 for ==; +1 for >

divModInteger#, quotRemInteger#
        :: Int# -> Int# -> ByteArray#
        -> Int# -> Int# -> ByteArray#
        -> PrelNum.Return2GMPs

integer2Int# :: Int# -> Int# -> ByteArray# -> Int# 

int2Integer#  :: Int#  -> Integer -- NB: no error-checking on these two!
word2Integer# :: Word# -> Integer

addr2Integer# :: Addr# -> Integer
        -- the Addr# is taken to be a `char *' string
        -- to be converted into an Integer.
\end{verbatim}


\subsubsection{Words and addresses}

A @Word#@ is used for bit-twiddling operations.  It is the same size as
an @Int#@, but has no sign nor any arithmetic operations.
\begin{verbatim}
type Word#      -- Same size/etc as Int# but *unsigned*
type Addr#      -- A pointer from outside the "Haskell world" (from C, probably);
                -- described under "arrays"
\end{verbatim}

@Word#@s and @Addr#@s have the usual comparison operations.
Other unboxed-@Word@ ops (bit-twiddling and coercions):
\begin{verbatim}
and#, or#, xor# :: Word# -> Word# -> Word#
        -- standard bit ops.

quotWord#, remWord# :: Word# -> Word# -> Word#
        -- word (i.e. unsigned) versions are different from int
        -- versions, so we have to provide these explicitly.

not# :: Word# -> Word#

shiftL#, shiftRA#, shiftRL# :: Word# -> Int# -> Word#
        -- shift left, right arithmetic, right logical

int2Word#       :: Int#  -> Word# -- just a cast, really
word2Int#       :: Word# -> Int#
\end{verbatim}

Unboxed-@Addr@ ops (C casts, really):
\begin{verbatim}
int2Addr#       :: Int#  -> Addr#
addr2Int#       :: Addr# -> Int#
\end{verbatim}

The casts between @Int#@, @Word#@ and @Addr#@ correspond to null
operations at the machine level, but are required to keep the Haskell
type checker happy.

Operations for indexing off of C pointers (@Addr#@s) to snatch values
are listed under ``arrays''.

\subsubsection{Arrays}

The type @Array# elt@ is the type of primitive, unboxed arrays of
values of type @elt@.

\begin{verbatim}
type Array# elt
\end{verbatim}

@Array#@ is more primitive than a Haskell array --- indeed, the
Haskell @Array@ interface is implemented using @Array#@ --- in that an
@Array#@ is indexed only by @Int#@s, starting at zero.  It is also
more primitive by virtue of being unboxed.  That doesn't mean that it
isn't a heap-allocated object --- of course, it is.  Rather, being
unboxed means that it is represented by a pointer to the array itself,
and not to a thunk which will evaluate to the array (or to bottom).
The components of an @Array#@ are themselves boxed.

The type @ByteArray#@ is similar to @Array#@, except that it contains
just a string of (non-pointer) bytes.

\begin{verbatim}
type ByteArray#
\end{verbatim}

Arrays of these types are useful when a Haskell program wishes to
construct a value to pass to a C procedure. It is also possible to
use them to build (say) arrays of unboxed characters for internal use
in a Haskell program.  Given these uses, @ByteArray#@ is deliberately
a bit vague about the type of its components.  Operations are provided
to extract values of type @Char#@, @Int#@, @Float#@, @Double#@, and
@Addr#@ from arbitrary offsets within a @ByteArray#@.  (For type
@Foo#@, the $i$th offset gets you the $i$th @Foo#@, not the @Foo#@ at
byte-position $i$.  Mumble.)  (If you want a @Word#@, grab an @Int#@,
then coerce it.)

Lastly, we have static byte-arrays, of type @Addr#@ [mentioned
previously].  (Remember the duality between arrays and pointers in C.)
Arrays of this types are represented by a pointer to an array in the
world outside Haskell, so this pointer is not followed by the garbage
collector.  In other respects they are just like @ByteArray#@.  They
are only needed in order to pass values from C to Haskell.

\subsubsubsection{Reading and writing.}

Primitive arrays are linear, and indexed starting at zero.

The size and indices of a @ByteArray#@, @Addr#@, and
@MutableByteArray#@ are all in bytes.  It's up to the program to
calculate the correct byte offset from the start of the array.  This
allows a @ByteArray#@ to contain a mixture of values of different
type, which is often needed when preparing data for and unpicking
results from C.  (Umm... not true of indices... WDP 95/09)

{\em Should we provide some @sizeOfDouble#@ constants?}

Out-of-range errors on indexing should be caught by the code which
uses the primitive operation; the primitive operations themselves do
{\em not} check for out-of-range indexes. The intention is that the
primitive ops compile to one machine instruction or thereabouts.

We use the terms ``reading'' and ``writing'' to refer to accessing
{\em mutable} arrays (see Section~\ref{sect:mutable}), and
``indexing'' to refer to reading a value from an {\em immutable}
array.

Immutable byte arrays are straightforward to index (all indices in bytes):
\begin{verbatim}
indexCharArray#   :: ByteArray# -> Int# -> Char#
indexIntArray#    :: ByteArray# -> Int# -> Int#
indexAddrArray#   :: ByteArray# -> Int# -> Addr#
indexFloatArray#  :: ByteArray# -> Int# -> Float#
indexDoubleArray# :: ByteArray# -> Int# -> Double#

indexCharOffAddr#   :: Addr# -> Int# -> Char#
indexIntOffAddr#    :: Addr# -> Int# -> Int#
indexFloatOffAddr#  :: Addr# -> Int# -> Float#
indexDoubleOffAddr# :: Addr# -> Int# -> Double#
indexAddrOffAddr#   :: Addr# -> Int# -> Addr#   
 -- Get an Addr# from an Addr# offset
\end{verbatim}
\index{indexCharArray# function}
\index{indexIntArray# function}
\index{indexAddrArray# function}
\index{indexFloatArray# function}
\index{indexDoubleArray# function}
\index{indexCharOffAddr# function}
\index{indexIntOffAddr# function}
\index{indexFloatOffAddr# function}
\index{indexDoubleOffAddr# function}
\index{indexAddrOffAddr# function}

The last of these, @indexAddrOffAddr#@, extracts an @Addr#@ using an offset
from another @Addr#@, thereby providing the ability to follow a chain of
C pointers.

Something a bit more interesting goes on when indexing arrays of boxed
objects, because the result is simply the boxed object. So presumably
it should be entered --- we never usually return an unevaluated
object!  This is a pain: primitive ops aren't supposed to do
complicated things like enter objects.  The current solution is to
return a lifted value, but I don't like it!
\begin{verbatim}
indexArray#       :: Array# elt -> Int# -> PrelBase.Lift elt  -- Yuk!
\end{verbatim}


\subsubsection{The state type}
\index{State# type}

The primitive type @State#@ represents the state of a state transformer.
It is parameterised on the desired type of state, which serves to keep
states from distinct threads distinct from one another.  But the {\em only}
effect of this parameterisation is in the type system: all values of type
@State#@ are represented in the same way.  Indeed, they are all 
represented by nothing at all!  The code generator ``knows'' to generate no 
code, and allocate no registers etc, for primitive states.
\begin{verbatim}
type State# s
\end{verbatim}


The type @GHC.RealWorld@ is truly opaque: there are no values defined
of this type, and no operations over it.  It is ``primitive'' in that
sense---but it is {\em not unboxed!} Its only role in life is to be
the type which distinguishes the @IO@ state transformer.
\begin{verbatim}
data RealWorld
\end{verbatim}

\subsubsubsection{State of the world}

A single, primitive, value of type @State# RealWorld@ is provided.
\begin{verbatim}
realWorld# :: State# GHC.RealWorld
\end{verbatim}
\index{realWorld# state object}

(Note: in the compiler, not a @PrimOp@; just a mucho magic
@Id@. Exported from @GHC@, though).

\subsubsection{State pairing types}
\label{sect:horrid-pairing-types}

This subsection defines some types which, while they aren't quite
primitive because we can define them in Haskell, are very nearly so.
They define constructors which pair a primitive state with a value of
each primitive type.  They are required to express the result type of
the primitive operations in the state monad.
\begin{verbatim}
data StateAndPtr#    s elt = StateAndPtr#    (State# s) elt 

data StateAndChar#   s     = StateAndChar#   (State# s) Char# 
data StateAndInt#    s     = StateAndInt#    (State# s) Int# 
data StateAndWord#   s     = StateAndWord#   (State# s) Word#
data StateAndFloat#  s     = StateAndFloat#  (State# s) Float# 
data StateAndDouble# s     = StateAndDouble# (State# s) Double#  
data StateAndAddr#   s     = StateAndAddr#   (State# s) Addr#

data StateAndStablePtr# s a = StateAndStablePtr#  (State# s) (StablePtr# a)
data StateAndForeignObj# s  = StateAndForeignObj# (State# s) ForeignObj#
data StateAndSynchVar#  s a = StateAndSynchVar#  (State# s) (SynchVar# a)

data StateAndArray#            s elt = StateAndArray#        (State# s) (Array# elt) 
data StateAndMutableArray#     s elt = StateAndMutableArray# (State# s) (MutableArray# s elt)  
data StateAndByteArray#        s = StateAndByteArray#        (State# s) ByteArray# 
data StateAndMutableByteArray# s = StateAndMutableByteArray# (State# s) (MutableByteArray# s)
\end{verbatim}

Hideous.

\subsubsection{Mutable arrays}
\label{sect:mutable}
\index{Mutable arrays}

Corresponding to @Array#@ and @ByteArray#@, we have the types of
mutable versions of each.  In each case, the representation is a
pointer to a suitable block of (mutable) heap-allocated storage.
\begin{verbatim}
type MutableArray# s elt
type MutableByteArray# s
\end{verbatim}

\subsubsubsection{Allocation}
\index{Mutable arrays, allocation}
\index{Allocation, of mutable arrays}

Mutable arrays can be allocated. Only pointer-arrays are initialised;
arrays of non-pointers are filled in by ``user code'' rather than by
the array-allocation primitive.  Reason: only the pointer case has to
worry about GC striking with a partly-initialised array.

\begin{verbatim}
newArray#       :: Int# -> elt -> State# s -> StateAndMutableArray# s elt 

newCharArray#   :: Int# -> State# s -> StateAndMutableByteArray# s 
newIntArray#    :: Int# -> State# s -> StateAndMutableByteArray# s 
newAddrArray#   :: Int# -> State# s -> StateAndMutableByteArray# s 
newFloatArray#  :: Int# -> State# s -> StateAndMutableByteArray# s 
newDoubleArray# :: Int# -> State# s -> StateAndMutableByteArray# s 
\end{verbatim}

The size of a @ByteArray#@ is given in bytes.

\subsubsubsection{Reading and writing}

%OLD: Remember, offsets in a @MutableByteArray#@ are in bytes.
\begin{verbatim}
readArray#       :: MutableArray# s elt -> Int# -> State# s -> StateAndPtr#    s elt
readCharArray#   :: MutableByteArray# s -> Int# -> State# s -> StateAndChar#   s
readIntArray#    :: MutableByteArray# s -> Int# -> State# s -> StateAndInt#    s
readAddrArray#   :: MutableByteArray# s -> Int# -> State# s -> StateAndAddr#   s 
readFloatArray#  :: MutableByteArray# s -> Int# -> State# s -> StateAndFloat#  s 
readDoubleArray# :: MutableByteArray# s -> Int# -> State# s -> StateAndDouble# s 

writeArray#       :: MutableArray# s elt -> Int# -> elt     -> State# s -> State# s 
writeCharArray#   :: MutableByteArray# s -> Int# -> Char#   -> State# s -> State# s 
writeIntArray#    :: MutableByteArray# s -> Int# -> Int#    -> State# s -> State# s 
writeAddrArray#   :: MutableByteArray# s -> Int# -> Addr#   -> State# s -> State# s 
writeFloatArray#  :: MutableByteArray# s -> Int# -> Float#  -> State# s -> State# s 
writeDoubleArray# :: MutableByteArray# s -> Int# -> Double# -> State# s -> State# s 
\end{verbatim}


\subsubsubsection{Equality.}

One can take ``equality'' of mutable arrays.  What is compared is the
{\em name} or reference to the mutable array, not its contents.
\begin{verbatim}
sameMutableArray#     :: MutableArray# s elt -> MutableArray# s elt -> Bool
sameMutableByteArray# :: MutableByteArray# s -> MutableByteArray# s -> Bool
\end{verbatim}


\subsubsubsection{Freezing mutable arrays}

Only unsafe-freeze has a primitive.  (Safe freeze is done directly in Haskell 
by copying the array and then using @unsafeFreeze@.) 
\begin{verbatim}
unsafeFreezeArray#     :: MutableArray# s elt -> State# s -> StateAndArray#     s elt
unsafeFreezeByteArray# :: MutableByteArray# s -> State# s -> StateAndByteArray# s
\end{verbatim}


\subsubsection{Stable pointers}
\index{Stable pointers}

A stable pointer is a name for a Haskell object which can be passed to
the external world.  It is ``stable'' in the sense that the name does
not change when the Haskell garbage collector runs --- in contrast to
the address of the object which may well change.

The stable pointer type is parameterised by the type of the thing
which is named.
\begin{verbatim}
type StablePtr# a
\end{verbatim}

A stable pointer is represented by an index into the (static)
@StablePointerTable@.  The Haskell garbage collector treats the
@StablePointerTable@ as a source of roots for GC.

The @makeStablePointer@ function converts a value into a stable
pointer.  It is part of the @IO@ monad, because we want to be sure
we don't allocate one twice by accident, and then only free one of the
copies.
\begin{verbatim}
makeStablePointer#  :: a -> State# RealWorld -> StateAndStablePtr# RealWorld a
freeStablePointer#  :: StablePtr# a -> State# RealWorld -> State# RealWorld
deRefStablePointer# :: StablePtr# a -> State# RealWorld -> StateAndPtr RealWorld a
\end{verbatim}

There is also a C procedure @FreeStablePtr@ which frees a stable pointer.

%{\em Andy's comment.} {\bf Errors:} The following is not strictly true: the current
%implementation is not as polymorphic as claimed.  The reason for this
%is that the C programmer will have to use a different entry-routine
%for each type of stable pointer.  At present, we only supply a very
%limited number (3) of these routines.  It might be possible to
%increase the range of these routines by providing general purpose
%entry points to apply stable pointers to (stable pointers to)
%arguments and to enter (stable pointers to) boxed primitive values.
%{\em End of Andy's comment.}

%
% Rewritten and updated for MallocPtr++ -- 4/96 SOF
%
\subsubsection{Foreign objects}
\index{Foreign objects}
\index{ForeignObj type}

A @ForeignObj#@ is a reference to an object outside the Haskell world
(i.e., from the C world, or a reference to an object on another
machine completely.), where the Haskell world has been told ``Let me
know when you're finished with this ...''.

\begin{verbatim}
type ForeignObj#
\end{verbatim}

GHC provides two primitives on @ForeignObj#@:

\begin{verbatim}
makeForeignObj# 
        :: Addr# -- foreign reference
        -> Addr# -- pointer to finalisation routine
        -> StateAndForeignObj# RealWorld ForeignObj#
writeForeignObj 
        :: ForeignObj#        -- foreign object
        -> Addr#              -- datum
        -> State# RealWorld
        -> State# RealWorld
\end{verbatim}

The module @Foreign@ (Section \ref{sec:foreign-obj}) provides a more
programmer-friendly interface to foreign objects.

\subsubsection{Synchronizing variables (M-vars)}
\index{Synchronising variables (M-vars)}
\index{M-Vars}

Synchronising variables are the primitive type used to implement
Concurrent Haskell's MVars (see the Concurrent Haskell paper for
the operational behaviour of these operations).

\begin{verbatim}
type SynchVar# s elt    -- primitive

newSynchVar#:: State# s -> StateAndSynchVar# s elt
takeMVar#   :: SynchVar# s elt -> State# s -> StateAndPtr# s elt
putMVar#    :: SynchVar# s elt -> State# s -> State# s
\end{verbatim}

\subsubsection{@spark#@ primitive operation (for parallel execution)}

{\em ToDo: say something}  It's used in the unfolding for @par@.

\subsubsection{The @errorIO#@ primitive operation}

The @errorIO#@ primitive takes an argument much like @IO@.  It aborts
execution of the current program, and continues instead by performing
the given @IO@-like value on the current state of the world.

\begin{verbatim}
errorIO# :: (State# RealWorld# -> a -> a
\end{verbatim}

\subsection{GHC/Hugs Extension Libraries}

The extension libraries provided by both GHC and Hugs are described in
a separate document ``The Hugs-GHC Extension Libraries''.

\subsection{GHC-only Extension Libraries}

If you rely on the implicit @import Prelude@ that GHC normally does
for you, and if you don't use any weird flags (notably
@-fglasgow-exts@), and if you don't import the Glasgow extensions
interface, @GlaExts@, then GHC should work {\em exactly} as the
Haskell report says, and the full user namespaces should be available
to you.

If you mess about with @import Prelude@... innocent hiding, e.g.,

\begin{verbatim}
import Prelude hiding ( fromIntegral )
\end{verbatim}

should work just fine.

% this should work now?? -- SDM

%There are some things you can do that will make GHC crash, e.g.,
%hiding a standard class:
%\begin{verbatim}
%import Prelude hiding ( Eq(..) )
%\end{verbatim}
%
%Don't do that.

If you turn on @-fglasgow-exts@, the compiler will recognise and parse
unboxed values properly. To get at the primitive operations described
herein, import the relevant interfaces.

\subsubsection{The @GlaExts@ interface}
\index{GlaExts interface (GHC extensions)}

The @GlaExts@ interface provides access to extensions that only GHC
implements.  These currently are: unboxed types, including the
representations of the primitive types (Int, Float, etc.), and the
GHC primitive operations (@+#@, @==#@, etc.).

This module used to provide access to all the Glasgow extensions, but
these have since been moved into separate libraries for compatibility
with Hugs (version 2.09: in fact, you can still get at this stuff via
@GlaExts@ for compatibility, but this facility will likely be removed
in the future).

\begin{verbatim}
-- the representation of some basic types:
data Char    = C# Char#
data Int     = I# Int#
data Addr    = A# Addr#
data Word    = W# Word#
data Float   = F# Float#
data Double  = D# Double#
data Integer = J# Int# Int# ByteArray#

module GHC  -- all primops and primitive types.
\end{verbatim}

\subsubsection{The @MutableArray@ interface}
\label{sec:mutable-array}
\index{MutableArray interface (GHC extensions)}

The @MutableArray@ interface defines a general set of operations over
mutable arrays (@MutableArray@) and mutable chunks of memory
(@MutableByteArray@):

\begin{verbatim}
data MutableArray s ix elt -- abstract
data MutableByteArray s ix -- abstract
                           -- instance of : CCallable
-- Creators:
newArray           :: Ix ix => (ix,ix) -> elt -> ST s (MutableArray s ix elt)
newCharArray       :: Ix ix => (ix,ix) -> ST s (MutableByteArray s ix) 
newAddrArray       :: Ix ix => (ix,ix) -> ST s (MutableByteArray s ix) 
newIntArray        :: Ix ix => (ix,ix) -> ST s (MutableByteArray s ix) 
newFloatArray      :: Ix ix => (ix,ix) -> ST s (MutableByteArray s ix) 
newDoubleArray     :: Ix ix => (ix,ix) -> ST s (MutableByteArray s ix) 

boundsOfArray      :: Ix ix => MutableArray s ix elt -> (ix, ix)  
boundsOfByteArray  :: Ix ix => MutableByteArray s ix -> (ix, ix)


readArray          :: Ix ix => MutableArray s ix elt -> ix -> ST s elt 

readCharArray      :: Ix ix => MutableByteArray s ix -> ix -> ST s Char 
readIntArray       :: Ix ix => MutableByteArray s ix -> ix -> ST s Int
readAddrArray      :: Ix ix => MutableByteArray s ix -> ix -> ST s Addr
readFloatArray     :: Ix ix => MutableByteArray s ix -> ix -> ST s Float
readDoubleArray    :: Ix ix => MutableByteArray s ix -> ix -> ST s Double

writeArray         :: Ix ix => MutableArray s ix elt -> ix -> elt -> ST s () 
writeCharArray     :: Ix ix => MutableByteArray s ix -> ix -> Char -> ST s () 
writeIntArray      :: Ix ix => MutableByteArray s ix -> ix -> Int  -> ST s () 
writeAddrArray     :: Ix ix => MutableByteArray s ix -> ix -> Addr -> ST s () 
writeFloatArray    :: Ix ix => MutableByteArray s ix -> ix -> Float -> ST s () 
writeDoubleArray   :: Ix ix => MutableByteArray s ix -> ix -> Double -> ST s () 

freezeArray        :: Ix ix => MutableArray s ix elt -> ST s (Array ix elt)
freezeCharArray    :: Ix ix => MutableByteArray s ix -> ST s (ByteArray ix)
freezeIntArray     :: Ix ix => MutableByteArray s ix -> ST s (ByteArray ix)
freezeAddrArray    :: Ix ix => MutableByteArray s ix -> ST s (ByteArray ix)
freezeFloatArray   :: Ix ix => MutableByteArray s ix -> ST s (ByteArray ix)
freezeDoubleArray  :: Ix ix => MutableByteArray s ix -> ST s (ByteArray ix)

unsafeFreezeArray     :: Ix ix => MutableArray s ix elt -> ST s (Array ix elt)  
unsafeFreezeByteArray :: Ix ix => MutableByteArray s ix -> ST s (ByteArray ix)
thawArray             :: Ix ix => Array ix elt -> ST s (MutableArray s ix elt)
\end{verbatim}

\subsubsection{The @ByteArray@ interface}
\label{sec:byte-array}
\index{ByteArray interface (GHC extensions)}

@ByteArray@s are chunks of immutable Haskell heap:

\begin{verbatim}
data ByteArray ix -- abstract
                  -- instance of: CCallable

indexCharArray     :: Ix ix => ByteArray ix -> ix -> Char 
indexIntArray      :: Ix ix => ByteArray ix -> ix -> Int
indexAddrArray     :: Ix ix => ByteArray ix -> ix -> Addr
indexFloatArray    :: Ix ix => ByteArray ix -> ix -> Float
indexDoubleArray   :: Ix ix => ByteArray ix -> ix -> Double

indexCharOffAddr   :: Addr -> Int -> Char
indexIntOffAddr    :: Addr -> Int -> Int
indexAddrOffAddr   :: Addr -> Int -> Addr
indexFloatOffAddr  :: Addr -> Int -> Float
indexDoubleOffAddr :: Addr -> Int -> Double
\end{verbatim}

\subsubsection{Stable pointers}

Nothing exciting here, just simple boxing up.
\begin{verbatim}
data StablePtr a = StablePtr (StablePtr# a)

makeStablePointer :: a -> StablePtr a
freeStablePointer :: StablePtr a -> IO ()
\end{verbatim}

\subsubsection{Foreign objects}
\label{sec:foreign-obj}
\index{Foreign objects}

This module provides the @ForeignObj@ type and wrappers around the
primitive operations on foreign objects.

\begin{verbatim}
data ForeignObj = ForeignObj ForeignObj#

makeForeignObj 
        :: Addr   -- object to be boxed up as a ForeignObj
        -> Addr   -- finaliser 
        -> IO ForeignObj

writeForeignObj 
        :: ForeignObj   -- previously created foreign object
        -> Addr         -- new value
        -> IO ()

\end{verbatim}
\index{ForeignObj type}
\index{makeForeignObj function}
\index{writeForeignObj function}

A typical use of @ForeignObj@ is in constructing Haskell bindings
to external libraries. A good example is that of writing a binding to
an image-processing library (which was actually the main motivation
for implementing @ForeignObj@'s precursor, @MallocPtr#@). The
images manipulated are not stored in the Haskell heap, either because
the library insist on allocating them internally or we (sensibly)
decide to spare the GC from having to heave heavy images around.

\begin{verbatim}
data Image = Image ForeignObj
\end{verbatim}

The @ForeignObj@ type is then used to refer to the externally
allocated image, and to acheive some type safety, the Haskell binding
defines the @Image@ data type. So, a value of type @ForeignObj@ is
used to ``box'' up an external reference into a Haskell heap object
that we can then indirectly reference:

\begin{verbatim}
createImage :: (Int,Int) -> IO Image
\end{verbatim}

So far, this looks just like an @Addr@ type, but @ForeignObj@ offers a
bit more, namely that we can specify a {\em finalisation routine} to
invoke when the @ForeignObj@ is discarded by the GC. The garbage
collector invokes the finalisation routine associated with the
@ForeignObj@, saying `` Thanks, I'm through with this now..'' For the
image-processing library, the finalisation routine could for the
images free up memory allocated for them. The finalisation routine has
currently to be written in C (the finalisation routine can in turn
call on @FreeStablePtr@ to deallocate a stable pointer).

Associating a finalisation routine with an external object is done by
calling @makeForeignObj@.  {\bf Note:} the foreign object value and
its finaliser are contained in the @ForeignObj@, so there's no danger
of an aggressive optimiser somehow separating the two (with the result
that the foreign reference would not be freed).

(Implementation: a linked list of all @ForeignObj#@s is maintained to
 allow the garbage collector to detect when a @ForeignObj#@ becomes
 garbage.)

Like @Array@, @ForeignObj#@s are represented by heap objects.

Upon controlled termination of the Haskell program, all @ForeignObjs@
are freed, invoking their respective finalisers before terminating.

\subsubsection{The @CCall@ module}

The @CCall@ module defines the classes @CCallable@ and @CReturnable@,
along with instances for the primitive types (@Int@, @Int#@, @Float@,
@Float#@ etc.)  GHC knows to import this module if you use @_ccall_@,
but if you need to define your own instances of these classes, you
will need to import @CCall@ explicitly.

More information on how to use @_ccall_@ can be found in Section
\ref{glasgow-ccalls}.

\begin{onlystandalone}
\end{document}
\end{onlystandalone}