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

\begin{onlystandalone}
\documentstyle[a4wide,grasp]{article}
\begin{rawlatex}
\renewcommand{\textfraction}{0.1}
\renewcommand{\floatpagefraction}{0.9}
\renewcommand{\dblfloatpagefraction}{0.9}

\sloppy
\renewcommand{\today}{March 1997}
\end{rawlatex}

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

\maketitle
\begin{rawlatex}
\tableofcontents
\end{rawlatex}
\end{onlystandalone}

\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 \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 @Prelude@.
\item @List@: more functions on lists.
\item @Char@: more functions on characters.
\item @Maybe@: more functions on @Maybe@ types.
\item @Complex@: interface defining complex number type and functions over it.
\item @Ratio@: functions on rational numbers.
\item @Monad@: functions on monads.
\item @Ix@: the @Ix@ class of indexing operations.
\item @Array@: monolithic arrays.
\item @IO@: additional input/output functions.
\item @Directory@: basic functions for accessing the file system.
\item @System@: basic operating-system interface functions.
\item @Numeric@: operations for reading and showing number values.
\end{itemize}

\item[@lib/glaExts@]  GHC extension libraries, currently comprising:
\begin{itemize}
\item @GlaExts@: collector interface to the various Glasgow
extensions, primitive operations.
\item @PackedString@: functions that manipulate strings packed efficiently, one character per byte.
\item @ST@: the state transformer monad.
\item @Foreign@: types and operations for GHC's foreign-language interface.
\item @ByteArray@: operations over immutable chunks of (heap allocated) bytes.
\item @MutableArray@: operations over mutable arrays.
\item @MutVar@: operations over mutable variables.
\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@, @Maybe@, @Either@.

\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 @mainPrimIO@, which is what {\em really} gets
        called by the runtime system.  @mainPrimIO@ in turn calls @main@.
\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 @IOBase.error@.

\subsection{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}
{gt,ge,eq,ne,lt,le}Char# :: Char# -> Char# -> Bool
    -- ditto for Int#, 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}
{plus,minus,times,quot,div,rem}Int# :: Int# -> Int# -> Int#
negateInt# :: Int# -> Int#
\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# :: Word# -> Word# -> Word#

not# :: Word# -> Word#

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

iShiftL#, iShiftRA#, iShiftRL# :: Int# -> Int# -> Int#
        -- same shift ops, but on Int#s

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}

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.

If you want to read/write a @Word#@, read an @Int#@ and coerce.

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}

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}

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 @PrimIO@ state transformer (see
Section~\ref{sect:io-spec}).
\begin{verbatim}
data RealWorld
\end{verbatim}

\subsubsubsection{States}

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

(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}

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.}

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}


\subsubsubsection{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 @PrimIO@ 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
%
\subsubsubsection{Foreign objects}

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 ...''.

%OLD:The @ForeignObj@ type is just a special @Addr#@ ({\em not} parameterised).
\begin{verbatim}
type ForeignObj#
\end{verbatim}

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#

instance CCallable Image
\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) -> PrimIO 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 
@makeForeignObj#@:

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

\bf{Note:} the foreign object value and its finaliser are contained
in the primitive value @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.

%\bf{ToDo:} Decide whether @FreeCHeapPointer@ is allowed to call on a
%stable pointer. (I sincerely hope not since we will still be in the
%GC at this point.)

\subsubsubsection{Synchronizing variables (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}

%\subsubsubsection{Controlling the garbage collector}

%The C function {\tt PerformGC\/}, allows the C world to force Haskell
%to do a garbage collection. It can only be called while Haskell is
%performing a C Call.

%Note that this function can be used to define a Haskell IO operation
%with the same effect:
%\begin{verbatim}
%>      performGCIO :: PrimIO ()
%>      performGCIO = _ccall_gc_ StgPerformGarbageCollection
%\end{verbatim}

%\bf{ToDo:} Is there any need for abnormal/normal termination to force
%a GC too?  Is there any need for a function that provides finer
%control over GC: argument = amount of space required; result = amount
%of space recovered.

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

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

\subsection{The @errorIO#@ primitive operation}

The @errorIO#@ primitive takes an argument much like @PrimIO@.  It
aborts execution of the current program, and continues instead by
performing the given @PrimIO@-like value on the current state of the
world.
\begin{verbatim}
errorIO# :: (State RealWorld -> ((), State RealWorld)) -> a
\end{verbatim}


\subsection{C Calls}

\bf{ToDo:} current implementation has state variable as second
argument not last argument.

The @ccall#@ primitive can't be given an ordinary type, because it has
a variable number of arguments.  The nearest we can get is:
\begin{verbatim}
ccall# :: CRoutine -> a1# -> ... -> an# -> State# RealWorld -> StateAndR# RealWorld
\end{verbatim}

where the type variables @a1#@\ldots@an#@ and @r#@ can be instantiated by any
primitive type, and @StateAndR#@ is the appropriate pairing type from 
Section~\ref{sect:horrid-pairing-types}.  The @CRoutine@ 
isn't a proper Haskell type at all; it just reminds us that @ccall#@ needs to 
know what C routine to call.

This notation is really short for a massive family of @ccall#@ primitives, one 
for each combination of types.  (Of course, the compiler simply remembers the 
types involved, and generates appropriate code when it finally spits out the C.)

Unlike all the other primitive operators, @ccall#@ is not bound to an in-scope 
identifier.  The only way it is possible to generate a @ccall#@ is via the 
@_ccall_@ construct.

All this applies equally to @casm#@:
\begin{verbatim}
casm#  :: CAsmString -> a1# -> ... -> an# -> State# RealWorld -> StateAndR# RealWorld
\end{verbatim}

%------------------------------------------------------------
\subsection{Library stuff built with the Really Primitive Stuff}

\subsubsection{The state transformer monad}

\subsubsubsection{Types}

A state transformer is a function from a state to a pair of a result
and a new state.
\begin{verbatim}
newtype ST s a = ST (State s -> (a, State s))
\end{verbatim}

The @ST@ type is {\em abstract}, so that the programmer cannot see its 
representation.  If he could, he could write bad things like:
\begin{verbatim}
bad :: ST s a
bad = ST $ \ s -> ...(f s)...(g s)...
\end{verbatim}

Here, @s@ is duplicated, which would be bad news.

A state is represented by a primitive state value, of type @State# s@, 
wrapped up in a @State@ constructor.  The reason for boxing it in this
way is so that we can be strict or lazy in the state.  (Remember, all 
primitive types are unboxed, and hence can't be bottom; but types built
with @data@ are all boxed.)
\begin{verbatim}
data State s = S# (State# s)
\end{verbatim}

\subsubsubsection{The state transformer combinators}

Now for the combinators, all of which live inside the @ST@
abstraction.  Notice that @returnST@ and @thenST@ are now strict
in the state. @ST@ is an instance of the @Monad@ type class.
\begin{verbatim}
returnST :: a -> ST s a
returnST a = ST (\ s@(S# _) -> (a, s)) -- strict in state.

thenST :: ST s a -> (a -> ST s b) -> ST s b
thenST m k =
  = ST $ \ s ->
    case (m s) of {(r, new_s) ->
    case (k r) of { ST k2 ->
    (k2 new_s) }}

instance Monad ST where 
  return = returnST
  (>>=)  = thenST

fixST :: (a -> ST s a) -> ST s a
fixST k s = ST $ \ s ->
            let (ST k_r)  = k r
                ans       = k_r s
                (r,new_s) = ans
            in
            ans

unsafeInterleaveST :: ST s a -> ST s a
unsafeInterleaveST (ST m) = ST $ \ s ->
    let
        (r, new_s) = m s
    in
    (r, s)
\end{verbatim}

The interesting one is, of course, @runST@.  We can't infer its type!
(It has a funny name because it must be wired into the compiler.)
\begin{verbatim}
-- runST :: forall a. (forall s. ST s a) -> a
runST m = case m (S# realWorld#) of
           (r,_) -> r
\end{verbatim}


\subsubsubsection{Other useful combinators}

There are various other standard combinators, all defined in terms the
fundamental combinators above. The @seqST@ combinator is like
@thenST@, except that it discards the result of the first state
transformer:
\begin{verbatim}
seqST :: ST s a -> ST s b -> ST s b
seqST m1 m2 = m1 `thenST` (\_ -> m2)
\end{verbatim}

We also have {\em lazy} (... in the state...) variants of the
then/return combinators (same types as their pals):
\begin{verbatim}
returnLazyST a = ST (\ s -> (a, s))

thenLazyST m k
 = ST $ \ s ->
   let (r, new_s) = m s
   in  
   k r new_s

seqLazyST m k
 = ST $ \ s ->
   let (_, new_s) = m s
   in  
   k new_s
\end{verbatim}

The combinator @listST@ is provided for backwards compatibility, its
behavior is captured in Haskell 1.3 (and later) by @Monad.accumulate@:
\begin{verbatim}
listST :: [ST s a] -> ST s [a]
listST ls = accumulate ls
\end{verbatim}

Another function provided for backwards compatibility is @mapST@, it
is just an instance of the more general monad combinator @Monad.mapM@:
\begin{verbatim}
mapST :: (a -> ST s b) -> [a] -> ST s [b]
mapST f ms = mapM f ms
\end{verbatim}

The @mapAndUnzipST@ combinator is similar to @mapST@, except that here the
function returns a pair:
\begin{verbatim}
mapAndUnzipST :: (a -> ST s (b,c)) -> [a] -> ST s ([b],[c])
mapAndUnzipST f ls = mapAndUnzipM f ls
\end{verbatim}


\bf{Note:} all the derived operators over @ST@ are implemented using
the {\em strict} @ST@ instance of @Monad@.

\subsubsection{The @PrimIO@ monad}
\label{sect:io-spec}

The @PrimIO@ type is defined in as a state transformer which manipulates the 
@RealWorld@.
\begin{verbatim}
type PrimIO a = ST RealWorld a      -- Transparent
\end{verbatim}

The @PrimIO@ type is an ordinary type synonym, transparent to the programmer.

The type @RealWorld@ and value @realWorld#@ do not need to be hidden (although 
there is no particular point in exposing them).  Even having a value of type 
@realWorld#@ does not compromise safety, since the type @ST@ is hidden. 

It is type-correct to use @returnST@ in an @PrimIO@ context, they're
in effect the same (ditto for the bind combinator):

\begin{verbatim}
returnPrimIO :: a -> PrimIO a
returnPrimIO v = returnST v

thenPrimIO  :: PrimIO a -> (a -> PrimIO b) -> PrimIO b
thenPrimIO m k = thenST m k

seqPrimIO   :: PrimIO a -> PrimIO b -> PrimIO b
seqPrimIO m k = seqST m k
\end{verbatim}

Why is it safe for @returnPrimIO@ to be strict in the state?  Because
every context in which an I/O state transformer is used will certainly
evaluate the resulting state; it is the state of the real world!

@fixPrimIO@ has to be lazy, though!
\begin{verbatim}
fixPrimIO  = fixST
\end{verbatim}

\subsubsubsection{@PrimIO@ combinators}



\begin{verbatim}
unsafePerformPrimIO     :: PrimIO a -> a
unsafeInterleavePrimIO  :: PrimIO a -> PrimIO a

unsafePerformPrimIO     = runST
unsafeInterleavePrimIO  = unsafeInterleaveST

listPrimIO        :: [PrimIO a] -> PrimIO [a]
mapPrimIO         :: (a -> PrimIO b) -> [a] -> PrimIO [b]
mapAndUnzipPrimIO :: (a -> PrimIO (b,c)) -> [a] -> PrimIO ([b],[c])
\end{verbatim}

The function @unsafePerformPrimIO@ is as the name suggests, {\em
unsafe}, placing a burden of proof on the programmer to ensure that
performing the I/O action does not break referential transparency.

\subsubsection{Arrays}

\subsubsubsection{Types}

\begin{verbatim}
data Array      ix elt = Array     (ix,ix) (Array# elt)
data ByteArray ix      = ByteArray (ix,ix) ByteArray#

data MutableArray     s ix elt = MutableArray     (ix,ix) (MutableArray# s elt)
data MutableByteArray s ix     = MutableByteArray (ix,ix) (MutableByteArray# s)
\end{verbatim}


\subsubsubsection{Operations on immutable arrays}

Ordinary array indexing is straightforward.
\begin{verbatim}
(!) :: Ix ix => Array ix elt -> ix -> elt
\end{verbatim}

QUESTIONs: should @ByteArray@s be indexed by Ints or ix?  With byte offsets
or sized ones? (sized ones [WDP])
\begin{verbatim}
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
\end{verbatim}

@Addr@s are indexed straightforwardly by @Int@s.  Unlike the primitive
operations, though, the offsets assume that the array consists entirely of the
type of value being indexed, and so there's an implicit multiplication by
the size of that value.  To access @Addr@s with mixed values requires
you to do a DIY job using the primitives.
\begin{verbatim}
indexAddrChar :: Addr -> Int -> Char
...etc...
indexStaticCharArray   :: Addr -> Int -> Char
indexStaticIntArray    :: Addr -> Int -> Int
indexStaticFloatArray  :: Addr -> Int -> Float
indexStaticDoubleArray :: Addr -> Int -> Double
indexStaticArray       :: Addr -> Int -> Addr
\end{verbatim}


\subsubsubsection{Operations on mutable arrays}
\begin{verbatim}
newArray     :: Ix ix => (ix,ix) -> elt -> ST s (MutableArray s ix elt)
newCharArray :: Ix ix => (ix,ix) -> ST s (MutableByteArray s ix) 
...
\end{verbatim}


\begin{verbatim}
readArray   :: Ix ix => MutableArray s ix elt -> ix -> ST s elt 
readCharArray   :: Ix ix => MutableByteArray s ix -> ix -> ST s Char 
...
\end{verbatim}

\begin{verbatim}
writeArray  :: Ix ix => MutableArray s ix elt -> ix -> elt -> ST s () 
writeCharArray  :: Ix ix => MutableByteArray s ix -> ix -> Char -> ST s () 
...
\end{verbatim}


\begin{verbatim}
freezeArray :: Ix ix => MutableArray s ix elt -> ST s (Array ix elt)
freezeCharArray :: Ix ix => MutableByteArray s ix -> ST s (ByteArray ix)
...
\end{verbatim}


We have no need on one-function-per-type for unsafe freezing:
\begin{verbatim}
unsafeFreezeArray :: Ix ix => MutableArray s ix elt -> ST s (Array ix elt)  
unsafeFreezeByteArray :: Ix ix => MutableByteArray s ix -> ST s (ByteArray ix)
\end{verbatim}


Sometimes we want to snaffle the bounds of one of these beasts:
\begin{verbatim}
boundsOfArray     :: Ix ix => MutableArray s ix elt -> (ix, ix)  
boundsOfByteArray :: Ix ix => MutableByteArray s ix -> (ix, ix)
\end{verbatim}


Lastly, ``equality'':
\begin{verbatim}
sameMutableArray     :: MutableArray s ix elt -> MutableArray s ix elt -> Bool
sameMutableByteArray :: MutableByteArray s ix -> MutableByteArray s ix -> Bool
\end{verbatim}


\subsubsection{Mutable Variables}

\subsubsubsection{Types}

Mutable variables are (for now anyway) implemented as arrays.  The
@MutableVar@ type is opaque, so we can change the implementation later
if we want.

\begin{verbatim}
type MutableVar s a = MutableArray s Int a
\end{verbatim}


\subsubsubsection{Operations}
\begin{verbatim}
newVar   :: a -> ST s (MutableVar s a)
readVar  :: MutableVar s a -> ST s a
writeVar :: MutableVar s a -> a -> ST s ()
sameVar  :: MutableVar s a -> MutableVar s a -> Bool
\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 -> PrimIO ()
\end{verbatim}

\subsubsection{Foreign objects}

Again, just boxing up.
\begin{verbatim}
data ForeignObj = ForeignObj ForeignObj#

makeForeignObj :: Addr   -- object to be boxed up as a ForeignObj
               -> Addr   -- finaliser 
               -> PrimIO ForeignObj
\end{verbatim}

\subsubsection{C calls}

Everything in this section goes for @_casm_@ too.

\bf{ToDo:} {\em mention @_ccall_gc_@ and @_casm_gc_@...}

The @_ccall_@ construct has the following form:
$$@_ccall_@~croutine~a_1~\ldots~a_n$$
This whole construct has type @PrimIO@~$res$.
The rules are these:
\begin{itemize}
\item
The first ``argument'', $croutine$, must be the literal name of a C procedure.
It cannot be a Haskell expression which evaluates to a string, etc; it must be 
simply the name of the procedure.
\item
The arguments $a_1, \ldots,a_n$ must be of {\em C-callable} type.
\item
The whole construct has type @PrimIO@~$ty$, where $ty$ is a {\em C-returnable} type.
\end{itemize}
A {\em boxed-primitive} type is both C-callable and C-returnable.
A boxed primitive type is anything declared by:
\begin{verbatim}
data T = C# t
\end{verbatim}

where @t@ is a primitive type.  Note that
programmer-defined boxed-primitive types are perfectly OK:
\begin{verbatim}
data Widget = W# Int#
data Screen = S# CHeapPtr#
\end{verbatim}


There are other types that can be passed to C (C-callable).  This
table summarises (including the standard boxed-primitive types):
\begin{verbatim}
Boxed             Type of transferd   Corresp.     Which is
Type              Prim. component     C type       *probably*...
------            ---------------     ------       -------------
Char              Char#               StgChar       unsigned char
Int               Int#                StgInt        long int
Word              Word#               StgWord       unsigned long int
Addr              Addr#               StgAddr       char *
Float             Float#              StgFloat      float
Double            Double#             StgDouble     double
                                       
Array             Array#              StgArray      StgPtr
ByteArray         ByteArray#          StgByteArray  StgPtr
MutableArray      MutableArray#       StgArray      StgPtr
MutableByteArray  MutableByteArray#   StgByteArray  StgPtr
                                      
State             State#              nothing!
                                      
StablePtr         StablePtr#          StgStablePtr  StgPtr
ForeignObj        ForeignObj#         StgForeignObj StgPtr
\end{verbatim}


All of the above are {\em C-returnable} except:
\begin{verbatim}
  Array, ByteArray, MutableArray, MutableByteArray, ForeignObj
\end{verbatim}

\bf{ToDo:} I'm pretty wary of @Array@ and @MutableArray@ being in
this list, and not too happy about @State@ [WDP].

\bf{ToDo:} Can code generator pass all the primitive types?  Should this be
extended to include {\tt Bool\/} (or any enumeration type?)

The type checker must be able to figure out just which of the C-callable/returnable
types is being used.  If it can't, you have to add type signatures. For example,
\begin{verbatim}
f x = _ccall_ foo x
\end{verbatim}

is not good enough, because the compiler can't work out what type @x@ is, nor 
what type the @_ccall_@ returns.  You have to write, say:
\begin{verbatim}
f :: Int -> PrimIO Float
f x = _ccall_ foo x
\end{verbatim}


\subsubsubsection{Implementation}

The desugarer unwraps the @_ccall_@ construct by inserting the
necessary evaluations etc to unbox the arguments.  For example, the
body of the definition of @f@ above would become:
\begin{verbatim}
        (\ s -> case x of { I# x# -> 
                case s of { S# s# ->
                case ccall# [Int#,Float#] x# s# of { StateAndFloat# f# new_s# ->
                (F# f#, S# new_s#)
                }}})
\end{verbatim}

Notice that the state, too, is unboxed.

%The code generator must deal specially with primitive objects which
%are stored on the heap.
%
%\begin{verbatim}
%... details omitted ...
%\end{verbatim}

%
%More importantly, it must construct a C Heap Pointer heap-object after
%a @_ccall_@ which returns a @MallocPtr#@.
%

%--------------------------------------------------------
\subsection{Non-primitive stuff that must be wired into GHC}

\begin{verbatim}
data Char    = C# Char#
data Int     = I# Int#
data Word    = W# Word#
data Addr    = A# Addr#

data Float   = F# Float#
data Double  = D# Double#
data Integer = J# Int# Int# ByteArray#

-- and the other boxed-primitive types:
    Array, ByteArray, MutableArray, MutableByteArray,
    StablePtr, ForeignObj

data Bool     = False | True
data Ordering = LT | EQ | GT  -- used in derived comparisons

data List a = [] | a : (List a)
-- tuples...

data Lift a = Lift a    -- used Yukkily as described elsewhere

type String  = [Char]    -- convenience, only
\end{verbatim}


%------------------------------------------------------------
\subsection{Programmer interface(s)}

\subsubsection{The bog-standard interface}

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.

\subsubsubsection{If you mess about with @import Prelude@...}

Innocent hiding, e.g.,
\begin{verbatim}
import Prelude hiding ( fromIntegral )
\end{verbatim}

should work just fine.

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.

\subsubsection{Turning on Glasgow extensions with @-fglasgow-exts@}

% Updated to tell the 2.02+ story  -- SOF

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 interface @GlaExts@.

% 1.3+ module system makes this a non-issue.
%%It is possible that some name conflicts between your code and the
%%wired-in things might spring to life (though we doubt it...).
%%Change your names :-)

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

The @GlaExts@ interface is the programmer gateway to most of the
programmer extensions GHC implement. Currently on tap:

\begin{verbatim}
  -- PrimIO monad (state transformer, no exceptions).
type PrimIO a = ST RealWorld a
  -- instances of: Monad
data RealWorld   -- abstract State value

thenPrimIO       :: PrimIO a -> (a -> PrimIO b) -> PrimIO b
returnPrimIO     :: a -> PrimIO a
seqPrimIO        :: PrimIO a -> PrimIO b -> PrimIO b

fixPrimIO        :: (a -> PrimIO a) -> PrimIO a
unsafePerformPrimIO    :: PrimIO a -> a
unsafeInterleavePrimIO :: PrimIO a -> PrimIO a

-- backwards compatibility
listPrimIO        :: [PrimIO a] -> PrimIO [a]
mapPrimIO         :: (a -> PrimIO b) -> [a] -> PrimIO [b]
mapAndUnzipPrimIO :: (a -> PrimIO (b,c)) -> [a] -> PrimIO ([b],[c])

-- Combining ST and PrimIO monads with IO
        stToIO,       -- :: ST RealWorld a -> IO a
        primIOToIO,   -- :: PrimIO a       -> IO a
        ioToST,       -- :: IO a -> ST RealWorld a
        ioToPrimIO,   -- :: IO a -> PrimIO       a
        thenIO_Prim,  -- :: PrimIO a -> (a -> IO b) -> IO b
        seqIO_Prim,   -- :: PrimIO a -> IO b -> IO b

-- 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#

-- misc
trace :: String -> a -> a

-- re-exported interfaces:
module ByteArray
module MutableArray
module GHC  -- all primops and primitive types.
\end{verbatim}


%************************************************************************
%*                                                                      *
\subsubsection{The @MutVar@ interface}
\index{MutVar interface (GHC extensions)}
%*                                                                      *
%************************************************************************

@MutVar@ defines an interface to mutable variables, defining type and
IO operations:

\begin{verbatim}
data  MutVar   -- abstract

newVar       :: a -> IO (MutVar a)
readVar      :: MutVar a -> IO a
writeVar     :: MutVar a -> a -> IO ()
sameVar      :: MutVar a -> MutVar a -> Bool
\end{verbatim}


%************************************************************************
%*                                                                      *
\subsubsection{The @MutableArray@ interface}
\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}
\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{The @Foreign@ interface}
\index{Foreign interface (GHC extensions)}
%*                                                                      *
%************************************************************************

The @Foreign@ interface define and export operations over @ForeignObj@
and @StablePtr@s:

\begin{verbatim}
-- semi-magic classes for pack/unpacking ccall arguments.
class CCallable a
 {- Instances defined for : 
      Char Char# Int Int# Float Float#
      Double Double# Addr Addr# Word Word#
      (MutableByteArray s ix) (MutableByteArray# s)
      (ByteArray ix) ByteArray#
      ForeignObj ForeignObj# 
      (StablePtr a)
      (StablePtr# a)
      [Char]
 -}
class CReturnable a
 {- Instances defined for : 
      Char Int Float Double Addr Word 
      (StablePtr a)
 -}

data ForeignObj = ForeignObj ForeignObj#
   -- instances of : CCallable Eq

eqForeignObj    :: ForeignObj  -> ForeignObj -> Bool
makeForeignObj  :: Addr        -> Addr       -> PrimIO ForeignObj
writeForeignObj :: ForeignObj  -> Addr       -> PrimIO ()

{- derived op - attaching a free() finaliser to a malloc() allocated reference. -}
makeMallocPtr   :: Addr        -> PrimIO ForeignObj

data StablePtr a = StablePtr (StablePtr# a)
  -- instances of : CCallable

makeStablePtr  :: a -> PrimIO (StablePtr a)
deRefStablePtr :: StablePtr a -> PrimIO a
freeStablePtr  :: StablePtr a -> PrimIO ()
performGC      :: PrimIO ()
\end{verbatim}

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