Sophie: ghc-0:6.10.4-1mdv2010.0 i586

ghc-6.10.4-1mdv2010.0.i586.rpm

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>base-4.1.0.0: Basic libraries</TD
><TD CLASS="topbut"
><A HREF="index.html"
>Contents</A
></TD
><TD CLASS="topbut"
><A HREF="doc-index.html"
>Index</A
></TD
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><TD CLASS="modulebar"
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><FONT SIZE="6"
>Foreign</FONT
></TD
><TD ALIGN="right"
><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="infohead"
>Portability</TD
><TD CLASS="infoval"
>portable</TD
></TR
><TR
><TD CLASS="infohead"
>Stability</TD
><TD CLASS="infoval"
>provisional</TD
></TR
><TR
><TD CLASS="infohead"
>Maintainer</TD
><TD CLASS="infoval"
>ffi@haskell.org</TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Description</TD
></TR
><TR
><TD CLASS="doc"
>A collection of data types, classes, and functions for interfacing
 with another programming language.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Synopsis</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
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><TD CLASS="decl"
>module <A HREF="Data-Bits.html"
>Data.Bits</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Data-Int.html"
>Data.Int</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Data-Word.html"
>Data.Word</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-Ptr.html"
>Foreign.Ptr</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-ForeignPtr.html"
>Foreign.ForeignPtr</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-StablePtr.html"
>Foreign.StablePtr</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-Storable.html"
>Foreign.Storable</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-Marshal.html"
>Foreign.Marshal</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AunsafePerformIO"
>unsafePerformIO</A
> ::  <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -&gt; a</TD
></TR
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></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Documentation</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Data-Bits.html"
>Data.Bits</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Data-Int.html"
>Data.Int</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Data-Word.html"
>Data.Word</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-Ptr.html"
>Foreign.Ptr</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-ForeignPtr.html"
>Foreign.ForeignPtr</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-StablePtr.html"
>Foreign.StablePtr</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-Storable.html"
>Foreign.Storable</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Foreign-Marshal.html"
>Foreign.Marshal</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
>For compatibility with the FFI addendum only.  The recommended
 place to get this from is <A HREF="System-IO-Unsafe.html"
>System.IO.Unsafe</A
>.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:unsafePerformIO"
><A NAME="v%3AunsafePerformIO"
></A
></A
><B
>unsafePerformIO</B
> ::  <A HREF="System-IO.html#t%3AIO"
>IO</A
> a -&gt; a</TD
></TR
><TR
><TD CLASS="doc"
><P
>This is the &quot;back door&quot; into the <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> monad, allowing
<TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> computation to be performed at any time.  For
this to be safe, the <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> computation should be
free of side effects and independent of its environment.
</P
><P
>If the I/O computation wrapped in <TT
><A HREF="Foreign.html#v%3AunsafePerformIO"
>unsafePerformIO</A
></TT
>
performs side effects, then the relative order in which those side
effects take place (relative to the main I/O trunk, or other calls to
<TT
><A HREF="Foreign.html#v%3AunsafePerformIO"
>unsafePerformIO</A
></TT
>) is indeterminate.  You have to be careful when 
writing and compiling modules that use <TT
><A HREF="Foreign.html#v%3AunsafePerformIO"
>unsafePerformIO</A
></TT
>:
</P
><UL
><LI
> Use <TT
>{-# NOINLINE foo #-}</TT
> as a pragma on any function <TT
>foo</TT
>
        that calls <TT
><A HREF="Foreign.html#v%3AunsafePerformIO"
>unsafePerformIO</A
></TT
>.  If the call is inlined,
        the I/O may be performed more than once.
</LI
><LI
> Use the compiler flag <TT
>-fno-cse</TT
> to prevent common sub-expression
        elimination being performed on the module, which might combine
        two side effects that were meant to be separate.  A good example
        is using multiple global variables (like <TT
>test</TT
> in the example below).
</LI
><LI
> Make sure that the either you switch off let-floating, or that the 
        call to <TT
><A HREF="Foreign.html#v%3AunsafePerformIO"
>unsafePerformIO</A
></TT
> cannot float outside a lambda.  For example, 
        if you say:
        <TT
>
           f x = unsafePerformIO (newIORef [])
        </TT
>
        you may get only one reference cell shared between all calls to <TT
>f</TT
>.
        Better would be
        <TT
>
           f x = unsafePerformIO (newIORef [x])
        </TT
>
        because now it can't float outside the lambda.
</LI
></UL
><P
>It is less well known that
<TT
><A HREF="Foreign.html#v%3AunsafePerformIO"
>unsafePerformIO</A
></TT
> is not type safe.  For example:
</P
><PRE
>     test :: IORef [a]
     test = unsafePerformIO $ newIORef []
     
     main = do
             writeIORef test [42]
             bang &lt;- readIORef test
             print (bang :: [Char])
</PRE
><P
>This program will core dump.  This problem with polymorphic references
is well known in the ML community, and does not arise with normal
monadic use of references.  There is no easy way to make it impossible
once you use <TT
><A HREF="Foreign.html#v%3AunsafePerformIO"
>unsafePerformIO</A
></TT
>.  Indeed, it is
possible to write <TT
>coerce :: a -&gt; b</TT
> with the
help of <TT
><A HREF="Foreign.html#v%3AunsafePerformIO"
>unsafePerformIO</A
></TT
>.  So be careful!
</P
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