There are two distinct possibilities: either

Both ways require you to bootstrap from intermediate HC files: these are the stylised C files generated by GHC when it compiles Haskell source. Basically the idea is to take the HC files for GHC itself to the target machine and compile them with gcc to get a working GHC, and go from there.

The Building Guide has all the details on how to bootstrap GHC on a new platform.

Yes. There are two reasons for this:

The subject of shared libraries has come up several times in the past — take a look through the mailing-list archives for some of the previous discussions. The upshot is that shared libraries wouldn't really buy much unless you really need to save the disk space: in all other considerations, static linking comes out better.

Unfortunately GHC-compiled libraries are very tightly coupled, which means it's unlikely you'd be able to swap out a shared library for a newer version unless it was compiled with exactly the same compiler and set of libraries as the old version.

If you're also using CPP, beware of the known pitfall with string gaps mentioned in Section 4.10.3.1, “CPP and string gaps”.

This probably means the .o files in question were compiled for profiling (with -prof). Workaround: recompile them without profiling. We really ought to detect this situation and give a proper error message.

The problem is that your system doesn't have the GMP library installed. If this is a RedHat distribution, install the RedHat-supplied gmp-devel package, and the gmp package if you don't already have it. There have been reports that installing the RedHat packages also works for SuSE (SuSE don't supply a shared gmp library).

The "correct" fix for this problem is to install the correct RPM for the particular flavour of Linux on your machine. If this isn't an option, however, there is a hack that might work: make a symbolic link from libreadline.so.4 to libreadline.so.3 in /usr/lib. We tried this on a SuSE 7.1 box and it seemed to work, but YMMV.

We suggest you try linking in some combination of the termcap, curses and ncurses libraries, by giving -ltermcap, -lcurses and -lncurses respectively. If you encounter this problem, we would appreciate feedback on it, since we don't fully understand what's going on here.

It has been reported that if you have multiple versions of the readline library installed on Linux, then this may cause the build to fail. If you have multiple versions of readline, try uninstalling all except the most recent version.

To build a working ghci, you need to build GHC 5.02 with itself; the above message appears if you build it with 4.08.X, for example. It'll still work fine for batch-mode compilation, though. Note that you really must build with exactly the same version of the compiler. Building 5.02 with 5.00.2, for example, may or may not give a working interactive system; it probably won't, and certainly isn't supported. Note also that you can build 5.02 with any older compiler, back to 4.08.1, if you don't want a working interactive system; that's OK, and supported.

You should use the -#include option to bring the correct prototype into scope (see Section 4.10.5, “Options affecting the C compiler (if applicable)”).

For utterly horrible reasons, programs that use more than 128Mb of heap won't work when compiled dynamically on Windows (they should be fine statically compiled).

Indeed not. You could change + to p or plus.

This is a consequence of the fact that GHC opens the FIFO in non-blocking mode. The behaviour varies from OS to OS: on Linux and Solaris you can wait for a writer by doing an explicit threadWaitRead on the file descriptor (gotten from Posix.handleToFd) before the first read, but this doesn't work on FreeBSD (although rumour has it that recent versions of FreeBSD changed the behaviour to match other OSs). A workaround for all systems is to open the FIFO for writing yourself, before (or at the same time as) opening it for reading.

This is a known bug in GHC versions prior to 5.02.2. GHC doesn't mask out the more significant bits of the result. It doesn't manifest with gcc 2.95, but apparently shows up with g++ and gcc 3.0.

Compile your program with -prof -auto-all (make sure you have the profiling libraries installed), and run it with +RTS -xc -RTS to get a “stack trace” at the point at which the exception was raised. See Section 4.14.4, “RTS options for hackers, debuggers, and over-interested souls” for more details.

See Section 4.14.5, ““Hooks” to change RTS behaviour”.

GHC doesn't ship with support for parallel execution, that support is provided separately by the GPH project.

We'll give two answers to this question, each of which may be helpful. These criteria are not rigorous in any real sense (you'd need a formal semantics for Haskell in order to give a proper answer to this question), but should give you a feel for the kind of things you can and cannot do with unsafePerformIO.

For more information, see this thread.

Linking a small program should take no more than a few seconds. Larger programs can take longer, but even linking GHC itself only takes 3-4 seconds on our development machines.

Long link times have been attributed to using Sun's linker on Solaris, as compared to GNU ld which appears to be much faster. So if you're on a Sun box, try switching to GNU ld. This article from the mailing list has more information.

This is a consequence of Unixy terminal semantics. Unix does line buffering on terminals in the kernel as part of the terminal processing, unless you turn it off. However, the Ctrl-D processing is also part of the terminal processing which gets turned off when the kernel line buffering is disabled. So GHC tries its best to get NoBuffering semantics by turning off the kernel line buffering, but as a result you lose Ctrl-D. C'est la vie.

The stdout handle is line-buffered by default, which means that output sent to the handle is only flushed when a newline (/n) is output, the buffer is full, or hFlush is called on the Handle. The right way to make the text appear without sending a newline is to use hFlush:

  import System.IO
  main = do
    putStr "how are you today? "
    hFlush stdout
    input &- hGetLine
    ...

You'll probably find that the behaviour differs when using GHCi: the hFlush isn't necessary to make the text appear. This is because in GHCi we turn off the buffering on stdout, because this is normally what you want in an interpreter: output appears as it is generated.

Chances are that your program is trying to write a message to stdout or stderr in the finalizer. Handles have finalizers themselves, and since finalizers don't keep other finalized values alive, the stdout and stderr Handles may be finalized before your finalizer runs. If this happens, your finalizer will block on the handle, and probably end up receiving a NonTermination exception (which is printed as <<loop>>).

No, extensible records are not implemented in GHC. Hugs implements TRex, one extensible record variant. The problem is that the record design space is large, and seems to lack local optima. And all reasonable variants break backward compatibility. As a result, nothing much happens.

Certain options, such as -O, turn on via-C compilation, instead of using the native code generator. Include files named by -#include options or in foreign import declarations are only used in via-C compilation mode. See Section 8.2.2.1, “Finding Header files” for more details.

You can select alternative suffixes for object files and interface files, so you can have several builds of the same code coexisting in the same directory. For example, to compile with profiling, you might do this:

ghc --make -prof -o foo-prof -osuf p.o -hisuf p.hi Main

See Section 4.6.4, “Redirecting the compilation output(s)” for more details on the -osuf and -hisuf options.

In order to load the readline package under GHCi on Windows, you need to make a version of the readline library that GHCi can load. Instructions for GHC 6.2.2. are here.

When running GHC under a Cygwin shell on Windows, Ctrl-C sometimes doesn't work. The workaround is to use Ctrl-Break instead.

An error message like this:

GHCi runtime linker: fatal error: I found a duplicate definition for symbol
       __module_registered
    whilst processing object file
       /usr/local/lib/ghc-6.2/HSfgl.o

probably indicates that when building a library for GHCi (HSfgl.o in the above example), you should use the -x option to ld.