This section has the answers to questions that get asked
regularly on the GHC mailing lists, in no particular order. Please
let us know if you think there's a question/answer that should be
How do I port GHC to platform X?
There are two distinct possibilities: either
The hardware architecture for your system is already
supported by GHC, but you're running an OS that isn't
supported (or perhaps has been supported in the past, but
currently isn't). This is the easiest type of porting
job, but it still requires some careful
Your system's hardware architecture isn't supported
by GHC. This will be a more difficult port (though by
comparison perhaps not as difficult as porting
Both ways require you to bootrap 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.
Guide has all the details on how to bootstrap GHC on a
Do I have to recompile all my code if I upgrade
Yes. There are two reasons for this:
GHC does a lot of cross-module optimisation, so
compiled code will include parts of the libraries it was
compiled against (including the Prelude), so will be
deeply tied to the actual version of those libraries it
was compiled against. When you upgrade GHC, the libraries
may change; even if the external interface of the
libraries doesn't change, sometimes internal details may
change because GHC optimised the code in the library
We sometimes change the ABI (application binary
interface) between versions of GHC. Code compiled with
one version of GHC is not necessarily compatible with code
compiled by a different version, even if you arrange to
keep the same libraries.
Why doesn't GHC use shared libraries?
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.
I can't get string gaps to work
If you're also using CPP, beware of the known pitfall
with string gaps mentioned in Section 188.8.131.52.
GHCi complains about missing symbols like
CC_LIST when loading a previously compiled .o
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.
Linking a program causes the following error on Linux:
/usr/bin/ld: cannot open -lgmp: No such file or
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
I Can't run GHCi on Linux, because it complains about a
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
/usr/lib. We tried this on a SuSE 7.1 box
and it seemed to work, but YMMV.
Solaris users may sometimes get link errors due to
libraries needed by GNU Readline.
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.
When I try to start ghci (probably one I compiled myself)
it says ghc-5.02: not built for interactive
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.
When I use a foreign function that takes or returns a
float, it gives the wrong answer, or crashes.
You should use the -#include option to
bring the correct prototype into scope (see Section 4.12.5).
My program that uses a really large heap crashes on
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).
GHC doesn't like filenames containing
Indeed not. You could change + to
p or plus.
When I open a FIFO (named pipe) and try to read from it, I
get EOF immediately.
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 behavour 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.
When I foreign import a function that
returns char or short, I
get garbage back.
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.
My program is failing with head , or
an array bounds error, or some other random error, and I have no
idea how to find the bug. Can you help?
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 4.16.3 for more
How do I increase the heap size permanently for a given
I'm trying to compile my program for parallel execution
with the -parallel, and GHC complains with an
error like “failed to load interface file for
GHC doesn't ship with support for parallel execution,
that support is provided separately by the GPH project.
When is it safe to use
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
It is safe to implement a function or API using
unsafePerformIO if you could imagine
also implementing the same function or API in Haskell
without using unsafePerformIO (forget
about efficiency, just consider the semantics).
In pure Haskell, the value of a function depends
only on the values of its arguments (and free variables,
if it has any). If you can implement the function using
unsafePerformIO and still retain this
invariant, then you're probably using
unsafePerformIO in a safe way. Note
that you need only consider the
observable values of the arguments
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
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
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