faq_406.shtml

---
title: GHC 4.06 Mini-FAQ
---

<h2>GHC 4.06 Mini-FAQ</h2>

This page lists known problems for the 4.06 release with workarounds.

<ul>
<li><p>I can't build GHC 4.06 with GHC 4.04!
<p>You need 4.04pl1. This is the version currently available from the 4.04
download pages.</li><p>
   <li>Linking a program causes the following error on Linux:
<pre>
   /usr/bin/ld: cannot open -lgmp: No such file or directory
</pre>
  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).
  </li><p>

  <li><p>My program fails with
<pre>
    fatal error: No threads to run!  Deadlock?
</pre>
  The primary cause of this message is trying to use <tt>trace</tt> at
  the same time as writing to <tt>stdout</tt> or <tt>stderr</tt>
  (eg. <tt>putStr (trace "foo" "bar")</tt> will illustrate the error).
  The problem is caused by overzealous locking in our I/O library, but
  we don't have a complete fix at this time.  A workaround is to start
  your program like this:
<pre>
    main = seq stderr $ do ...
</pre>
  This allows you to trace while writing to <tt>stdout</tt>, but
  tracing while writing to stderr will still cause a deadlock.
  </li>
  <p><li><p>My program loops while trying to evaluate a large expression.
  <p>GHC can only allocate 4096 bytes at a time. If your expression (e.g. a
  large literal list) would take more, the run-time system loops trying to
  allocate space for it. The workaround is to compile with <tt>-O</tt>,
  which pulls the expression out to the top level and makes it static
  (hopefully).
  </li>
  <p><li><p>GHC complains about bogus overlapping string patterns in my program!
  <p>I'm afraid so.
<pre>
  module X where
  x a = case a of
              "aa"            -> a
              "{"             -> a
</pre>
  gives the warning
<pre>
  X.hs:2: Pattern match(es) are overlapped in a group of case 
  alternatives beginning
                                             "aa":
            "{" -> ...
</pre>
  </li>
  <p>There's no fix at the moment.

<p><li><p>GHC has problems parsing interface files it's generated, giving
messages like

<pre>
TysPrim.lhs:52:
    Type.hi:172 Interface file parse error; on input  #) '

TysPrim.lhs:52: Could not find valid interface file Type'
</pre>

<p>Apparently 4.06 has a heart attack if it is faced with an empty unboxed
tuple <tt>(# #)</tt>. Unfortunately, it also generates such things. The
workaround is not to use optimisation.
</li>
</ul>