9.2. When your program “does the wrong thing”
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## 9.2. When your program “does the wrong thing”

(For advice about overly slow or memory-hungry Haskell programs, please see Chapter 6, Advice on: sooner, faster, smaller, thriftier).

“Help! My program crashed!”

  (e.g., a `segmentation fault' or `core dumped')

  If your program has no foreign calls in it, and no calls to known-unsafe functions (such as `unsafePerformIO`) then a crash is always a BUG in the GHC system, except in one case: If your program is made of several modules, each module must have been compiled after any modules on which it depends (unless you use `.hi-boot` files, in which case these must be correct with respect to the module source).

  For example, if an interface is lying about the type of an imported value then GHC may well generate duff code for the importing module. This applies to pragmas inside interfaces too! If the pragma is lying (e.g., about the “arity” of a value), then duff code may result. Furthermore, arities may change even if types do not.

  In short, if you compile a module and its interface changes, then all the modules that import that interface must be re-compiled.

  A useful option to alert you when interfaces change is `-hi-diffs`. It will run diff on the changed interface file, before and after, when applicable.

  If you are using make, GHC can automatically generate the dependencies required in order to make sure that every module is up-to-date with respect to its imported interfaces. Please see Section 4.6.11, “Dependency generation”.

  If you are down to your last-compile-before-a-bug-report, we would recommend that you add a `-dcore-lint` option (for extra checking) to your compilation options.

  So, before you report a bug because of a core dump, you should probably:

      % rm *.o        # scrub your object files
      % make my_prog  # re-make your program; use -hi-diffs to highlight changes;
                      # as mentioned above, use -dcore-lint to be more paranoid
      % ./my_prog ... # retry...

  Of course, if you have foreign calls in your program then all bets are off, because you can trash the heap, the stack, or whatever.

“My program entered an `absent' argument.”

  This is definitely caused by a bug in GHC. Please report it (see Section 1.2, “Reporting bugs in GHC”).

“What's with this `arithmetic (or `floating') exception' ”?

  `Int`, `Float`, and `Double` arithmetic is unchecked. Overflows, underflows and loss of precision are either silent or reported as an exception by the operating system (depending on the platform). Divide-by-zero may cause an untrapped exception (please report it if it does).

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