Find the source of an Haskell exception

Question

I am trying to figure out where an exception is thrown. These are my first experiences with exception handling in Haskell. I am trying to call an XML-RPC function on a remote host, that is accessed using https:

ghci> import Network.XmlRpc.Client
ghci> import Network.XmlRpc.Internals
ghci> remote "https://rpc.ote.gandi.net/xmlrpc/" "domain.count" (ValueString "01234567890ABCDEF")
*** Exception: user error (https not supported)

In order to figure out if I just forgot to enable SSL support in some package or if it's something different, I would like to know which package throws the exception.

I started by following the instructions in the GHC docs , but it is not working out as expected:

ghci> :set -fbreak-on-exception
ghci> :trace remote "https://rpc.ote.gandi.net/xmlrpc/" "domain.count" (ValueString "01234567890ABCDEF")
Stopped at <exception thrown>
_exception :: e = _
ghci> :hist
Empty history. Perhaps you forgot to use :trace?

All relevant packages should be compiled with --enable-library-profiling .

How to locate the exception?

Answer 1

The reason you couldn't get any good information out of it is that :trace can't go into library code -- we need to interpret any code we want to trace. Whether it was compiled with profiling is irrelevant. After installing some dependencies, I did this to get some more information:

    % cabal unpack haxr
    % cd haxr-3000.8.5
    % ghci Network/XmlRpc/Client.hs -XOverlappingInstances -XTypeSynonymInstances -XFlexibleInstances -XTemplateHaskell
    *Network.XmlRpc.Client> :set -fbreak-on-exception
    *Network.XmlRpc.Client> :trace remote "https://rpc.ote.gandi.net/xmlrpc/" "domain.count" (ValueString "01234567890ABCDEF")
    Stopped at <exception thrown>
    _exception :: e = _
    [<exception thrown>] *Network.XmlRpc.Client> :hist
    -1  : authHdr (Network/XmlRpc/Client.hs:169:27-33)
    -2  : request:parseUserInfo (Network/XmlRpc/Client.hs:161:34-40)
    -3  : request:parseUserInfo (Network/XmlRpc/Client.hs:161:31-73)
    -4  : request:parseUserInfo:(...) (Network/XmlRpc/Client.hs:159:55-70)
    -5  : request:parseUserInfo:(...) (Network/XmlRpc/Client.hs:159:39-51)
    -6  : request:parseUserInfo:(...) (Network/XmlRpc/Client.hs:159:39-70)
    -7  : request:parseUserInfo (Network/XmlRpc/Client.hs:160:34-39)
    -8  : request:parseUserInfo (Network/XmlRpc/Client.hs:160:31-64)
    -9  : request:parseUserInfo (Network/XmlRpc/Client.hs:(160,29)-(161,74))
    -10 : request:parseUserInfo (Network/XmlRpc/Client.hs:(159,5)-(161,74))
    -11 : authHdr (Network/XmlRpc/Client.hs:(169,1)-(175,60))
    -12 : request:headers (Network/XmlRpc/Client.hs:158:33-47)
    -13 : request:headers (Network/XmlRpc/Client.hs:158:33-63)
    -14 : request:headers (Network/XmlRpc/Client.hs:158:33-70)
    -15 : request:headers (Network/XmlRpc/Client.hs:158:20-71)
    -16 : request:headers (Network/XmlRpc/Client.hs:157:16-65)
    -17 : request:headers (Network/XmlRpc/Client.hs:156:16-47)
    -18 : request:headers (Network/XmlRpc/Client.hs:155:16-44)
    -19 : request:headers (Network/XmlRpc/Client.hs:(155,15)-(158,71))
    -20 : request (Network/XmlRpc/Client.hs:(149,28)-(152,54))
    ...

Hopefully that gets you started. You may find that this leads you to another library boundary -- if so, you'll need to unpack and interpret that library to go deeper. Good luck!