Run doctest on Link.hs and succeeds. Run doctest on Person.hs, which runs Link.hs and fails with failed unpickling.

.ghci

-- GHC 7.6.3
:load Test/Person.hs
fmap (unpickleDoc' xpAuthorElem) . runLA xread $ "<author><link/></author>" -- this succeeds
:reload
fmap (unpickleDoc' xpLinkElem) . runLA xread $ "<link/>" -- this fails
fmap (unpickleDoc' xpAuthorElem) . runLA xread $ "<author><link/></author>" -- this fails in the same way, even though it is the same expression
:reload
fmap (unpickleDoc' xpAuthorElem) . runLA xread $ "<author><link/></author>" -- now it is working again

Link.hs

-- Test/Link.hs

module Test.Link where

import Text.XML.HXT.Core

data Link =
  Link 
  deriving Show

-- |
--
-- >>> fmap (unpickleDoc' xpLinkElem) . runLA xread $ "<link/>"
-- [Right Link]
xpLinkElem ::
  PU Link
xpLinkElem =
  xpElem "link"
    (xpWrap (const Link, const ())
      xpUnit)

Person.hs

-- Test/Person.hs

module Test.Person where

import Test.Link(Link, xpLinkElem)
import Text.XML.HXT.Core

data Person =
  Person Link
  deriving Show

-- |
--
-- >>> fmap (unpickleDoc' xpAuthorElem) . runLA xread $ "<author><link/></author>"
-- [Right (Person Link)]
xpAuthorElem::
  PU Person
xpAuthorElem =
  xpElem "author"
    (xpWrap
      (Person, \(Person l) -> l)
      xpLinkElem)

reproduce.sh

#/bin/sh

cabal sandbox init
cabal install hxt-9.3.1.1
mkdir -p Test
wget https://gist.github.com/tonymorris/7095376/raw/c2bb6d107ae97e52c89807877baa04db56153084/Link.hs -O Test/Link.hs -q -c
wget https://gist.github.com/tonymorris/7095376/raw/cbc4aa4bbbbca38979e18cb8620c5fb7748cc28b/Person.hs -O Test/Person.hs -q -c
wget -c -q https://gist.github.com/tonymorris/7095376/raw/6f605e841ccb94d37626ea999c944a592e4d9247/.ghci -O .ghci -q -c
ghci -package-conf `find .cabal-sandbox -name "*-packages.conf.d"`

result

% ghc --version                                                                                    [issue-1]
The Glorious Glasgow Haskell Compilation System, version 7.6.3

% cabal sandbox hc-pkg list | grep hxt                                                             [issue-1]
    hxt-9.3.1.1
    hxt-charproperties-9.1.1.1
    hxt-regex-xmlschema-9.1.0
    hxt-unicode-9.0.2.1
% cabal sandbox hc-pkg list | grep doctest                                                         [issue-1]
    doctest-0.9.9

% ./.cabal-sandbox/bin/doctest Test/Link.hs                            
Examples: 1  Tried: 1  Errors: 0  Failures: 0

% ./.cabal-sandbox/bin/doctest Test/Person.hs
### Failure in ./Test/Link.hs:13: expression `fmap (unpickleDoc' xpLinkElem) . runLA xread $ "<link/>"'
expected: [Right Link]
 but got: [Left "xpElem: got element name \"link\", but expected \"link\"\ncontext:    element \"/\""]
Examples: 2  Tried: 2  Errors: 0  Failures: 1