The attached patch seems to fix the FTBFS. However, I did not do any tests.
diff -ruN orig/haskell-hsql-1.6/debian/rules haskell-hsql-1.6/debian/rules --- orig/haskell-hsql-1.6/debian/rules 2006-04-01 12:09:10.000000000 +0200 +++ haskell-hsql-1.6/debian/rules 2006-04-01 12:10:30.000000000 +0200 @@ -58,7 +58,7 @@ cd HSQL && dh_haskell_prep -plibghc6-hsql-dev cd HSQL && dh_haskell_build -plibghc6-hsql-dev # Register HSQL - sed s,/usr/lib/haskell-packages/ghc6/lib/hsql-$(version),`pwd`/HSQL/dist/build,g < HSQL/.installed-pkg-config > debian/tmp-installed-pkg + sed s,/usr/lib/haskell-packages/ghc6/lib/hsql-$(version)/ghc-[0-9\.]*,`pwd`/HSQL/dist/build,g < HSQL/.installed-pkg-config > debian/tmp-installed-pkg echo '[]' > debian/tmp-package-conf ghc-pkg -f debian/tmp-package-conf register debian/tmp-installed-pkg