Hello! I hate every Haskell package related thing, so take what I say with a grain of salt. Have you considered using cblrepo? It's working fine for me, even for packages they didn't have yet.
They have binary packages in an unofficial repository (https://wiki.archlinux.org/index.php/Unofficial_user_repositories#haskell-core). I have haskell-pandoc from there, and it seems they agree with you: $ ldd /bin/pandoc linux-vdso.so.1 (0x00007fff96912000) libz.so.1 => /usr/lib/libz.so.1 (0x00007f7fdc07c000) librt.so.1 => /usr/lib/librt.so.1 (0x00007f7fdbe74000) libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f7fdbc70000) libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f7fdba52000) libgmp.so.10 => /usr/lib/libgmp.so.10 (0x00007f7fdb7bf000) libm.so.6 => /usr/lib/libm.so.6 (0x00007f7fdb4ad000) libc.so.6 => /usr/lib/libc.so.6 (0x00007f7fdb107000) /lib64/ld-linux-x86-64.so.2 (0x00007f7fdc293000) 9 lines, to contrast with the binary in community/pandoc (136 lines, I'm not listing that here, plus half of them point to "not found"). Best luck with your compiling, João Miguel
