Haddock Install Failed

I tried to install pandoc the markdown utility plus alpha, but I couldn't install the dependent library haddock.

$ sudo port install pandoc
--->  Building haddock
Error: Target org.macports.build returned: shell command "cd /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_ports_devel_haddock/work/haddock-2.4.1 && ./Setup build -v" returned error 1
Command output: Creating dist/build/autogen (and its parents)
Preprocessing library haddock-2.4.1...
Preprocessing executables for haddock-2.4.1...
Building haddock-2.4.1...
Building library...
Creating dist/build (and its parents)
/opt/local/bin/ghc -package-name haddock-2.4.1 --make -hide-all-packages -no-user-package-conf -i -idist/build -isrc -idist/build/autogen -Idist/build/autogen -Idist/build -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package Cabal-1.6.0.3 -package array-0.2.0.0 -package base-4.1.0.0 -package containers-0.2.0.1 -package directory-1.0.0.3 -package filepath-1.1.0.2 -package ghc-6.10.2 -package ghc-paths-0.1.0.5 -package haskell98-1.0.1.0 -package pretty-1.0.1.0 -O -XCPP -XPatternGuards -XDeriveDataTypeable -XScopedTypeVariables -XMagicHash Distribution.Haddock Haddock.Types Haddock.InterfaceFile Haddock.Exception Haddock.Utils

src/Haddock/Utils.hs:1:13:
    Warning: -XPatternSignatures is deprecated: use -XScopedTypeVariables or pragma {-# LANGUAGE ScopedTypeVariables#-} instead
Linking...
/usr/bin/ar q dist/build/libHShaddock-2.4.1.a dist/build/Distribution/Haddock.o dist/build/Haddock/Types.o dist/build/Haddock/InterfaceFile.o dist/build/Haddock/Exception.o dist/build/Haddock/Utils.o
ar: creating archive dist/build/libHShaddock-2.4.1.a
/usr/bin/ld -x -r -o dist/build/HShaddock-2.4.1.o.tmp dist/build/Distribution/Haddock.o dist/build/Haddock/Types.o dist/build/Haddock/InterfaceFile.o dist/build/Haddock/Exception.o dist/build/Haddock/Utils.o
Building executable: haddock...
Creating dist/build/haddock (and its parents)
Creating dist/build/haddock/haddock-tmp (and its parents)
/opt/local/bin/ghc -o dist/build/haddock/haddock --make -hide-all-packages -no-user-package-conf -i -idist/build/haddock/haddock-tmp -isrc -idist/build/autogen -Idist/build/autogen -Idist/build/haddock/haddock-tmp -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build/haddock/haddock-tmp -hidir dist/build/haddock/haddock-tmp -stubdir dist/build/haddock/haddock-tmp -package Cabal-1.6.0.3 -package array-0.2.0.0 -package base-4.1.0.0 -package containers-0.2.0.1 -package directory-1.0.0.3 -package filepath-1.1.0.2 -package ghc-6.10.2 -package ghc-paths-0.1.0.5 -package haskell98-1.0.1.0 -package pretty-1.0.1.0 -O -funbox-strict-fields -O2 -XCPP -XPatternGuards -XDeriveDataTypeable -XScopedTypeVariables -XMagicHash src/Main.hs

src/Main.hs:1:39:
    Warning: -XPatternSignatures is deprecated: use -XScopedTypeVariables or pragma {-# LANGUAGE ScopedTypeVariables#-} instead

src/Haddock/Utils.hs:1:13:
    Warning: -XPatternSignatures is deprecated: use -XScopedTypeVariables or pragma {-# LANGUAGE ScopedTypeVariables#-} instead
[15 of 24] Compiling Haddock.Interface.AttachInstances ( src/Haddock/Interface/AttachInstances.hs, dist/build/haddock/haddock-tmp/Haddock/Interface/AttachInstances.o )

src/Haddock/Interface/AttachInstances.hs:94:17:
    Ambiguous occurrence `funTyConName'
    It could refer to either `Haddock.Interface.AttachInstances.funTyConName', defined at src/Haddock/Interface/AttachInstances.hs:108:0
                          or `TypeRep.funTyConName', imported from TypeRep at src/Haddock/Interface/AttachInstances.hs:26:0-13

Error: The following dependencies failed to build: haddock
Error: Status 1 encountered during processing.

It is summarized as follows:

src/Haddock/Interface/AttachInstances.hs:94:17:
    Ambiguous occurrence `funTyConName'
    It could refer to either `Haddock.Interface.AttachInstances.funTyConName', defined at src/Haddock/Interface/AttachInstances.hs:108:0
                          or `TypeRep.funTyConName', imported from TypeRep at src/Haddock/Interface/AttachInstances.hs:26:0-13

I don't understand why Haddock developpers have made this mistake.