### Please describe the problem.
I have problems building with yesod 1.4

### What steps will reproduce the problem?
Building git annex in a clean sandbox. 

### What version of git-annex are you using? On what operating system?
5.20140927 on OS X i.e. Trying to upgrade the homebrew recipe to the most recent version of git-annex

### Please provide any additional information below.
Error messages below are discussed in the following SO-thread:
https://stackoverflow.com/questions/26225991/illegal-view-pattern-frompathpiece-just-dyn-abdd-when-using-parameters-on


[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log
[310 of 470] Compiling Assistant.WebApp.Types ( Assistant/WebApp/Types.hs, dist/dist-sandbox-52ca649e/build/git-annex/git-annex-tmp/Assistant/WebApp/Types.o )

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_aceZO
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_aceZW
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf02
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0c
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0e
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0f
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0h
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0j
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0l
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0n
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0p
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0r
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0u
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0w
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0y
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0z
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0C
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0D
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0F
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0H
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0J
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0L
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0M
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0O
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0R
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0T
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf0U
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf11
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf13
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf18
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1a
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1c
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1e
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1g
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1i
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1k
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1m
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1o
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1q
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1s
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1v
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1x
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1z
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1B
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1D
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1G
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1I
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1J
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1L
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1M
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1O
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1R
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1U
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1X
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf1Y
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf20
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf22
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf25
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf27
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf28
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf2b
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf2d
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf2f
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf2h
    Use ViewPatterns to enable view patterns

Assistant/WebApp/Types.hs:40:1:
    Illegal view pattern:  fromPathPiece -> Just dyn_acf2j
    Use ViewPatterns to enable view patterns
cabal: Error: some packages failed to install:
git-annex-5.20140927


# End of transcript or log.
"""]]