Hi,

thanks for the patch!  I applied it, tried it, and ran into some build
issues.  Maybe I missed something; here is (roughly) what I tried:

$ # apply diff to current master
$ git checkout a57c4eff6bbdcff79294fa15ecb95ab2b3c55bb4
$ git checkout origin/wip-haskell -- guix/build/haskell-build-system.scm
$ # bump ghc-attoparsec version
$ git diff gnu/packages/haskell-xyz.scm
diff --git a/gnu/packages/haskell-xyz.scm b/gnu/packages/haskell-xyz.scm
index db653f8c93..c4da66aa4a 100644
--- a/gnu/packages/haskell-xyz.scm
+++ b/gnu/packages/haskell-xyz.scm
@@ -589,7 +589,7 @@ (define-public ghc-atomic-write-0.2.0.7
 (define-public ghc-attoparsec
   (package
     (name "ghc-attoparsec")
-    (version "0.13.2.5")
+    (version "0.14.4")
     (source
      (origin
        (method url-fetch)
@@ -599,10 +599,12 @@ (define-public ghc-attoparsec
              ".tar.gz"))
        (sha256
         (base32
-         "0vv88m5m7ynjrg114psp4j4s69f1a5va3bvn293vymqrma7g7q11"))))
+         "0v4yjz4qi8bwhbyavqxlhsfb1iv07v10gxi64khmsmi4hvjpycrz"))))
     (build-system haskell-build-system)
     (arguments
-     `(#:phases
+     `(#:cabal-revision
+       ("2" "00jyrn2asz1kp698l3fyh19xxxz4npf1993y041x9b9cq239smn0")
+       #:phases
        (modify-phases %standard-phases
          (add-after 'unpack 'patch-for-newer-quickcheck
            (lambda _
$ # build ghc-attoparsec-iso8601 which depends on ghc-attoparsec
$ guix shell -D guix help2man git strace --pure # & bootstrap & ...
$ ./pre-inst-env guix build ghc-attoparsec-iso8601

After some time, this build failed with:

> phase `setup-compiler' succeeded after 0.1 seconds
> starting phase `configure'
> running "runhaskell Setup.hs" with command "configure" and parameters ("--prefix=/gnu/store/70ddgy9ayd4239dp37b7kvcsdxkg5nrj-ghc-attoparsec-iso8601-1.0.2.0" "--libdir=/gnu/store/70ddgy9ayd4239dp37b7kvcsdxkg5nrj-ghc-attoparsec-iso8601-1.0.2.0/lib" "--docdir=/gnu/store/70ddgy9ayd4239dp37b7kvcsdxkg5nrj-ghc-attoparsec-iso8601-1.0.2.0/share/doc/ghc-attoparsec-iso8601-1.0.2.0" "--libsubdir=$compiler/$pkg-$version" "--package-db=/tmp/guix-build-ghc-attoparsec-iso8601-1.0.2.0.drv-0/package.conf.d" "--global" "--enable-tests" "--enable-shared" "--enable-executable-dynamic" "--ghc-option=-fPIC" "--ghc-option=-optl=-Wl,-rpath=/gnu/store/70ddgy9ayd4239dp37b7kvcsdxkg5nrj-ghc-attoparsec-iso8601-1.0.2.0/lib/$compiler/$pkg-$version")
> Configuring attoparsec-iso8601-1.0.2.0...
> Error:
>     The following packages are broken because other packages they depend on are missing. These broken packages must be rebuilt before they can be used.
> installed package attoparsec-0.14.4 is broken due to missing package scientific-0.3.7.0-9XG3zUjXOw970JFcruv0cZ


Ignorant of the patch provided here, I tried to build newer versions of
ghc-attoparsec / its dependants and ended up with the attached patch.
Using this patch, the build succeeds (but I am not sure if the patch's
approach is sensible).