I get the following error message when using the newest GHC HEAD:
Illegal instance declaration for ‛BufferType String’
(All instance types must be of the form (T a1 ... an)
where a1 ... an are *distinct type variables*,
and each type variable appears at most once in the instance head.
Use -XFlexibleInstances if you want to disable this.)
In the instance declaration for ‛BufferType String’
Failed to install HTTP-4000.0.7
4000.2.8 compiles fine for me with GHC 7.7.20130720.
base needs a version bump now as of 7.7.20130828
I opened a pull request for this in issue 46.
Same error with The Glorious Glasgow Haskell Compilation System, version 7.7.20130904 for 4000.2.8
Allow newer versions of dependencies
I have the same issue, cabal versioning is forcing an attempt to compile 4000.0.7.
As far as I understand this is fixed or not reproducible in the latest version.
If there's a problem with cabal selecting an old version I'm not sure if there's anything much the HTTP package can do about it.