Hello,
The issue was brought up on this list [1] that the released versions of GHC 7.8 do not work on El Capitan due to an older `unix` package. In a subsequent GHC weekly news [2], it was announced that since El Capitan wasn't yet released, it wasn't worthwhile to put more developer time into 7.8.
Now that El Capitan is out, and the existing 7.8 binaries continue to not work, I'd like to reopen this issue and request a point release with an updated `unix` package.
Thank you,
Adam Foltzer
Research Engineer
Galois, Inc.