To reproduce, just guix environment --ad-hoc ghc -- ghci. firstname.lastname@example.org works. Seems like this may be related to some Prelude changes upstream (https://gitlab.haskell.org/ghc/ghc/issues/16563), but I'm not familiar enough with GHC internals to really tell what's going on. Interestingly, when I try to run plain old GHC on some source files with 8.6.5 I get another seemingly unrelated error:
Bad interface file: /gnu/store/8v1sn5ns7r5n02aip0b0ypyyzb2y1i1a-ghc-8.4.3/lib/ghc-8.4.3/base-18.104.22.168/Prelude.hi mismatched interface file versions (wanted "8065", got "8043") | 1 | import Test.Hspec (Spec, it, shouldBe) | ^ Again, I'm afraid I'm not entirely familiar with GHC internals and the function of interfaces files. Nevertheless, it seems not all is well in Guix-Haskell-land.