On guix-devel¹, I had given an incomplete patch to update idris to
1.2.0. After further investigation, I concluded this was the wrong
approach because our current version of GHC is 8.0.2, so the right
version of LTS Haskell to follow is https://www.stackage.org/lts-9.4,
which includes Idris 1.0, not 1.2.

Big thank you to sclv on #hackage for all their help.

¹ https://lists.gnu.org/archive/html/guix-devel/2018-03/msg00100.html