[Nix-dev] haskell: overriding mtl/any builtin

[Peter Simons]

Hi Mateusz,

 > Notably if I specify mtl = mtl_2_2_1; then it complains that it needs
 > transformers == 0.4 but there seems to be no clues in nixpkgs as to how
 > to achieve this. Currently mtl = 2.2.1 for HEAD but I know it should
 > work with 7.8.3

transformers is a core library in GHC 7.8.3. We cannot override that
package, because it's shipped with the compiler and other core libraries
are linked against that version.

Best regards,
Peter