cabal install --- global constraint requires installed instance
cabal, cabal-install, haskell
Solution
You should have a `cabal.config` file in your current directory. Open it with any text editor and serach for the `transformers installed` string.
Once you have found it, replace it with `transformers ==0.4.3.0` or any other version you like.
Problem
My cabal dependencies are listed as ``` base ==4.6.*, containers ==0.5.*, bytestring ==0.10.*, binary ==0.5.*, parsec ==3.1.* ``` i did, ``` cabal sandbox init wget http://www.stackage.org/lts/cabal.config cabal install --only-dependencies ``` To overcome dependency conflicts, i am using sandbox installation. But still, cabal is talking about some global constraints which is confusing for me. Cabal dependency tree resolution gets screwed as, ``` trying: parsec-3.1.8 (dependency of myproj-0.1.0.0) next goal: mtl (dependency of parsec-3.1.8) rejecting: mtl-2.2.1, 2.2.0.1, 2.2 (global constraint requires ==2.1.3.1) trying: mtl-2.1.3.1 next goal: transformers (dependency of mtl-2.1.3.1) rejecting: transformers-0.4.2.0, 0.4.1.0, 0.3.0.0, 0.2.2.1, 0.2.2.0, 0.2.1.0, 0.2.0.0, 0.1.4.0, 0.1.3.0, 0.1.1.0, 0.1.0.1, 0.1.0.0, 0.0.1.0, 0.0.0.0, 0.4.0.0 (global constraint requires installed instance) ```