Tested on Ubuntu 18.04. NB: The following commands should not be run in a directory containing a stack.yaml file.
Install Stack from https://haskellstack.org
$ curl -sSL https://get.haskellstack.org/ | sh
Run
$ stack setup --resolver=lts-12.19
Install Clash
$ stack install --resolver=lts-12.19 clash-ghc-0.99.3
Verify
$ stack exec --resolver=lts-12.19 clashi