Cabal で Reactive をインストールしようとすると、次のようになります。
src/FRP/Reactive/PrimReactive.hs:79:8:
Could not find module `Control.Comonad'
It is a member of the hidden package `comonad-1.1.1.6'.
Perhaps you need to add `comonad' to the build-depends in your .cabal file.
It is a member of the hidden package `comonad-3.0.0.1'.
Perhaps you need to add `comonad' to the build-depends in your .cabal file.
thisによると、実際には comonad 依存関係はありません。ただし、PrimReactive.hsで Control.Comonad を使用します。