Why can't I cabal install template-haskell-2.17.0.0?

Question

I am trying to install the specific version template-haskell-2.17.0.0. I run this command, but get an error during dependency resolution:

❯ cabal install template-haskell-2.17.0.0 --lib
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] next goal: template-haskell (user goal)
[__0] rejecting: template-haskell-2.18.0.0 (constraint from user target
requires ==2.17.0.0)
[__0] rejecting: template-haskell-2.17.0.0 (constraint from user target
requires ==2.16.0.0)
[__0] rejecting: template-haskell-2.16.0.0/installed-2.16.0.0,
template-haskell-2.16.0.0, template-haskell-2.15.0.0,
template-haskell-2.14.0.0, template-haskell-2.13.0.0,
template-haskell-2.12.0.0, template-haskell-2.11.1.0,
template-haskell-2.11.0.0, template-haskell-2.10.0.0,
template-haskell-2.9.0.0, template-haskell-2.8.0.0, template-haskell-2.7.0.0,
template-haskell-2.6.0.0, template-haskell-2.5.0.0, template-haskell-2.4.0.1,
template-haskell-2.4.0.0, template-haskell-2.3.0.1, template-haskell-2.3.0.0,
template-haskell-2.2.0.0 (constraint from user target requires ==2.17.0.0)
[__0] fail (backjumping, conflict set: template-haskell)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: template-haskell

I'm using the latest cabal version:

❯ cabal --version
cabal-install version 3.6.2.0
compiled using version 3.6.3.0 of the Cabal library

Answer 1

Some packages come with GHC and cannot be reinstalled. These are known as "boot" packages. The template-haskell package is one of these boot packages. You can find a complete list of them here: https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/libraries/version-history.

For your particular case, template-haskell-2.17.0.0 is part of GHC 9.0.2 (or 9.0.1). The only way to install that version of template-haskell is to use the appropriate version of GHC.

Based on the error output, I'm guessing that you're using some version of GHC 8.10.x, probably 8.10.7. You don't mention how you installed GHC, so I don't know exactly how to tell you to upgrade. If you used GHCup, then you should be able to upgrade GHC with this command:

ghcup install ghc 9.0.2 --set