Various Haskell things that I can never remember

Making Cabal fetch a dependency from a version control repository

Upstream documentation here.

TLDR: put this in cabal.project:

source-repository-package
    type: git
    location: https://github.com/hvr/HsYAML.git
    tag: e70cf0c171c9a586b62b3f75d72f1591e4e6aaa1

The tag is the Git commit.

Making Cabal fetch a dependency from a directory

Add another packages stanza to cabal.project. Example:

packages: ../futhark-server-haskell

Ignoring upper bounds

Put this in cabal.project:

allow-newer: hashable:ghc-bignum

This lets hashable use a newer ghc-bignum.

Using packages from a different version of GHC in Nix

Use e.g. pkgs.haskell.packages.ghc921.ormolu_0_4_0_0 where pkgs is the Nixpkgs package set. You don't need the version number on the package itself; but if you're using a nonstandard compiler version, you're probably trying to use a newer version of the package than is the default in Nixpkgs.