Guide users in filtering their source so that evaluations don’t happen too often.

[Profpatsch]

If you do a naive src = ./. somewhere, nix will have to re-import the whole directory into the store each time any file in ./. changes, which usually includes things like ./.git or ./dist or ./target, meaning lorri will start many useless evaluations.

See #6 as an example.

You can use builtins.filterSource and the functions in nixpkgs/lib/sources.nix to filter which files nix imports (and thus which files lorri watches), but they are extremely unintuitive and notoriously hard to use and debug.
pkgs/build-support/nix-gitignore or https://github.com/Profpatsch/nixperiments/blob/master/filterSourceGitignore.nix are another practical approach that uses the projects .gitignore files.

So we basically need three (four) things:

• nicely usable wrappers around builtins.filterSource (Improve sources.nix in nixpkgs to make it easier to ignore stuff #84)
• documentation about how to use them (Improve sources.nix in nixpkgs to make it easier to ignore stuff #84)
• user-friendly messages in lorri to prevent common mistakes (Display warnings if certain well-known directories (e.g. .git) are imported  #85)
• (default ignores in lorri, maybe)

Related: #49