cabal file https imports should be cached

[geekosaur]

Describe the feature request
It seems that cabal always re-downloads import files, which is silly for e.g. Stackage (the suggested use in the manual) because snapshots are immutable. For the cases where they aren't, an If-Modified-Since header would avoid doing a full download when the upstream is unchanged.

[chreekat]

As a design constraint, I would suggest that if cabal does cache external resources, it needs to expose a way to bust the cache.

For instance, Nix has a --refresh option to some commands, like nix run --refresh <target>, that forces a redownload of the target when it's a remote resource.

[ulysses4ever]

cabal file https imports

Do you mean project file?

[geekosaur]

Probably. (Still discombobulated here)