cabal sandbox tips

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In case you missed it, starting from version 1.18 cabal-install has awesome sandboxing capabilities. Here I share a couple of tricks that will make your sandboxing experience even better.

Location-independent sandboxes

By default, cabal uses sandbox only in the directory where cabal.sandbox.config is present. This is inconvenient when sharing a sandbox among multiple projects, and in general makes it somewhat fragile.

With cabal 1.19 (i.e. cabal HEAD as of now) you can set the CABAL_SANDBOX_CONFIG environment variable to the path to your cabal.sandbox.config, and the corresponding sandbox will be used regardless of your current directory.

I’ve defined convenience functions for myself such as

for every sandbox I commonly use.

Notice how I also set the sandbox_name variable to the human-readable name of the sandbox. It can be displayed in the prompt as follows:

(sandbox name in the prompt idea is due to /u/cameleon)

Sandbox-aware ghc

Sandboxes only affect cabal, but not ghc or ghci when those are invoked directly. At some point in the future we’ll be able to write

% cabal exec ghc ...

For now I’ve defined the following sandbox-aware wrappers for ghc and ghci:

Clone the repo somewhere

% git clone ghc_sandbox

and include in your .bashrc or .zshrc

. ~/path/to/ghc_sandbox/

(Why am I wrapping ghci instead of using cabal repl? cabal repl has some side-effects, like re-installing packages, that are not always desirable. And ghci is much faster to start, too.)