Courtesy of Brian McKenna a zsh script to show the sandbox status of the current directory in our shell -- http://dev.stephendiehl.com/hask/

cabal-sandbox.sh

# a zsh script to show the sandbox status of the current directory in our shell.

function cabal_sandbox_info() {
    cabal_files=(*.cabal(N))
    if [ $#cabal_files -gt 0 ]; then
        if [ -f cabal.sandbox.config ]; then
            echo "%{$fg[green]%}sandboxed%{$reset_color%}"
        else
            echo "%{$fg[red]%}not sandboxed%{$reset_color%}"
        fi
    fi
}
 
RPROMPT="\$(cabal_sandbox_info) $RPROMPT"