if [ -n "${VIRTHUALENV}" ]; then
    if [ "<VIRTHUALENV>" = "${VIRTHUALENV}" ]; then
        echo "${VIRTHUALENV_NAME} Virtual Haskell Environment is already active."
    else
        echo "There is already ${VIRTHUALENV_NAME} Virtual Haskell Environment activated."
        echo "Deactivate it first (using command 'deactivate'), to activate"
        echo "<VIRTHUALENV_NAME> environment."
    fi
    return 1
fi

export VIRTHUALENV="<VIRTHUALENV>"
export VIRTHUALENV_NAME="<VIRTHUALENV_NAME>"

echo "Activating ${VIRTHUALENV_NAME} Virtual Haskell Environment (at ${VIRTHUALENV})."
echo ""
echo "Use regular Haskell tools (ghc, ghci, ghc-pkg, cabal) to manage your Haskell environment."
echo ""
echo "To exit from this virtual environment, enter command 'deactivate'."

export "VIRTHUALENV_PATH_BACKUP"="${PATH}"
export "VIRTHUALENV_PS1_BACKUP"="${PS1}"

deactivate() {
    echo "Deactivating ${VIRTHUALENV_NAME} Virtual Haskell Environment (at ${VIRTHUALENV})."
    echo "Restoring previous environment settings."

    export "PATH"="${VIRTHUALENV_PATH_BACKUP}"
    unset -v "VIRTHUALENV_PATH_BACKUP"
    export "PS1"="${VIRTHUALENV_PS1_BACKUP}"
    unset -v "VIRTHUALENV_PS1_BACKUP"

    unset -v VIRTHUALENV
    unset -v VIRTHUALENV_NAME
    unset -v GHC_PACKAGE_PATH
    unset -f deactivate

    if [ -n "$BASH" -o -n "$ZSH_VERSION" ]; then
        hash -r
    fi
}

PATH_PREPENDIX="$(cat <VIRTHUALENV_DIR>/path_var_prependix)"
export PATH="${PATH_PREPENDIX}:${PATH}"
unset -v PATH_PREPENDIX

export GHC_PACKAGE_PATH="$(cat <VIRTHUALENV_DIR>/ghc_package_path_var)"
export PS1="(${VIRTHUALENV_NAME})${PS1}"

if [ -n "$BASH" -o -n "$ZSH_VERSION" ]; then
    hash -r
fi