diff --git a/.envrc b/.envrc
index 1d953f4..2bfa87c 100644
--- a/.envrc
+++ b/.envrc
@@ -1 +1,153 @@
-use nix
+# Usage: use_nix [...]
+#
+# Load environment variables from `nix-shell`.
+# If you have a `default.nix` or `shell.nix` one of these will be used and
+# the derived environment will be stored at ./.direnv/env-<hash>
+# and symlink to it will be created at ./.direnv/default.
+# Dependencies are added to the GC roots, such that the environment remains persistent.
+#
+# The resulting environment is cached for better performance.
+#
+# To trigger switch to a different environment:
+# `rm -f .direnv/default`
+#
+# To derive a new environment:
+# `rm -rf .direnv/env-$(md5sum {shell,default}.nix 2> /dev/null | cut -c -32)`
+#
+# To remove cache:
+# `rm -f .direnv/dump-*`
+#
+# To remove all environments:
+# `rm -rf .direnv/env-*`
+#
+# To remove only old environments:
+# `find .direnv -name 'env-*' -and -not -name `readlink .direnv/default` -exec rm -rf {} +`
+#
+
+set -eo pipefail
+
+use_nix() {
+    # define all local variables
+    local shell f env_hash dir default wd drv dump path_backup
+    local files_to_watch=()
+
+        declare opt
+        declare OPTARG
+        declare OPTIND
+
+    while getopts ":s:w:" opt; do
+        case "${opt}" in
+            s)
+                shell="${OPTARG}"
+                files_to_watch=("${files_to_watch[@]}" "${shell}")
+                ;;
+            w)
+                files_to_watch=("${files_to_watch[@]}" "${OPTARG}")
+                ;;
+            :)
+                >&2 echo "Invalid option: $OPTARG requires an argument"
+                ;;
+            \?)
+                >&2 echo "Invalid option: $OPTARG"
+                exit 1
+                ;;
+        esac
+    done
+    shift $((OPTIND -1))
+
+    if [[ -z "${shell}" ]]; then
+        >&2 echo "ERR: no shell was given"
+        exit 1
+    fi
+
+    for f in "${files_to_watch[@]}"; do
+        if ! [[ -f "${f}" ]]; then
+            >&2 echo "cannot watch file ${f} because it does not exist"
+            exit 1
+        fi
+    done
+
+    # compute the hash of all the files that makes up the development environment
+    env_hash="$(hashContents "${files_to_watch[@]}")"
+
+    dir="$(direnv_layout_dir)"
+    default="${dir}/default"
+    if [[ ! -L "${default}" ]] || [[ ! -d $(readlink "${default}") ]]; then
+        wd="${dir}/env-${env_hash}"
+        mkdir -p "${wd}"
+
+        drv="${wd}/env.drv"
+        if [[ ! -f "${drv}" ]]; then
+            log_status "use nix: deriving new environment"
+            IN_NIX_SHELL=1 nix-instantiate --add-root "${drv}" --indirect "${shell}" > /dev/null
+            nix-store -r $(nix-store --query --references "${drv}") --add-root "${wd}/dep" --indirect > /dev/null
+        fi
+
+        rm -f "${default}"
+        ln -s $(basename "${wd}") "${default}"
+    fi
+
+    drv=$(readlink "${default}/env.drv")
+    dump="${dir}/dump-$(hashFile ".envrc")-$(hashFile ${drv})"
+
+    if [[ ! -f "${dump}" ]] || [[ "${XDG_CONFIG_DIR}/direnv/direnvrc" -nt "${dump}" ]]; then
+        log_status "use nix: updating cache"
+
+        old=$(find "${dir}" -name 'dump-*')
+        nix-shell --pure "${drv}" --show-trace --run "$(join_args "$direnv" dump bash)" > "${dump}"
+        rm -f ${old}
+    fi
+
+    # evaluate the dump created by nix-shell earlier, but have to merge the PATH
+    # with the current PATH
+    # NOTE: we eval the dump here as opposed to direnv_load it because we don't
+    # want to persist environment variables coming from the shell at the time of
+    # the dump. See https://github.com/direnv/direnv/issues/405 for context.
+    path_backup="${PATH}"
+    eval $(cat "${dump}")
+    export PATH="${PATH}:${path_backup}"
+
+    for f in "${files_to_watch[@]}"; do
+        watch_file "${f}"
+    done
+}
+
+hashContents() {
+    if has md5sum; then
+        cat "${@}" | md5sum | cut -c -32
+    elif has md5; then
+        cat "${@}" | md5 -q
+    fi
+}
+
+hashFile() {
+    if has md5sum; then
+        md5sum "${@}" | cut -c -32
+    elif has md5; then
+        md5 -q "${@}"
+    fi
+}
+
+fail() {
+    log_error "${@}"
+    exit 1
+}
+
+validateVersion() {
+    local version="$("${direnv}" version)"
+    local major="$(echo "${version}" | cut -d. -f1)"
+    local minor="$(echo "${version}" | cut -d. -f2)"
+    local patch="$(echo "${version}" | cut -d. -f3)"
+
+    if [[ "${major}" -gt 2 ]]; then return 0; fi
+    if [[ "${major}" -eq 2 ]] && [[ "${minor}" -gt 18 ]]; then return 0; fi
+    if [[ "${major}" -eq 2 ]] && [[ "${minor}" -eq 18 ]] && [[ "${patch}" -ge 2 ]]; then return 0; fi
+    return 1
+}
+
+if ! validateVersion; then
+    echo "This .envrc requires direnv version 2.18.2 or above."
+    exit 1
+fi
+
+use_nix -s shell.nix
diff --git a/.gitignore b/.gitignore
index 120c4f5..f5d01ab 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 _*
 src/archive.org
+.direnv/