Hacky nix-instantiate with convenient pure evaluation mode

nix-instantiate-pure

#!/usr/bin/env bash

# This is emulating flake's pure evaluation mode
purePathExpr() {
  local arg=$1
  if [[ -f "$arg" ]]; then
    root=$(dirname "$arg")
  else
    root=$arg
  fi

  if gitdir=$(git -C "$root" rev-parse --show-toplevel 2>/dev/null); then
    root=$(nix-instantiate --eval --read-write-mode --expr "(fetchGit $gitdir).outPath" | tr -d \")
    subpath=$(realpath "$arg" --relative-to="$gitdir")
  else
    subpath=$(realpath "$arg" --relative-to="$root")
  fi

  sha256=$(nix-hash --type sha256 --base32 "$root")
  echo "import (
    builtins.path {
      path = $root;
      sha256 = \"$sha256\";
    }
    + \"/$subpath\"
  )"
}

system=$(nix-instantiate --eval --expr builtins.currentSystem | tr -d \")
args=("--pure-eval" "--argstr" "system" "$system")

# This is all just messy and incomplete argument parsing
# to turn all paths into --expr's with purePathExpr
hasSomething=
while [[ "$#" != 0 ]]; do
  arg=$1
  shift
  if [[ "$arg" == "--expr" || "$arg" == "-E" ]]; then
    hasSomething=1
    args+=("$arg" "$1")
    shift
  elif [[ "$arg" == "-A" ]]; then
    args+=("$arg" "$1")
    shift
  elif [[ ! "$arg" =~ ^- ]]; then
    hasSomething=1
    args+=("--expr" "$(purePathExpr "$arg")")
  else
    args+=("$arg")
  fi
done

if [[ -z "$hasSomething" ]]; then
  args+=("--expr" "$(purePathExpr "$PWD")")
fi


set -x
nix-instantiate --pure-eval "${args[@]}"