convert markdown string into html file

default.nix

with import <nixpkgs> {};

{ string ? "# hello"}:
let
 input = writeTextFile {
   name = "index.md";
   text = string;
 };
in
stdenv.mkDerivation {
  name = "simple-md";
  src = input;

  unpackPhase = "true";
  installPhase = ''
    mkdir $out
    cp $src $out/index.md
    cp $src $out/index.html
  '';
}