ocaml - How to use nix-shell properly and avoid "dumping very large path"? -

based on readings (notably the wiki , this blog post), have come following default.nix load nix-shell:

with import <nixpkgs> {};  let emacs =   emacswithpackages (p : [ p.tuareg ]); in  stdenv.mkderivation rec {     name = "env";      src = ./.;      # customizable development requirements     buildinputs = [         pkgconfig         ocaml         ocamlpackages.merlin         ocamlpackages.findlib         ocamlpackages.lablgtk         ocamlpackages.camlp5_transitional         ncurses         emacs     ];      # customizable development shell setup     shellhook = ''         export path=`pwd`/bin:$path     ''; } 

but prints warning:

warning: dumping large path (> 256 mib); may run out of memory 

and takes quite long load (about 45 seconds first time call nix-shell after start-up, 2 seconds on subsequent calls).

what meaning of message? when on google, find few github issues not expressed in way easy understand layman.

can speed load , remove message? seems me i'm doing wrong.

are there general recommendations on writing kind of development environment might not aware of?

probably src attribute (the current directory) big. nix-shell copy nix store on each invocation, not want/need. workaround write:

src = if lib.innixshell null else nix; 

(where lib comes nixpkgs).

this way, ./. copied when invoke nix-build, not when run nix-shell.