diff --git a/.envrc b/.envrc
new file mode 100644
index 00000000..1e71ffbc
--- /dev/null
+++ b/.envrc
@@ -0,0 +1,4 @@
+# shellcheck shell=bash
+# For use with direnv.
+# Installing nix-direnv will ensure a smoother experience.
+use flake
\ No newline at end of file
diff --git a/.nix/catala.nix b/.nix/catala.nix
deleted file mode 100644
index 06869e25..00000000
--- a/.nix/catala.nix
+++ /dev/null
@@ -1,81 +0,0 @@
-{ lib
-, alcotest
-, ansiterminal
-, benchmark
-, bindlib
-, buildDunePackage
-, cmdliner
-, cppo
-, dates_calc
-, js_of_ocaml
-, js_of_ocaml-ppx
-, menhir
-, menhirLib
-, ocamlgraph
-, pkgs
-, ppx_deriving
-, ppx_yojson_conv
-, re
-, sedlex
-, ubase
-, unionfind
-, visitors
-, z3
-, zarith
-, zarith_stubs_js
-, ocaml-crunch
-, cohttp-lwt-unix
-, ocolor
-, dune-build-info
-}:
-
-buildDunePackage {
-  pname = "catala";
-  version = "0.8.0"; # TODO parse `catala.opam` with opam2json
-
-  minimumOCamlVersion = "4.11";
-
-  src = ../.;
-
-  duneVersion = "3";
-
-  nativeBuildInputs = [ cppo menhir ocaml-crunch ];
-
-  propagatedBuildInputs = [
-    alcotest
-    ansiterminal
-    benchmark
-    bindlib
-    cmdliner
-    dates_calc
-    js_of_ocaml
-    js_of_ocaml-ppx
-    menhirLib
-    ocamlgraph
-    pkgs.z3
-    ppx_deriving
-    ppx_yojson_conv
-    re
-    sedlex
-    ubase
-    unionfind
-    visitors
-    z3
-    zarith
-    zarith_stubs_js
-    cohttp-lwt-unix
-    ocolor
-    dune-build-info
-  ];
-
-  # Currently there is no unit tests in catala and Cram tests are handled by clerk
-  doCheck = false;
-
-  meta = with lib; {
-    homepage = "https://catala-lang.org";
-    description =
-      "Catala is a domain-specific programming language designed for deriving correct-by-construction implementations from legislative texts.";
-    license = licenses.asl20;
-    maintainers = with maintainers; [ ];
-  };
-}
diff --git a/.nix/clerk.nix b/.nix/clerk.nix
deleted file mode 100644
index 7d269b78..00000000
--- a/.nix/clerk.nix
+++ /dev/null
@@ -1,45 +0,0 @@
-{ lib
-, buildDunePackage
-, odoc
-, re
-, ansiterminal
-, cmdliner
-, ninja_utils
-, alcotest
-, catala
-, ninja
-, colordiff
-}:
-
-buildDunePackage {
-  pname = "clerk";
-  version = "0.8.0"; # TODO parse `catala.opam` with opam2json
-
-  minimumOCamlVersion = "4.11";
-
-  src = ../.;
-
-  duneVersion = "3";
-
-  propagatedBuildInputs = [
-    odoc
-    re
-    ansiterminal
-    cmdliner
-    ninja_utils
-    alcotest
-    catala
-  ];
-
-  # todo: the current colordiff in nixpkgs always prints the banner. This make the logs totally unreadable.
-  nativeBuildInputs = [ catala ninja colordiff ];
-  doCheck = true;
-
-  meta = with lib; {
-    homepage = "https://github.com/CatalaLang/catala";
-    description =
-      "Build system for Catala, a specification language for tax and social benefits computation rules";
-    license = licenses.asl20;
-    maintainers = with maintainers; [ ];
-  };
-}
diff --git a/.nix/dates_calc.nix b/.nix/dates_calc.nix
deleted file mode 100644
index d32e7c7b..00000000
--- a/.nix/dates_calc.nix
+++ /dev/null
@@ -1,16 +0,0 @@
-{ lib, fetchurl, buildDunePackage }:
-
-buildDunePackage rec {
-  pname = "dates_calc";
-  version = "0.0.4";
-
-  minimumOCamlVersion = "4.11.0";
-
-  duneVersion = "2";
-
-  src = fetchurl
-    {
-      url = "https://github.com/CatalaLang/dates-calc/archive/${version}.tar.gz";
-      sha256 = "sha256-lWhNBK50r3qhY3PXRHZVvqDee6Y4+3vC51ZfjLxGryg=";
-    };
-}
diff --git a/.nix/french_law.nix b/.nix/french_law.nix
deleted file mode 100644
index ff5c73e0..00000000
--- a/.nix/french_law.nix
+++ /dev/null
@@ -1,32 +0,0 @@
-{ lib
-, buildDunePackage
-, odoc
-, catala
-}:
-
-buildDunePackage {
-  pname = "french_law";
-  version = "0.8.0"; # TODO parse `catala.opam` with opam2json
-
-  minimumOCamlVersion = "4.11";
-
-  src = ../.;
-
-  duneVersion = "3";
-
-  propagatedBuildInputs = [
-    odoc
-    catala
-  ];
-
-  nativeBuildInputs = [ catala ];
-  doCheck = true;
-
-  meta = with lib; {
-    homepage = "https://github.com/CatalaLang/catala";
-    description =
-      "Build system for Catala, a specification language for tax and social benefits computation rules";
-    license = licenses.asl20;
-    maintainers = with maintainers; [ ];
-  };
-}
diff --git a/.nix/ninja_utils.nix b/.nix/ninja_utils.nix
deleted file mode 100644
index 36262477..00000000
--- a/.nix/ninja_utils.nix
+++ /dev/null
@@ -1,30 +0,0 @@
-{ lib
-, buildDunePackage
-, odoc
-, re
-}:
-
-buildDunePackage rec {
-  pname = "ninja_utils";
-  version = "0.8.0"; # TODO parse `catala.opam` with opam2json
-
-  minimumOCamlVersion = "4.11";
-
-  src = ../.;
-
-  duneVersion = "3";
-
-  propagatedBuildInputs = [
-    odoc
-    re
-  ];
-  doCheck = true;
-
-  meta = with lib; {
-    homepage = "https://catala-lang.org";
-    description =
-      "A collection of utility functions used to generate Ninja build files";
-    license = licenses.asl20;
-    maintainers = with maintainers; [ ];
-  };
-}
diff --git a/.nix/ocolor.nix b/.nix/ocolor.nix
deleted file mode 100644
index 4157841d..00000000
--- a/.nix/ocolor.nix
+++ /dev/null
@@ -1,39 +0,0 @@
-{ lib
-, buildDunePackage
-, odoc
-, cppo
-, ounit
-, fetchurl
-}:
-
-buildDunePackage rec {
-  pname = "ocolor";
-  version = "1.3.0";
-  
-  minimumOCamlVersion = "4.11";
-
-  src = fetchurl
-    {
-      url = "https://github.com/marc-chevalier/${pname}/archive/${version}.tar.gz";
-      sha256 = "sha256-V7xuBRNbWWi3OpDRe7WakOO4Vz6nyoV8/gyn45fpmYA=";
-    };
-
-  duneVersion = "3";
-
-  propagatedBuildInputs = [
-    odoc
-    ounit
-    cppo
-  ];
-
-  nativeBuildInputs = [ cppo ];
-  doCheck = true;
-
-  meta = with lib; {
-    homepage = "https://github.com/marc-chevalier/ocolor";
-    description =
-      "OColor is an OCaml library which help to format nicely using ANSI escape codes.";
-    license = licenses.mit;
-    maintainers = [ ];
-  };
-}
diff --git a/.nix/packages.nix b/.nix/packages.nix
deleted file mode 100644
index 26c25e75..00000000
--- a/.nix/packages.nix
+++ /dev/null
@@ -1,15 +0,0 @@
-{ ocamlPackages, fetchurl }:
-
-ocamlPackages.overrideScope' (self: super: {
-  alcotest = (super.alcotest.override {}).overrideAttrs (_: {
-    doCheck = false;
-  });
-  catala = self.callPackage ./catala.nix { };
-  unionfind = self.callPackage ./unionfind.nix { };
-  ninja_utils = self.callPackage ./ninja_utils.nix { };
-  clerk = self.callPackage ./clerk.nix { };
-  ubase = self.callPackage ./ubase.nix { };
-  dates_calc = self.callPackage ./dates_calc.nix { };
-  french_law = self.callPackage ./french_law.nix { };
-  ocolor = self.callPackage ./ocolor.nix { };
-})
diff --git a/.nix/ubase.nix b/.nix/ubase.nix
deleted file mode 100644
index e2f6acbd..00000000
--- a/.nix/ubase.nix
+++ /dev/null
@@ -1,20 +0,0 @@
-{ lib, fetchurl, buildDunePackage, uutf }:
-
-buildDunePackage rec {
-  pname = "ubase";
-  version = "0.05";
-
-  minimumOCamlVersion = "4.05.0";
-
-  useDune2 = true;
-
-  propagatedBuildInputs = [
-    uutf
-  ];
-
-  src = fetchurl
-    {
-      url = "https://github.com/sanette/${pname}/archive/${version}.tar.gz";
-      sha256 = "sha256-D7/aCobZDS9/e5hLxd6pO9MJ4xXaSTACUXeQU4j5u0E=";
-    };
-}
diff --git a/.nix/unionfind.nix b/.nix/unionfind.nix
deleted file mode 100644
index cb26f67e..00000000
--- a/.nix/unionfind.nix
+++ /dev/null
@@ -1,23 +0,0 @@
-{ lib, fetchurl, buildDunePackage }:
-
-buildDunePackage rec {
-  pname = "unionFind";
-  version = "20220122";
-
-  minimumOCamlVersion = "4.0.8";
-
-  useDune2 = true;
-
-  src = fetchurl {
-    url =
-      "https://gitlab.inria.fr/fpottier/${pname}/-/archive/${version}/archive.tar.gz";
-    hash = "sha256-85+5KNYKXsNAH568qR8/AFC9UDviLJEO/Fztc9cRHZA=";
-  };
-
-  meta = with lib; {
-    homepage = "https://gitlab.inria.fr/fpottier/unionFind/";
-    description = "Implementations of the union-find data structure";
-    license = licenses.gpl2;
-    maintainers = [ ];
-  };
-}
diff --git a/flake.lock b/flake.lock
index c6adec9f..79a73a9b 100644
--- a/flake.lock
+++ b/flake.lock
@@ -1,5 +1,21 @@
 {
   "nodes": {
+    "flake-compat": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1627913399,
+        "narHash": "sha256-hY8g6H2KFL8ownSiFeMOjwPC8P0ueXpCVEbxgda3pko=",
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "rev": "12c64ca55c1014cdc1b16ed5a804aa8576601ff2",
+        "type": "github"
+      },
+      "original": {
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
     "flake-utils": {
       "inputs": {
         "systems": "systems"
@@ -18,26 +34,141 @@
         "type": "github"
       }
     },
-    "nixpkgs": {
+    "flake-utils_2": {
       "locked": {
-        "lastModified": 1690640159,
-        "narHash": "sha256-5DZUYnkeMOsVb/eqPYb9zns5YsnQXRJRC8Xx/nPMcno=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "e6ab46982debeab9831236869539a507f670a129",
+        "lastModified": 1638122382,
+        "narHash": "sha256-sQzZzAbvKEqN9s0bzWuYmRaA03v40gaJ4+iL1LXjaeI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "74f7e4319258e287b0f9cb95426c9853b282730b",
         "type": "github"
       },
       "original": {
-        "owner": "NixOS",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "mirage-opam-overlays": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1661959605,
+        "narHash": "sha256-CPTuhYML3F4J58flfp3ZbMNhkRkVFKmBEYBZY5tnQwA=",
+        "owner": "dune-universe",
+        "repo": "mirage-opam-overlays",
+        "rev": "05f1c1823d891ce4d8adab91f5db3ac51d86dc0b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "dune-universe",
+        "repo": "mirage-opam-overlays",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1682362401,
+        "narHash": "sha256-/UMUHtF2CyYNl4b60Z2y4wwTTdIWGKhj9H301EDcT9M=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "884ac294018409e0d1adc0cae185439a44bd6b0b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
         "ref": "nixos-unstable",
         "repo": "nixpkgs",
         "type": "github"
       }
     },
+    "opam-nix": {
+      "inputs": {
+        "flake-compat": "flake-compat",
+        "flake-utils": "flake-utils_2",
+        "mirage-opam-overlays": "mirage-opam-overlays",
+        "nixpkgs": "nixpkgs",
+        "opam-overlays": "opam-overlays",
+        "opam-repository": [
+          "opam-repository"
+        ],
+        "opam2json": "opam2json"
+      },
+      "locked": {
+        "lastModified": 1716292162,
+        "narHash": "sha256-UOJNCbqvxABD56JZtZkv3C9ufdqrs7/Ep4AKkCHgPuo=",
+        "owner": "tweag",
+        "repo": "opam-nix",
+        "rev": "1d3cbd6d3f247db77cb581c88c9a1d72e4acad60",
+        "type": "github"
+      },
+      "original": {
+        "owner": "tweag",
+        "repo": "opam-nix",
+        "type": "github"
+      }
+    },
+    "opam-overlays": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1654162756,
+        "narHash": "sha256-RV68fUK+O3zTx61iiHIoS0LvIk0E4voMp+0SwRg6G6c=",
+        "owner": "dune-universe",
+        "repo": "opam-overlays",
+        "rev": "c8f6ef0fc5272f254df4a971a47de7848cc1c8a4",
+        "type": "github"
+      },
+      "original": {
+        "owner": "dune-universe",
+        "repo": "opam-overlays",
+        "type": "github"
+      }
+    },
+    "opam-repository": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1722612394,
+        "narHash": "sha256-djcSMr+mKRyTAm2QEICJBmqRT4ftsy4th4/OR1HWGjM=",
+        "owner": "ocaml",
+        "repo": "opam-repository",
+        "rev": "fe8dffac61bbab376897c0f4e7cc5b7b8c4eecc1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "ocaml",
+        "repo": "opam-repository",
+        "type": "github"
+      }
+    },
+    "opam2json": {
+      "inputs": {
+        "nixpkgs": [
+          "opam-nix",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1671540003,
+        "narHash": "sha256-5pXfbUfpVABtKbii6aaI2EdAZTjHJ2QntEf0QD2O5AM=",
+        "owner": "tweag",
+        "repo": "opam2json",
+        "rev": "819d291ea95e271b0e6027679de6abb4d4f7f680",
+        "type": "github"
+      },
+      "original": {
+        "owner": "tweag",
+        "repo": "opam2json",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
         "flake-utils": "flake-utils",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": [
+          "opam-nix",
+          "nixpkgs"
+        ],
+        "opam-nix": "opam-nix",
+        "opam-repository": "opam-repository"
       }
     },
     "systems": {
diff --git a/flake.nix b/flake.nix
index 300a3be2..ca2e7af5 100644
--- a/flake.nix
+++ b/flake.nix
@@ -1,44 +1,63 @@
 {
   inputs = {
+    opam-repository = {
+      url = "github:ocaml/opam-repository";
+      flake = false;
+    };
+    opam-nix.url = "github:tweag/opam-nix";
+    opam-nix.inputs.opam-repository.follows = "opam-repository";
     flake-utils.url = "github:numtide/flake-utils";
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    nixpkgs.follows = "opam-nix/nixpkgs";
   };
-
-  outputs = {nixpkgs, flake-utils, ...}:
-    let
-      systems = nixpkgs.lib.systems.flakeExposed;
-    in flake-utils.lib.eachSystem systems (system:
+  outputs = { self, flake-utils, opam-nix, nixpkgs, ... }@inputs:
+    flake-utils.lib.eachDefaultSystem (system:
       let
-        pkgs = import nixpkgs { inherit system; };
-        ocamlPackages = pkgs.callPackage ./.nix/packages.nix {};
-      in
-        rec {
-          packages = {
-            catala = ocamlPackages.catala;
-            clerk = ocamlPackages.clerk;
-            french_law = ocamlPackages.french_law;
+        pkgs = nixpkgs.legacyPackages.${system};
+        on = opam-nix.lib.${system};
+        localPackagesQuery = builtins.mapAttrs (_: pkgs.lib.last)
+          (on.listRepo (on.makeOpamRepo ./.));
+        devPackagesQuery = {
+          # You can add "development" packages here. They will get added to the devShell automatically.
+          ocaml-lsp-server = "*";
+          ocamlformat = "*";
+        };
+        query = devPackagesQuery // {
+          ## You can force versions of certain packages here, e.g:
+          ## - force the ocaml compiler to be taken from opam-repository:
+          # ocaml-base-compiler = "*";
+          ## - or force the compiler to be taken from nixpkgs and be a certain version:
+          # ocaml-system = "4.14.0";
+          ## - or force ocamlfind to be a certain version:
+          # ocamlfind = "1.9.2";
+        };
+        scope = on.buildOpamProject' { } ./. query;
+        overlay = final: prev:
+          {
+            conf-ninja = prev.conf-ninja.overrideAttrs (old: {
+              nativeBuildInputs = old.nativeBuildInputs ++ [ pkgs.ninja ];
+            });
+            # You can add overrides here
           };
-          defaultPackage = packages.clerk;
-          devShell = pkgs.mkShell {
-            inputsFrom = [ packages.clerk packages.catala ];
-            buildInputs = [
-              pkgs.inotify-tools
-              ocamlPackages.merlin
-              pkgs.ocamlformat_0_26_0
-              ocamlPackages.ocp-indent
-              ocamlPackages.utop
-              ocamlPackages.odoc
-              ocamlPackages.ocaml-lsp
-              pkgs.groff
-              pkgs.obelisk
-              pkgs.ninja
-              pkgs.colordiff
-              pkgs.pandoc
-              pkgs.python3.pkgs.pygments
-              pkgs.nodejs
-              pkgs.nodePackages.npm
-            ];
-          };
-        }
-    );
+        scope' = scope.overrideScope' overlay;
+        # Packages from devPackagesQuery
+        devPackages = builtins.attrValues
+          (pkgs.lib.getAttrs (builtins.attrNames devPackagesQuery) scope');
+        # Packages in this workspace
+        packages =
+          pkgs.lib.getAttrs (builtins.attrNames localPackagesQuery) scope';
+      in {
+        legacyPackages = scope';
+
+        #inherit packages;
+
+        ## If you want to have a "default" package which will be built with just `nix build`, do this instead of `inherit packages;`:
+        packages = packages // { default = packages.catala; };
+
+        devShells.default = pkgs.mkShell {
+          inputsFrom = builtins.attrValues packages;
+          buildInputs = devPackages ++ [
+            # You can add packages from nixpkgs here
+          ];
+        };
+      });
 }