provide nix flake overlay

flake.nix

@@ -0,0 +1,116 @@
+{
+  description = "concat";
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-21.11";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachSystem flake-utils.lib.allSystems (system:
+      let
+        haskellLib = (import nixpkgs { inherit system; }).haskell.lib;
+
+        excludedPackages = [ "concat-hardware" ];
+        noHaddockPackages =
+          [ "concat-examples" "concat-inline" "concat-plugin" ];
+        # need display, graphviz for testing. disable test for now.
+        noCheckPackages = [ "concat-graphics" "concat-plugin" ];
+
+        parseCabalProject = import ./parse-cabal-project.nix;
+        concatPackages = let parsed = parseCabalProject ./cabal.project;
+        in builtins.filter
+        ({ name, ... }: !(builtins.elem name excludedPackages)) parsed;
+        concatPackageNames = builtins.map ({ name, ... }: name) concatPackages;
+
+        haskellOverlay = self: super:
+          builtins.listToAttrs (builtins.map ({ name, path }: {
+            inherit name;
+            value = let
+              p = self.callCabal2nix name (./. + "/${path}") { };
+              p1 = if builtins.elem name noHaddockPackages then
+                haskellLib.dontHaddock p
+              else
+                p;
+              p2 = if builtins.elem name noCheckPackages then
+                haskellLib.dontCheck p1
+              else
+                p1;
+            in p2;
+          }) concatPackages);
+
+        # see these issues and discussions:
+        # - https://github.com/NixOS/nixpkgs/issues/16394
+        # - https://github.com/NixOS/nixpkgs/issues/25887
+        # - https://github.com/NixOS/nixpkgs/issues/26561
+        # - https://discourse.nixos.org/t/nix-haskell-development-2020/6170
+        fullOverlay = final: prev: {
+          haskellPackages = prev.haskellPackages.override (old: {
+            overrides =
+              final.lib.composeExtensions (old.overrides or (_: _: { }))
+              haskellOverlay;
+          });
+        };
+      in {
+        # This package set is only useful for CI build test.
+        # In practice, users will create a development environment composed by overlays.
+        packages = let
+          packagesOnGHC = ghcVer:
+            let
+              overlayGHC = final: prev: {
+                haskellPackages = prev.haskell.packages.${ghcVer};
+              };
+
+              newPkgs = import nixpkgs {
+                overlays = [ overlayGHC fullOverlay ];
+                inherit system;
+              };
+
+              individualPackages = builtins.listToAttrs (builtins.map
+                ({ name, ... }: {
+                  name = ghcVer + "_" + name;
+                  value = builtins.getAttr name newPkgs.haskellPackages;
+                }) concatPackages);
+
+              allEnv = let
+                hsenv = newPkgs.haskellPackages.ghcWithPackages (p:
+                  let
+                    deps =
+                      builtins.map ({ name, ... }: p.${name}) concatPackages;
+                  in deps);
+              in newPkgs.buildEnv {
+                name = "all-packages";
+                paths = [ hsenv ];
+              };
+            in individualPackages // { "${ghcVer}_all" = allEnv; };
+
+        in packagesOnGHC "ghc8107" // packagesOnGHC "ghc884"
+        // packagesOnGHC "ghc901" // packagesOnGHC "ghc921"
+        // packagesOnGHC "ghcHEAD";
+
+        overlay = fullOverlay;
+
+        devShells = let
+          mkDevShell = ghcVer:
+            let
+              overlayGHC = final: prev: {
+                haskellPackages = prev.haskell.packages.${ghcVer};
+              };
+
+              newPkgs = import nixpkgs {
+                overlays = [ overlayGHC fullOverlay ];
+                inherit system;
+              };
+
+            in newPkgs.haskellPackages.shellFor {
+              packages = ps: builtins.map (name: ps.${name}) concatPackageNames;
+              buildInputs = [ newPkgs.haskellPackages.cabal-install ] ++
+                # haskell-language-server on GHC 9.2.1 is broken yet.
+                newPkgs.lib.optional (ghcVer != "ghc921")
+                [ newPkgs.haskell-language-server ];
+              withHoogle = false;
+            };
+        in {
+          "ghc8107" = mkDevShell "ghc8107";
+          "ghc921" = mkDevShell "ghc921";
+        };
+      });
+}

parse-cabal-project.nix

@@ -0,0 +1,19 @@
+# Ideally, parsing cabal.project should be done via official tools
+# Related discussion here:
+# https://github.com/NixOS/cabal2nix/issues/286
+
+cabalProject:
+let
+  content = builtins.readFile cabalProject;
+  lines = builtins.filter builtins.isString (builtins.split ''
+    [
+    ]'' content);
+  matches =
+    builtins.map (builtins.match "[[:space:]]*[.]/(.*)/(.*)[.]cabal$") lines;
+  projects = builtins.concatMap (match:
+    if builtins.isList match && builtins.length match == 2 then [{
+      name = builtins.elemAt match 1;
+      path = builtins.elemAt match 0;
+    }] else
+      [ ]) matches;
+in projects