Add Haskell.nix and use it to provide a shell.nix (#3)

cabal.project

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+packages: *.cabal
+package dear-imgui
+  flags: +sdl2 +opengl

default.nix

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+{ # Fetch haskell.nix and import its default.nix
+haskellNix ? (import (import ./nix/sources.nix)."haskell.nix" { })
+
+# haskell.nix provides access to the nixpkgs pins which are used by our CI,
+# hence you will be more likely to get cache hits when using these.
+# But you can also just use your own, e.g. '<nixpkgs>'.
+, nixpkgsSrc ? haskellNix.sources.nixpkgs-2009
+
+  # haskell.nix provides some arguments to be passed to nixpkgs, including some
+  # patches and also the haskell.nix functionality itself as an overlay.
+, nixpkgsArgs ? haskellNix.nixpkgsArgs
+, compiler-nix-name ? "ghc884"
+}:
+let
+  pkgs = import nixpkgsSrc nixpkgsArgs;
+in pkgs.haskell-nix.project {
+  inherit compiler-nix-name;
+  # 'cleanGit' cleans a source directory based on the files known by git
+  src = pkgs.haskell-nix.haskellLib.cleanGit {
+    name = "dear-imgui";
+    src = ./.;
+  };
+}

nix/sources.json

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+{
+    "haskell.nix": {
+        "branch": "master",
+        "description": "Alternative Haskell Infrastructure for Nixpkgs",
+        "homepage": "https://input-output-hk.github.io/haskell.nix",
+        "owner": "input-output-hk",
+        "repo": "haskell.nix",
+        "rev": "ef4aef4ce2060dc1a41b2690df1f54f986e0f9ab",
+        "sha256": "0537fbjh4mcnywa33h4hl135kw7i8c0j8qndyzv5i82j7mc8wjvs",
+        "type": "tarball",
+        "url": "https://github.com/input-output-hk/haskell.nix/archive/ef4aef4ce2060dc1a41b2690df1f54f986e0f9ab.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "niv": {
+        "branch": "master",
+        "description": "Easy dependency management for Nix projects",
+        "homepage": "https://github.com/nmattia/niv",
+        "owner": "nmattia",
+        "repo": "niv",
+        "rev": "3cd7914b2c4cff48927e11c216dadfab7d903fe5",
+        "sha256": "1agq4nvbhrylf2s77kb4xhh9k7xcwdwggq764k4jgsbs70py8cw3",
+        "type": "tarball",
+        "url": "https://github.com/nmattia/niv/archive/3cd7914b2c4cff48927e11c216dadfab7d903fe5.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs": {
+        "branch": "nixos-19.09",
+        "description": "DEPRECATED! This is an obsolete, read-only mirror of the NixOS/nixpkgs repository.",
+        "homepage": "https://github.com/NixOS/nixpkgs",
+        "owner": "NixOS",
+        "repo": "nixpkgs-channels",
+        "rev": "75f4ba05c63be3f147bcc2f7bd4ba1f029cedcb1",
+        "sha256": "157c64220lf825ll4c0cxsdwg7cxqdx4z559fdp7kpz0g6p8fhhr",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs-channels/archive/75f4ba05c63be3f147bcc2f7bd4ba1f029cedcb1.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}

nix/sources.nix

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+# This file has been generated by Niv.
+
+let
+
+  #
+  # The fetchers. fetch_<type> fetches specs of type <type>.
+  #
+
+  fetch_file = pkgs: spec:
+    if spec.builtin or true then
+      builtins_fetchurl { inherit (spec) url sha256; }
+    else
+      pkgs.fetchurl { inherit (spec) url sha256; };
+
+  fetch_tarball = pkgs: spec:
+    if spec.builtin or true then
+      builtins_fetchTarball { inherit (spec) url sha256; }
+    else
+      pkgs.fetchzip { inherit (spec) url sha256; };
+
+  fetch_git = spec:
+    builtins.fetchGit { url = spec.repo; inherit (spec) rev ref; };
+
+  fetch_builtin-tarball = spec:
+    builtins.trace
+      ''
+        WARNING:
+          The niv type "builtin-tarball" will soon be deprecated. You should
+          instead use `builtin = true`.
+
+          $ niv modify <package> -a type=tarball -a builtin=true
+      ''
+      builtins_fetchTarball { inherit (spec) url sha256; };
+
+  fetch_builtin-url = spec:
+    builtins.trace
+      ''
+        WARNING:
+          The niv type "builtin-url" will soon be deprecated. You should
+          instead use `builtin = true`.
+
+          $ niv modify <package> -a type=file -a builtin=true
+      ''
+      (builtins_fetchurl { inherit (spec) url sha256; });
+
+  #
+  # Various helpers
+  #
+
+  # The set of packages used when specs are fetched using non-builtins.
+  mkPkgs = sources:
+    let
+      sourcesNixpkgs =
+        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) {};
+      hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;
+      hasThisAsNixpkgsPath = <nixpkgs> == ./.;
+    in
+      if builtins.hasAttr "nixpkgs" sources
+      then sourcesNixpkgs
+      else if hasNixpkgsPath && ! hasThisAsNixpkgsPath then
+        import <nixpkgs> {}
+      else
+        abort
+          ''
+            Please specify either <nixpkgs> (through -I or NIX_PATH=nixpkgs=...) or
+            add a package called "nixpkgs" to your sources.json.
+          '';
+
+  # The actual fetching function.
+  fetch = pkgs: name: spec:
+
+    if ! builtins.hasAttr "type" spec then
+      abort "ERROR: niv spec ${name} does not have a 'type' attribute"
+    else if spec.type == "file" then fetch_file pkgs spec
+    else if spec.type == "tarball" then fetch_tarball pkgs spec
+    else if spec.type == "git" then fetch_git spec
+    else if spec.type == "builtin-tarball" then fetch_builtin-tarball spec
+    else if spec.type == "builtin-url" then fetch_builtin-url spec
+    else
+      abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
+
+  # Ports of functions for older nix versions
+
+  # a Nix version of mapAttrs if the built-in doesn't exist
+  mapAttrs = builtins.mapAttrs or (
+    f: set: with builtins;
+    listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set))
+  );
+
+  # fetchTarball version that is compatible between all the versions of Nix
+  builtins_fetchTarball = { url, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchTarball;
+    in
+      if lessThan nixVersion "1.12" then
+        fetchTarball { inherit url; }
+      else
+        fetchTarball attrs;
+
+  # fetchurl version that is compatible between all the versions of Nix
+  builtins_fetchurl = { url, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchurl;
+    in
+      if lessThan nixVersion "1.12" then
+        fetchurl { inherit url; }
+      else
+        fetchurl attrs;
+
+  # Create the final "sources" from the config
+  mkSources = config:
+    mapAttrs (
+      name: spec:
+        if builtins.hasAttr "outPath" spec
+        then abort
+          "The values in sources.json should not have an 'outPath' attribute"
+        else
+          spec // { outPath = fetch config.pkgs name spec; }
+    ) config.sources;
+
+  # The "config" used by the fetchers
+  mkConfig =
+    { sourcesFile ? ./sources.json
+    , sources ? builtins.fromJSON (builtins.readFile sourcesFile)
+    , pkgs ? mkPkgs sources
+    }: rec {
+      # The sources, i.e. the attribute set of spec name to spec
+      inherit sources;
+
+      # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
+      inherit pkgs;
+    };
+in
+mkSources (mkConfig {}) // { __functor = _: settings: mkSources (mkConfig settings); }

shell.nix

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+let
+  hsPkgs = import ./default.nix {};
+in
+  hsPkgs.shellFor {
+    # Builds a Hoogle documentation index of all dependencies,
+    # and provides a "hoogle" command to search the index.
+    withHoogle = true;
+
+    # You might want some extra tools in the shell (optional).
+
+    # Some common tools can be added with the `tools` argument
+    tools = { cabal = "3.2.0.0"; };
+
+    # Prevents cabal from choosing alternate plans, so that
+    # *all* dependencies are provided by Nix.
+    exactDeps = true;
+  }