Include dhall-nix in stack and cabal project configs (#1178)

Note that `dhall-nix` cannot be built on Windows due to its transitive
dependency on the `unix` package.

Also:

* Nix: Enable `-Werror` for `dhall-nix`

* Add support for `toMap` in `dhall-nix`:

    \(x : { a : Bool }) -> toMap x

is translated as

    x:
      (kvs:
        map (k:
          {
            mapKey = k;
            mapValue = builtins.getAttr k kvs;
            }) (builtins.attrNames kvs)) x

cabal.project

@@ -1 +1 @@
-packages: ./dhall ./dhall-bash ./dhall-json ./dhall-lsp-server
+packages: ./dhall ./dhall-bash ./dhall-json ./dhall-lsp-server ./dhall-nix

dhall-nix/src/Dhall/Nix.hs

@@ -490,6 +490,17 @@ dhallToNix e = loop (Dhall.Core.normalize e)
         a' <- loop a
         b' <- loop b
         return (Fix (NBinary NApp b' a'))
+    loop (ToMap a _) = do
+        a' <- loop a
+        let ks = Fix (NBinary NApp "builtins.attrNames" "kvs")
+        let v = Fix (NBinary NApp (Fix (NBinary NApp "builtins.getAttr" "k")) "kvs")
+        let setBindings =
+                [ NamedVar [StaticKey "mapKey"] "k" Nix.nullPos
+                , NamedVar [StaticKey "mapValue"] v Nix.nullPos
+                ]
+        let map_ = Fix (NBinary NApp "map" (Fix (NAbs "k" (Fix (NSet setBindings)))))
+        let toMap = Fix (NAbs "kvs" (Fix (NBinary NApp map_ ks)))
+        return (Fix (NBinary NApp toMap a'))
     loop (Prefer a b) = do
         a' <- loop a
         b' <- loop b

nix/shared.nix

@@ -151,6 +151,7 @@ let
                     "dhall"
                     "dhall-bash"
                     "dhall-json"
+                    "dhall-nix"
                   ];
 
                 extension =

stack.yaml

@@ -3,11 +3,17 @@ packages:
   - dhall
   - dhall-bash
   - dhall-json
+  - dhall-nix
   - dhall-lsp-server
 extra-deps:
   - repline-0.2.1.0
   - th-lift-instances-0.1.13@sha256:2852e468511805cb25d9e3923c9e91647d008ab4a764ec0921e5e40ff8a8e874,2625
   - th-lift-0.8.0.1@sha256:cceb81b12c0580e02a7a3898b6d60cca5e1be080741f69ddde4f12210d8ba7ca,1960
+  - hnix-0.6.1@sha256:13da97c09489ccf0b908fbe7817401137559eebc377c00cd7b8a66b77e8c251b,24519
+  - hashing-0.1.0.1@sha256:98861f16791946cdf28e3c7a6ee9ac8b72d546d6e33c569c7087ef18253294e7,2816
+  - hnix-store-core-0.1.0.0@sha256:719a261870367b3ce2e2e8d11502e7170faa664109a14486b95b555bc3672807,2541
+  - monadlist-0.0.2@sha256:978305e3f03dd5f65c673b551124dac0c39a247c2c14d84739ca9a3405fbb63a,904
+  - ref-tf-0.4.0.1@sha256:369e5c6237e4b6fe330fde021b94a9f98971690cbec5b910faa9af3cbcb15610,1133
 nix:
   packages:
     - ncurses