add matrix.nix for matrix build using all packages and compilers

Uses ci.dhall as a single source of truth
(ci.dhall.frozen which is now-autoupdated via ci.sh)

Author: sorki

.github/workflows/ci.dhall.frozen

@@ -0,0 +1,31 @@
+let haskellCi =
+      https://raw.githubusercontent.com/sorki/github-actions-dhall/main/haskell-ci.dhall
+        sha256:9e49c0d57a38ab766825b3d2de6656d998b15eec873982db97d590ef58298601
+
+let defSteps = haskellCi.defaultCabalSteps
+
+in    haskellCi.generalCi
+        ( haskellCi.withNix
+            ( defSteps
+              with extraSteps.pre
+                   =
+                    defSteps.extraSteps.pre
+                  # [ haskellCi.installCachixStep "hnix-store"
+                    , haskellCi.BuildStep.NameIf
+                        { name = "Install libsodium"
+                        , run = "sudo apt install libsodium-dev"
+                        , `if` = "matrix.os == 'ubuntu-latest'"
+                        }
+                    ]
+            )
+        )
+        haskellCi.DhallMatrix::{
+        , ghc =
+          [ haskellCi.GHC.GHC963
+          , haskellCi.GHC.GHC947
+          , haskellCi.GHC.GHC902
+          , haskellCi.GHC.GHC8107
+          ]
+        , os = [ haskellCi.OS.Ubuntu, haskellCi.OS.MacOS ]
+        }
+    : haskellCi.CI.Type

.github/workflows/ci.sh

@@ -4,6 +4,12 @@
 set -eo pipefail
 cd "$( dirname "${BASH_SOURCE[0]}" )"
 
+which dhall || cabal install dhall
 which dhall-to-yaml || cabal install dhall-yaml
+
+echo "cp haskellCi.dhall -> ci.dhall.frozen"
+cp ci.dhall ci.dhall.frozen
+echo "dhall freez-ing ci.dhall.frozen"
+dhall freeze ci.dhall.frozen
 echo "regenerating ci.yaml"
 dhall-to-yaml-ng --generated-comment --file ci.dhall > ci.yaml

matrix.nix

@@ -0,0 +1,33 @@
+# Matrix build for all packages and compilers
+#
+# Uses ./.github/workflows/ci.dhall as a single source of
+# truth for a list of supported compilers
+#
+# The dhall expression needs to be frozen via dhall freeze
+# which is done automatically when we update it via ./.github/workflows/ci.sh
+# and dhallDirectoryToNix uses ci.dhall.frozen instead
+{ pkgs ? import <nixpkgs> {}
+}:
+let
+  lib = pkgs.lib;
+  ciDhallNix = pkgs.dhallDirectoryToNix { src = ./.github/workflows; file = "ci.dhall.frozen"; };
+  ciCompilers = ciDhallNix.jobs.build.strategy.matrix.ghc;
+
+  # from e.g. 9.6.3 to ghc963
+  convertCompilers = cs:
+    map (x: "ghc${lib.strings.replaceStrings ["."] [""] x}") cs;
+
+  compilers =
+    lib.traceValFn (cs:
+      let
+        prettyCs = lib.concatMapStringsSep "\n" (c: "- ${c}") cs;
+      in
+        "building for compilers:\n${prettyCs}")
+      (convertCompilers ciCompilers);
+in
+pkgs.lib.recurseIntoAttrs
+  (pkgs.lib.genAttrs compilers (compiler:
+    pkgs.lib.recurseIntoAttrs
+      (import ./. { inherit pkgs compiler; })
+    )
+  )