run doctests as part of cabal test

.gitignore

@@ -2,6 +2,7 @@
 dist
 dist-*
 *.cabal
+.ghc.*
 
 # nix
 result

.hspec

@@ -0,0 +1 @@
+--color

README.md

@@ -116,3 +116,13 @@ Nix (or more precisely, Cabal2nix) uses a `.cabal` file if it is present, otherw
 - `crm.cabal`
 
 This file is automatically generated from the `package.yaml` file and should not be committed to Git history.
+
+### tool-specific files
+
+- `fourmolu.yaml`
+
+Configuration file for the [`fourmolu`](https://github.com/fourmolu/fourmolu) formatting tool.
+
+- `.hspec`
+
+Using a dedicated file to specify options for [`hspec`](https://hspec.github.io) allows passing options only to it. Using `cabal test --test-options` would pass options to all test stanzas instead.

doctest/Main.hs

@@ -0,0 +1,8 @@
+module Main where
+
+import "base" System.Environment (getArgs)
+import "doctest-parallel" Test.DocTest (mainFromCabal)
+
+main :: IO ()
+main = do
+  mainFromCabal "crm" =<< getArgs

flake.nix

@@ -76,8 +76,10 @@
         "cabal build";
 
       # Trigger a test execution every time a file changes
+      # the --write-ghc-environment-files=always is required by doctest-parallel
+      # see https://github.com/martijnbastiaan/doctest-parallel/blob/main/example/README.md#cabalproject
       test-watch = watch "test-watch"
-        "cabal test --test-show-details=streaming --test-option=--color";
+        "cabal test --test-show-details=streaming --write-ghc-environment-files=always";
     in
     rec {
       packages = {

nix/haskell-source.nix

@@ -1,4 +1,5 @@
 [
+  "doctest"
   "src"
   "spec"
   "package.yaml"

package.yaml

@@ -92,13 +92,13 @@ ghc-options:
 
 dependencies:
   - base
-  - singletons-base
-  - text
 
 library:
   source-dirs:      src
   dependencies:
     - profunctors
+    - singletons-base
+    - text
   # Disable adding Paths_crm to other-modules, because it does not conform to our style guide.
   # https://github.com/sol/hpack#handling-of-paths_-modules
   when:
@@ -112,6 +112,21 @@ tests:
     dependencies:
       - crm
       - hspec
+      - singletons-base
+      - text
+    when:
+    - condition: false
+      other-modules: Paths_crm
+
+  crm-doctests:
+    source-dirs:    doctest
+    main:           Main.hs
+    ghc-options:
+      - -threaded
+      - -Wno-unused-packages
+    dependencies:
+      - crm
+      - doctest-parallel
     when:
     - condition: false
       other-modules: Paths_crm

src/CRM/Graph.hs

@@ -10,10 +10,10 @@ newtype Graph a = Graph [(a, a)]
  It has as vertices the product of the set of vertices of the initial graph.
  It has as edge from `(a1, b1)` to `(a2, b2)` if and only if there is an edge
  from `a1` to `a2` and an edge from `b1` to `b2`
--}
 
--- >>> productGraph (Graph [('a', 'b')]) (Graph [('c', 'd')])
--- Graph [(('a','c'),('b','d'))]
+ >>> productGraph (Graph [('a', 'b')]) (Graph [('c', 'd')])
+ Graph [(('a','c'),('b','d'))]
+-}
 productGraph :: Graph a -> Graph b -> Graph (a, b)
 productGraph (Graph edges1) (Graph edges2) =
   Graph $