nix: Add a script to generate a graph of Haskell imports (#1751)

.gitignore

@@ -21,3 +21,4 @@ __pycache__
 *.tix
 coverage
 .hpc
+imports.png

nix/devtools.nix

@@ -3,6 +3,7 @@
 , checkedShellScript
 , devCabalOptions
 , entr
+, graphviz
 , silver-searcher
 , style
 , tests
@@ -96,6 +97,41 @@ let
         ${style}/bin/postgrest-lint
         ${style}/bin/postgrest-style-check
       '';
+
+  importsgraph =
+    checkedShellScript
+      {
+        name = "postgrest-importsgraph";
+        docs =
+          ''
+            Render the imports between PostgREST modules as a graph.
+
+            Output is written to 'imports.png' in the root directory of the project.
+          '';
+        inRootDir = true;
+      }
+      ''
+        imports=$(
+          grep -rE 'import .*PostgREST\.' src main \
+            | sed -E \
+                -e 's|/|\.|g' \
+                -e 's/(src|main)\.(.*)\.hs:import .*(PostgREST\.\S+)( .*)?/"\2" -> "\3"/'
+        )
+
+        labels=$(
+          grep -rE '^(--)?\s*Description\s*:' src main \
+            | sed -E \
+                -e 's|/|\.|g' \
+                -e 's/^(src|main)\.(.*)\.hs:(--)?\s*Description\s*:\s*(.*)$/"\2" [label="\2\\n\4"]/'
+        )
+
+        cat <<EOF | ${graphviz}/bin/dot -Tpng > imports.png
+          digraph {
+            $imports
+            $labels
+          }
+        EOF
+      '';
 in
 buildEnv {
   name = "postgrest-devtools";
@@ -106,5 +142,6 @@ buildEnv {
     run.bin
     clean.bin
     check.bin
+    importsgraph.bin
   ];
 }