implement correct-by-construction relative links

content/default.nix

@@ -7,7 +7,7 @@ in
 
   collections.news.type = config.content-types.article;
 
-  pages.index = {
+  pages.index = { link, ... }: {
     title = "Fediversity";
     locations = [
       "index.html"
@@ -21,13 +21,13 @@ in
 
       ${pages.fediversity.summary}
 
-      [Learn more about Fediversity](${pages.fediversity})
+      [Learn more about Fediversity](${link pages.fediversity})
 
       # Fediversity grants
 
       ${pages.grants.summary}
 
-      [Learn more about Fediversity grants](${pages.grants})
+      [Learn more about Fediversity grants](${link pages.grants})
 
       # Consortium
 
@@ -38,7 +38,7 @@ in
 
       ${partner.description}
 
-      [Read more about ${partner.title}](${partner})
+      [Read more about ${partner.title}](${link partner})
       '') (with pages; [ nlnet oid tweag nordunet ]))}
 
       # News

lib.nix

@@ -46,6 +46,25 @@ rec {
       (map (x: if x == "" then x else "${prefix}${x}") (tail lines))
     );
 
+  relativePath = path1': path2':
+    let
+      inherit (lib.path) subpath;
+      inherit (lib) lists;
+
+      path1 = subpath.components path1';
+      path2 = subpath.components path2';
+
+      commonPrefixLength = with lists;
+        findFirstIndex (i: i.fst != i.snd)
+          { fst = null; snd = null; }
+          (zipLists path1 path2);
+
+      relativeComponents = with lists;
+        [ "." ] ++ (replicate (length path1 - commonPrefixLength - 1) "..") ++
+        (drop commonPrefixLength path2);
+    in
+    join "/" relativeComponents;
+
   types = rec {
     collection = elemType:
       let

structure/content-types.nix

@@ -14,7 +14,8 @@ in
     };
   };
   config.content-types = {
-    document = { name, config, ... }: {
+    document = { name, config, link, ... }: {
+      config._module.args.link = config.link;
       options = {
         name = mkOption {
           description = "Symbolic name, used as a human-readable identifier";
@@ -37,7 +38,7 @@ in
         link = mkOption {
           description = "Helper function for transparent linking to other pages";
           type = with types; functionTo str;
-          default = target: "TODO: compute the relative path based on `locations`";
+          default = target: with lib; relativePath (head config.locations) (head target.locations);
         };
         # TODO: may not need it when using `link`; could repurpose it to render the default template
         outPath = mkOption {