Add frontmatter decoder module.

src/Pages/FrontmatterDecoder.elm

@@ -0,0 +1,51 @@
+module Pages.FrontmatterDecoder exposing (FrontmatterDecoder, decodeList, decoder, resolve)
+
+import Json.Decode as Decode exposing (Decoder)
+
+
+type FrontmatterDecoder a
+    = FrontmatterDecoder (List Decode.Value -> Decoder a)
+
+parse : List String -> FrontmatterDecoder a -> Result String a
+parse (FrontmatterDecoder rawDecoder) =
+    Decode.decodeString rawDecoder
+
+decoder : Decoder a -> FrontmatterDecoder a
+decoder rawDecoder =
+    FrontmatterDecoder (\values -> rawDecoder)
+
+
+resolve : (List a -> Decoder b) -> FrontmatterDecoder a -> FrontmatterDecoder b
+resolve resolveFn (FrontmatterDecoder rawDecoder) =
+    (\values ->
+        let
+            listDecoder : Decoder (List a)
+            listDecoder =
+                decodeList values (rawDecoder values)
+        in
+        listDecoder
+            |> Decode.andThen
+                (\list ->
+                    resolveFn list
+                )
+    )
+        |> FrontmatterDecoder
+
+
+decodeList : List Decode.Value -> Decoder a -> Decoder (List a)
+decodeList values rawDecoder =
+    List.foldl
+        (\value soFar ->
+            soFar
+                |> Decode.andThen
+                    (\list ->
+                        case Decode.decodeValue rawDecoder value of
+                            Ok decoded ->
+                                Decode.succeed (decoded :: list)
+
+                            Err error ->
+                                Decode.fail "TODO error message"
+                    )
+        )
+        (Decode.succeed [])
+        values