Merge pull request graphql#988 from kutyel/mu-haskell

Add mu-haskell as another Haskell GraphQL Server alternative! 🚀

Author: orta

src/content/code/language-support/haskell/server/mu-haskell.md

@@ -0,0 +1,56 @@
+---
+name: Mu-Haskell with Mu-GraphQL
+description: A Haskell library for building microservices (gRPC, HTTP) and GraphQL APIs.
+url: https://higherkindness.io/mu-haskell/
+github: higherkindness/mu-haskell
+---
+
+Example implementation of a GraphQL server with type-level representation of the schema auto-generated:
+
+```haskell
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE PartialTypeSignatures #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+
+-- imports omitted for brevity...
+
+graphql "Library" "library.graphql" -- all the magic happens here! 🪄🎩
+
+-- ... a bit more code...
+
+libraryServer :: SqlBackend -> ServerT ObjectMapping i Library ServerErrorIO _
+libraryServer conn =
+  resolver
+    ( object @"Book"
+        ( field @"id" bookId,
+          field @"title" bookTitle,
+          field @"author" bookAuthor,
+          field @"imageUrl" bookImage
+        ),
+      object @"Author"
+        ( field @"id" authorId,
+          field @"name" authorName,
+          field @"books" authorBooks
+        ),
+      object @"Query"
+        ( method @"authors" allAuthors,
+          method @"books" allBooks
+        ),
+      object @"Mutation"
+        ( method @"newAuthor" newAuthor,
+          method @"newBook" newBook
+        ),
+      object @"Subscription"
+        (method @"allBooks" allBooksConduit)
+    )
+  where
+    bookId :: Entity Book -> ServerErrorIO Integer
+    bookId (Entity (BookKey k) _) = pure $ toInteger k
+    -- ... more resolvers...
+```
+
+See [our docs](https://higherkindness.io/mu-haskell/graphql/) for more information about how to build your own GraphQL server and [the library example](https://github.com/higherkindness/mu-graphql-example-elm) for a more end-to-end example that includes a client written in Elm!