ParameterizedSubData.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE FunctionalDependencies #-}
-- ^ Useful for debugging generated code; can be removed once stable

-- | Test case for mkYesodSubData and mkYesodSubDispatch with
-- parameterized/constrained types. Reproduces bugs reported by
-- l0neGamer on PR #1887:
--
-- 1. mkYesodSubData with a constrained type like
--    @"(SomeClass subsite master) => SubsiteData subsite"@
--    where 'master' appears in the context but NOT as a type parameter
--    of the subsite, produces "Out of scope type variable" errors in
--    family instances.
--
-- 2. mkYesodSubDispatch with the same setup produces "Expecting one
--    more argument to 'NestedR'" because isInstance checks and Proxy
--    types weren't fully applied. See ParameterizedSubDispatch.hs.
module YesodCoreTest.ParameterizedSubData where

import Yesod.Core

type RouteConstraints t = (Eq t, Read t, Show t, PathPiece t)

-- | A multi-param type class where 'master' is NOT a parameter of the
-- subsite type, mimicking the pattern:
--   @mkYesodSubData "(SubsiteClass subsite master) => SubsiteData subsite"@
-- The fundep ensures 'master' is determined by 'subsite', which is
-- typical for real-world subsite classes.
class (RouteConstraints (AssocType subsite)) => ParamSubsiteClass subsite master | subsite -> master where
  type AssocType subsite
  getSubsiteValue :: subsite -> master -> String

newtype ParamSubsite subsite = ParamSubsite subsite

mkYesodSubData "(ParamSubsiteClass subsite master) => ParamSubsite subsite" [parseRoutes|
/ ParamSubHomeR GET
/item/#Int ParamSubItemR GET
!/#{AssocType subsite}/nested NestedR:
    / NestedHomeR GET
    /detail/#Int NestedDetailR GET
|]