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{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances, FlexibleContexts #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE OverloadedStrings #-}
module T7332 where
import GHC.Exts( IsString(..) )
import Data.Monoid
newtype DC d = DC d
deriving (Show, Monoid)
instance IsString (DC String) where
fromString = DC
class Monoid acc => Build acc r where
type BuildR r :: * -- Result type
build :: (acc -> BuildR r) -> acc -> r
instance Monoid dc => Build dc (DC dx) where
type BuildR (DC dx) = DC dx
build tr acc = tr acc
instance (Build dc r, a ~ dc) => Build dc (a->r) where
type BuildR (a->r) = BuildR r
build tr acc s = build tr (acc `mappend` s)
-- The type is inferred
-- tspan :: (Monoid d, Build (DC d) r, BuildR r ~ DC d) => r
tspan :: (Build (DC d) r, BuildR r ~ DC d) => r
tspan = build (id :: DC d -> DC d) mempty
{- Wanted:
Build acc0 r0
Monid acc0
acc0 ~ DC d0
DC d0 ~ BuildR r0
==>
Build (DC d0) r0
Monoid (DC d0) --> Monoid d0
DC d- ~ BuildR r0
In fact Monoid (DC d0) is a superclass of (Build (DC do) r0)
But during inference we do not take upserclasses of wanteds
-}
foo = tspan "aa"
foo1 = tspan (tspan "aa")
bar = tspan "aa" :: DC String
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