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ghc-8.8 does not infer kind

    • Closed Issue created by jwaldmann @jwaldmann

    Rejected by ghc-8.8.4 with

    should.hs:27:12: error:
    • Expected kind ‘* -> *’, but ‘solver’ has kind ‘*’
    • In the first argument of ‘Solver’, namely ‘solver’
      In the type ‘(Solver solver, Queue q, Transformer t) =>
                   Int -> q -> t -> EvalState t -> solver (Int, [a])’
      In the type declaration for ‘ContinueSig’

    but accepted by 8.6.5, 8.10.2. 9.0.1-alpha1.

    
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE GADTs #-}

class Monad solver => Solver solver where
  type Constraint solver  :: *
  type Label solver       :: *

class Queue q

data Tree s a where
  NewVar :: Term s t => (t -> Tree s a) -> Tree s a

class Solver solver => Term solver term

class Transformer t where
  type EvalState t :: *
  type TreeState t :: *
  type ForSolver t :: (* -> *)
  type ForResult t :: *
  nextT :: SearchSig (ForSolver t) q t (ForResult t)
  returnT :: ContinueSig solver q t (ForResult t) 

type ContinueSig solver  q t a =
  ( Solver solver, Queue q, Transformer t  )
  => Int -> q -> t -> EvalState t
  -> solver (Int, [a])

type SearchSig solver q t a =
     (Solver solver, Queue q, Transformer t     ) 
     => Int -> Tree solver a -> q -> t -> EvalState t -> TreeState t
     -> solver (Int,[a])

    8.8 accepts this after introducing this:

    type ContinueSig (solver :: * -> *)  q t a = 
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