Opened 6 years ago
Closed 2 years ago
#2695 closed bug (fixed)
bogus "syntactically distinct contexts" error
Reported by: | conal | Owned by: | |
---|---|---|---|
Priority: | low | Milestone: | ⊥ |
Component: | Compiler (Type checker) | Version: | 6.11 |
Keywords: | Cc: | conal | |
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | None/Unknown | Test Case: | |
Blocked By: | Blocking: | ||
Related Tickets: | Differential Revisions: |
Description
{-# LANGUAGE TypeOperators, FlexibleContexts , MultiParamTypeClasses, FunctionalDependencies , TypeFamilies -- , ScopedTypeVariables #-} -- The ScopedTypeVariables is there just as a bug work-around. Without: -- -- Mutually dependent functions have syntactically distinct contexts -- When matching the contexts of the signatures for -- dZero :: forall b a s. -- (AdditiveGroup b, HasBasis a s, HasTrie (Basis a)) => -- a :> b -- pureD :: forall b a s. -- (AdditiveGroup b, HasBasis a s, HasTrie (Basis a)) => -- b -> a :> b -- The signature contexts in a mutually recursive group should all be identical -- When generalising the type(s) for dZero, pureD -- -- This bug was introduced between ghc 6.9.20080622 and 6.10.0.20081007. {-# OPTIONS_GHC -fno-warn-missing-methods #-} import Control.Applicative class AdditiveGroup v where zeroV :: v (^+^) :: v -> v -> v negateV :: v -> v class AdditiveGroup v => VectorSpace v s | v -> s where (*^) :: s -> v -> v -- | Mapping from all elements of @a@ to the results of some function class HasTrie a where data (:->:) a :: * -> * instance HasTrie a => Functor ((:->:) a) instance HasTrie a => Applicative ((:->:) a) class VectorSpace v s => HasBasis v s where type Basis v :: * -- | Linear map, represented a as a memo function from basis to values. type u :-* v = Basis u :->: v data a :> b = D { powVal :: b, derivative :: a :-* (a :> b) } dZero :: (AdditiveGroup b, HasBasis a s, HasTrie (Basis a)) => a:>b dZero = pureD zeroV pureD :: (AdditiveGroup b, HasBasis a s, HasTrie (Basis a)) => b -> a:>b pureD b = b `D` pure dZero
Change History (4)
comment:1 Changed 6 years ago by conal
comment:2 Changed 6 years ago by igloo
- Component changed from Compiler to Compiler (Type checker)
- difficulty set to Unknown
- Milestone set to 6.10.2
Thanks for the report
comment:3 Changed 6 years ago by simonpj
- Milestone changed from 6.10.2 to _|_
- Priority changed from normal to low
Good point. The requirement that the contexts of all the declarations in a mutually recursive group must be identical, even though really we have polymorphic recursion, is a very tiresome Haskell 98 thing. What is biting us here is that we're trying to unify the two contexts, but since Basis is a type function, knowing that (Basis a1) = (Basis a2) does not allow us to conclude that a1 = a2.
We could solve this by writing a syntactic-identity checker (oh, but it has to be subject to alpha-renaming...), but it just doesn't seem worth the bother.
The solution is to use -XRelaxedPolyRec, which lifts the restriction altogether. Haskell Prime will probably have this as the default.
Meanwhile I'll leave this open at low priority.
Simon
comment:4 Changed 2 years ago by monoidal
- Resolution set to fixed
- Status changed from new to closed
- Type of failure set to None/Unknown
As of GHC 7.6, the code compiles fine. -XRelaxedPolyRec is the default and cannot be switched off.
see also bug #2696