Opened 8 years ago

Closed 4 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 Rev(s): | ||

Wiki Page: |

### 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 8 years ago by

### comment:2 Changed 8 years ago by

Component: | Compiler → Compiler (Type checker) |
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difficulty: | → Unknown |

Milestone: | → 6.10.2 |

Thanks for the report

### comment:3 Changed 8 years ago by

Milestone: | 6.10.2 → _|_ |
---|---|

Priority: | normal → 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 4 years ago by

Resolution: | → fixed |
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Status: | new → closed |

Type of failure: | → None/Unknown |

As of GHC 7.6, the code compiles fine. -XRelaxedPolyRec is the default and cannot be switched off.

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see also bug #2696