Opened 5 years ago

Closed 5 years ago

## #8390 closed bug (invalid)

# regression in handling of type variables in constraints on instances which do not appear in the instance head

Reported by: | aavogt | Owned by: | |
---|---|---|---|

Priority: | normal | Milestone: | |

Component: | Compiler | Version: | 7.7 |

Keywords: | Cc: | ||

Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |

Type of failure: | GHC rejects valid program | Test Case: | |

Blocked By: | Blocking: | ||

Related Tickets: | Differential Rev(s): | ||

Wiki Page: |

### Description

ghc-7.7.20130720 (from here http://darcs.haskell.org/ghcBuilder/uploads/igloo-m/) rejects instances which work with ghc-7.6.2.

{-# LANGUAGE FlexibleInstances, ImplicitParams, MultiParamTypeClasses, ScopedTypeVariables, TypeFamilies, UndecidableInstances #-} class Fun f a b where fun :: f -> a -> b instance (b ~ Int, a ~ Int) => Fun F a b where fun _ = (+1) data F = F data Compose a b = Compose a b -- ghc-7.6 version instance (Fun f b c, Fun g a b) => Fun (Compose f g) a c where fun (Compose f g) a = fun f (fun g a :: b) {- | ghc >= 7.7 accepts this second instance, which is an ugly workaround >>> fun (Compose F F) 2 4 unsatisfactory ghc-77 workaround: >>> let ?b = undefined in fun (Compose F F) 2 4 -} instance (Fun f b c, Fun g a b, ?b :: b) => Fun (Compose f g) a c where fun (Compose f g) a = fun f (fun g a `asTypeOf` ?b)

### Attachments (1)

### Change History (9)

### comment:1 Changed 5 years ago by

### Changed 5 years ago by

explicitly computing argument/result types with a type family

### comment:2 Changed 5 years ago by

While Compose may not be so interesting, the same thing prevents HList from having both a single class for polymorphic functions (Fun above) and having higher-order functions that work for some instances of that class (ones like F).

Some faking can be done by having some associated types for Fun to calculate the argument from the result type etc. But that is uglier (see attached inf2).

### comment:3 Changed 5 years ago by

Resolution: | → invalid |
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Status: | new → closed |

I found -XAllowAmbiguousTypes, added by commit 97db0edc4e637dd61ec635d1f9b6b6dd25ad890c, which allows the old behavior.

### comment:4 Changed 5 years ago by

Resolution: | invalid |
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Status: | closed → new |

Though my head goes a little swimmy when I look at this for too long, I think the ambiguity checker in 7.7 is being overeager here. I can use the `Compose`

instance for `Fun`

without any type annotations, so therefore, its type is not ambiguous. It seems that the equality constraints on the `F`

instance induce what are effectively functional dependencies. Once we know that the first parameter to `Fun`

is `F`

, we know that the next two must be `Int`

.

I'm still a little lost as to how to characterize this behavior, and I could understand an argument saying that the ambiguity checker should reject this code. But, we should then also admit that the ambiguity checker is somewhat liberal, rejecting more than is necessary.

### comment:5 Changed 5 years ago by

goldfire, you're asking for a message `suggested fix: -XAllowAmbiguousTypes'? The reasoning for rejecting the Compose f g instance by default makes sense here.

### comment:6 Changed 5 years ago by

No, not quite. I think that, given that uses of the `Compose`

instance work just fine without extra annotations, then that means that the instance is not ambiguous, tautologically by my operating definition of "ambiguous". So, I think we need to choose between these two design alternatives:

- Permit the instance for
`Compose`

without`-XAllowAmbiguousTypes`

.

- Admit that the ambiguity checker is over-eager and sometimes rejects code that is unambiguous.

Or, I suppose

- Define ambiguous differently.

It's possible that I'm very confused about something here, so if that's the case, please try to enlighten me!

### comment:7 Changed 5 years ago by

Consider:

f :: Num a => Int -> Int f = blah instance Num a => C Int where blah

Both cases, calling `f`

or using the instance, will give rise to an ambiguous constraint `Num a`

. So both are rejected in the same way.

But functions with an ambiguous times may *sometimes* be callable. Eg

class C a b where ... f :: C a b => a -> Int f = blah instance C a b => Eq [a] where blah

The type of `f`

is ambiguous, in the sense that if we said

g :: C a b => a -> Int -- Identical g = f

type checking would fail even. But at *certain* types, typechecking might succeed. Example

instance C Char b where blah foo = f 'x'

The `-XAllowAmbiguousTypes`

flag therefore tells GHC to accept f's type even if it's ambiguous.

Thus, GHC's definition of "ambiguous" is "over-eager" in the sense that the function *can* be called at some types, although perhaps not at others. Hence the flag.

I don't know a less-eager-but-still-useful definition.

Certainly, I got lots of Trac tickets before saying "I had a definition f=e; I got ghci to tell me its type; I pasted in that type as a type signature for f, and it didn't typecheck". That was because the inferred type was ambiguous.

Same with instance decls. The error message now says

T8390.hs:14:10: Could not deduce (Fun g a b0) arising from the ambiguity check for an instance declaration from the context (Fun f b c, Fun g a b) bound by an instance declaration: (Fun f b c, Fun g a b) => Fun (Compose f g) a c at T8390.hs:14:10-56 The type variable ‛b0’ is ambiguous In the ambiguity check for: forall f g a c b. (Fun f b c, Fun g a b) => Fun (Compose f g) a c To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the instance declaration for ‛Fun (Compose f g) a c’

and adding `-XAllowAmbiguousTypes`

makes it go through.

OK?

Simon

### comment:8 Changed 5 years ago by

Resolution: | → invalid |
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Status: | new → closed |

That is option 3, above, fixing my definition of ambiguous.

Yes, OK. Many thanks.

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If it helps, the error with ghc-7.7 is: