# Simplify the rules for implicit quantification

This thread http://www.haskell.org/pipermail/glasgow-haskell-users/2010-October/019360.html convinced me that GHC's rules for implicit quantification are unnecessarily complicated.

== The current spec == The current spec seems to be this:

- Define an "implicit quantification point" to be (a) the type in a type signature
`f :: type`

if`type`

does not start with`forall`

; or (b) a type of form`(context => type)`

, that is not immediately enclosed by an explicit`forall`

- At each implicit quantification point 'ty', working outside in, GHC finds all the type variables a,b,c in 'ty' that are not already in scope, and transforms 'ty' to (forall a,b,c. ty).

Note that

- The argument of a constructor is not an implicit quantification point, so that

`data Foo = MkFoo (a -> a)`

is an error, and does not mean

`data Foo = MkFoo (forall a. a->a)`

- Implicit quantification points may be nested but the inner ones are effectively no-ops. Example

`f :: Int -> (Eq a => a -> a) -> Int`

There are two quantification points: the whole type, and the

`(Eq a => ...)`

. But when the outer quantification point wraps forall a around it, the inner quantification point has no free variables to quantify. So we get

`f :: forall a. Int -> (Eq a => a -> a) -> Int`

== The new proposal ==

The proposed new rule is this:

- Implicit quantification applies only to an entire user type signature that does not start with
`forall`

. - For such signatures, find all the type variables a,b,c in the signature that are not already in scope, and prefix the signature with
`forall a,b,c.`

I believe that the only observable changes in behaviour would be

- In a data type declaration

`data Foo = MkFoo (Eq a => a -> a)`

you'd get an error "a is not in scope", just as you would with

`data Foo = MkFoo (a->a)`

To me this seems more consistent behavour.

- Inside an
*explicit*`forall`

you would get no*implicit*`foralls`

:

`f :: forall a. (Eq b => b->b) -> a -> a`

would yield "b is not in scope", whereas at present it behaves like

`f :: forall a. (forall b. Eq b => b->b) -> a -> a`

## Trac metadata

Trac field | Value |
---|---|

Version | 6.12.3 |

Type | FeatureRequest |

TypeOfFailure | OtherFailure |

Priority | normal |

Resolution | Unresolved |

Component | Compiler |

Test case | |

Differential revisions | |

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Blocking | |

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Operating system | |

Architecture |