Implicit 'forall' in data declarations

Sebastian Fischer fischer at
Thu Oct 21 04:57:28 EDT 2010

Hi Simon,

thank you for your explanations. Now I understand why type variables  
are quantified at the outermost position in function types.

My confusion was caused by implicit quantifications in data type  
declarations. These are not (explicitly) mentioned in the  
documentation that you have linked and they only occur when a type  
class context is given.

> In a data type decl
>    data Foo = Foo (Eq a => a)
> the "top of the type" is done separately for each argument.  After
> all, Foo (Eq a => a) isn't a type.  So you get
>    data Foo = Foo (forall a. Eq a => a)

This was a surprise as

     data Bar = Bar (a -> a)

is illegal and *not* equivalent to

     data Bar = Bar (forall a . a -> a)

(at least in GHC 6.12.3)

This lead me into thinking that the type class context causes  
quantification which was apparently wrong.

In fact, I think according to the documentation of implicit  
quantification it is unclear if the definitions of Foo and Bar  
(without explicit forall) are legal. I expected both to be illegal and  
am surprised that one is legal and the other is not.

If "the top level of user written types" includes data constructor  
arguments, then probably both should be legal. On the other hand it  
would probably be surprising if one could write

     data Id type = Id typ

without getting an error message.

> Suppose you wrote
>     bar :: (Eq a => a) -> a
> Then which of these two do you want?
> 	bar :: forall a. (forall a. Eq a => a) -> a
>      bar :: forall a. (Eq a => a) -> a
> And then what's the rule lexically scoped tyvars?

I'm afraid I don't understand your question. But I'm fine with the  
current behaviour of implicit quantification in function types.


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