[Haskell-cafe] advanced class constraints in Haskell 98?

[Miguel Mitrofanov]

On 8 Jan 2009, at 23:59, Henning Thielemann wrote:

>
> GHC accepts a class declaration like
>  class Monad (m Maybe) => C m where
>     ...
> without having any language extension switched on. But it isn't  
> Haskell 98, is it?

It is.

 From Report:

========================

A class assertion has form qtycls tyvar, and indicates the membership  
of the type tyvar in the class qtycls. A class identifier begins with  
an uppercase letter. A context consists of zero or more class  
assertions, and has the general form

( C1 u1, ..., Cn un )

where C1, ..., Cn are class identifiers, and each of the u1, ..., un  
is either a type variable, or the application of type variable to one  
or more types.

========================

"atype" is defined as

========================

atype 	 ->     gtycon
	| 	tyvar
	| 	( type1 , ... , typek ) 	(tuple type, k>=2)
	| 	[ type ] 	(list type)
	| 	( type ) 	(parenthesised constructor)

========================

> Hugs 2005 also accepts this.
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