[Haskell-cafe] advanced class constraints in Haskell 98?
lemming at henning-thielemann.de
Thu Jan 8 16:27:59 EST 2009
On Thu, 8 Jan 2009, Henning Thielemann wrote:
> On Fri, 9 Jan 2009, Miguel Mitrofanov wrote:
>> 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
> A nice. I jumped into 4.3 and found
> § § R 32 ©
... copying from Haskell 98 report did not only insert rubbish, but also
triggered sending the e-mail. I hope it did not more damage ...
scontext -> simpleclass
| (simpleclass_1, ..., simpleclass_n)
simpleclass -> qtycls tyvar
So it must be 'atype' instead of 'tyvar'? Haskell 98 is really mighty.
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