Christian Sievers <[EMAIL PROTECTED]>
writes (I format the text)
> The report says explicit that instance declarations like
>
> instance C (a,a) where ...,
>
> or for (Int,a) or for [[a]]
>
> are not allowed. I tried to understand this by thinking these
> types are too complex, but I had to learn that a type may also
> be too simple, i.e. just writing
> instance D a
> is not allowed either.
> ...
As to `instance D a',
it is not a loss. Because `instance D a' is the same as
`class D a' - supplied with the default definition. For example,
the illegal declaration pair
class C a => D a where d :: a -> a
instance C a => D a where d = <definition>
can be replaced with the legal and simpler declaration
class C a => D a where d :: a -> a
d = <definition>
Correct me please, if this is a mistake, I am not much of an
expert in Haskell (though keep on programming for two years!)
As to instance ... => C (a ,a) where ...,
... => C (Int,a) where ...,
and such,
they might express a very meaningful mathematical sense, these
constructions are highly desirable.
And, yes, the *overlapping instances* too.
The developers often say these overlaps conflict with the separate
compilation. But separate compilation is rather a technical detail.
Besides, might it occure some way-out ... - ?
-------------------------------------
Sergey Mechveliani [EMAIL PROTECTED]
