> I've also been experiencing this a lot in class instances, such as:
>
> class Foo f where
> foo :: a -> f a
>
> data Bar f a = Foo f => Bar {bar :: f a}
>
> instance Foo (Bar f) where
> foo a = Bar (foo a)
>
> Is there any way to avoid repeating the Foo f constraint in the Bar f
> instance?
Dear Harry, et al,
The problem here is that GADT-constructors simply "carry" the appropriate
dictionaries. When you *produce* an element of a GADT where the constructor
requires a dictionary, you must provide it there. In this case, for a variable
f, you don't have a dictionary, so you could read the constraint and instance
head as "if you give me a Foo-dictionary for f, then I will wrap it in a Bar."
Another way of looking at it is that the type "Bar f" only 'exists' for 'f's
that are 'Foo's. If you don't know whether a particular 'f' is a 'Foo', you
don't know whether Bar f exists.
In short, I think that there is no such way.
Regards,
Philip
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