Re: [isabelle] Arity limitations of type classes?
This is an inherent limitation which allows to infer unique most general
types. The technical details are explained for example here:
Sorry about that.
Rafal Kolanski wrote:
I am working with a situation where I want to state that words with
certain properties on their lengths are members of a type class.
However, there seems to be no way to state that a type constructor (in
this case bit0) takes a type class into two different ones.
So if we have words_of_length_2, we can say that:
instance bit0 :: (words_of_length_4) words_of_length_2
If the length is 4, then we know it's divisible by 4:
instance bit0 :: (words_of_length_2) words_div_4
instance bit0 :: (words_div_2) words_div_4
at this point, we are stuck, as the above line will have problems with
In general, anything of this form doesn't seem to work:
instance bit0 :: (a) b sorry
instance bit0 :: (c) b oops (* FAIL *)
Looking at the papers didn't really enlighten. Is this a fundamental
limitation on type classes? Is there any way to get around it?
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