[Haskell-cafe] About instance
jules at jellybean.co.uk
Tue Jan 18 17:03:05 EST 2005
On 18 Jan 2005, at 21:45, Ulises Juarez Martinez wrote:
> Who can I do an instance of Eq (Ocurrence -> Bool)? Is there another
> option to avoid the error?
In general, you can't define one. To define equality on functions you
want to check the value on every possible input, and since Ocurrence is
an infinite type, that can't be done in a finite time.
In this specific case, you are dealing with indicator functions, which
are isomorphic to (possibly infinite) Sets of Ocurrences. If you happen
to know that they will always be finite (or, perhaps, always cofinite)
you might solve the problem by using that representation.
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