On Sat, Mar 3, 2018 at 6:41 PM, Francesco Ariis
On Sat, Mar 03, 2018 at 06:31:47PM -0800, Hilco Wijbenga wrote:
In any case, _when_ I flip "success" and "failure" the Functor instance no longer compiles. Which probably makes sense because I did not tell the compiler to interpret "Result failure" as "Result * failure"?
I wonder if you are talking about failure (type parameter) or Failure (data constructor).
I believe I am talking about "failure" the type parameter.
This instance obviously work
instance Functor (Result success) where fmap f (Success value) = Success (f value) fmap _ (Failure error) = Failure error
Flipping in `data` of course means you are to flip one of: a) instance or b) data constructor, e.g.:
instance Functor (Result success) where fmap f (Failure error) = Failure (f error) fmap _ (Success value) = Success value
Yes, indeed. But that's what I meant with 'now "a" has a different meaning'. I understand that to the compiler there is no practical difference between Result a b and Result b a ... but there is to me. :-) So am I to understand then that to be able to do the kind of "fmap" I want (i.e. one that affects the "success" value), I _have to_ make sure that I use "Result failure success" (and not "Result success failure")?