On Mon, Mar 9, 2009 at 10:37 AM, Eugene Kirpichov
I mean, there is no way to write a firstCoord function so that it would work, for example, on '\y -> P 42 y' and yield 42.
Except for this one:
firstCoord proj = case (proj undefined) of P x y -> x
However, this requires proj to be non-strict in its remaining argument. But this will actually work if you pass "P x" to it for some x, because it *is* non-strict in the remaining argument.
That's brilliant. Any function which behaves like "P x" but is strict in the second argument is in fact a different function. So this trick is a semidecidable pattern for this function. Bring in the unamb, so we can do interesting things with semidecidable predicates :-) (I still don't like the proof obligation of unamb, and would like to see it picked up to a higher level of abstraction where the usage is always correct) Luke