For 2 datatype like this.
datatype flight = F of int * int;
datatype flights = Fs of flight list;
I want to make a function that can check if (a,b) is in flights or not.
Example:
val reachable = fn : flights * (int * int) -> bool
reachable (Fs [F(0,1), F(1,0)], (0, 1));
val it = true : bool
I have no idea how can i compare a (int*int) to flights.
I use
fun get_f_x (F(x,y)) = x;
to get the first integer in flight.
But when i try to do the same thing to flights.
I do it like below:
fun test_hd(Fs[i,_]) = i;
In order to get the first flight out for the flights. But it can only take flights with 2 flight only (Fs [F(0,1), F(1,0)]) If flight have more than 2 element, it show error.
Similarly
fun test_tl(Fs[_,i]) = i;
have the same problem.
How can I make a hd and tl for flights? Or what is the correct way to think about this problem?
Not having seen the errors you're encountering, it's hard to say, but looking at the code you've shown, I suspect in-exhaustive pattern matching is a factor. With that in mind, let's look at a way of breaking down the recursion and pattern matching involved.
If you want to see if a particular flight is in flights, first we need to start with what we absolutely know: If a flights value contains no values in its list, then the answer must be false, no matter what we're looking for.
fun reachable(Fs([]), _) = false
What if there is something in that list? Well, let's check the first element to see if it's what we are looking for.
fun reachable(Fs([]), _) = false
| reachable(Fs(F(x) :: xs), flt) = flt = x
But what about the rest, if that one doesn't match? We'd need to check the rest of the list.
fun reachable(Fs([]), _) = false
| reachable(Fs(F(x) :: xs), flt) =
flt = x orelse reachable(Fs(xs), flt)