Gaining an Element from a List of Tuples in Haskell

Question

So I have a list of tuples defined like so:

type Domino = (Int,Int)
data End =  L|R  
type Board = [Domino]
d :: Domino
d= (4,5)
b :: Board
b= [(1,3),(3,3),(3,4)]

In my function I need to be able to gain the first part of the board. So for example I can head the board to get the domino (1,3) as a tuple but I've been trying to get the integer one from this and simply failing. I need to be able to compare that integer value. My question is simply how do you acquire the first part of a tuple in an a list as everything I have done and searched keeps failing. Apologies if this is really simple, I am new to haskell. This is my function code, obviously with a bunch of errors

goesP :: Domino->Board->End-> Bool
goesP _ []_ = True
goesP dom bor L = (if head bor fst  == fst dom then True else if last bor == snd then True else False)

Answer 1

From your question, it doesn't look like you're interested in a generalised function, so these will work:

fst $ head b will get the very first Int in that list, and snd $ last b will get the very last.

How you compare them then is up to you.

Answer 2

Something as simple as

goesP :: Domino -> Board -> End -> Bool
goesP _ [] _ = True
goesP _ ((a,b):doms) _ = a

will work, as you can pattern match for the list being empty, and then being a pair cons the rest of a list, and extract the first element out of that pair.

I'm not sure what you're trying to do with the End type in there as well, but I left it in there in my example (although I do nothing with it).

Answer 3

As you may or may not know fst and snd only work for 2-element tuples so for you it should be enough:

fst (a,b) = a

You can also write you own:

get3th (_,_,a,_,_,_,_,_,_,_) = a

As you can see you may want to define your own type.