I have such a data type :
data Node a = Node
{ label :: a,
adjacent :: [(a,Int)] } deriving Show
Example : ( Node 'a' [ ( 'b' , 3 ) , ( 'c' ,2 ) ] )
I want to get the label from this structure, I wrote this function (and several other combinations which I thought might work) :
giveLabel Node a [(c,b)] = a;
But I keep getting errors. Can you tell me how should I change my function? Thanks
giveLabel (Node a [(c,b)]) = a
Is the syntax you want - defining functions uses the same rules of precedence as calling them, and according to those rules, you defined a function giveLabel with three arguments (Node, a, and [c,b]); and that was illegal because in that context Node was missing arguments.
Even that probably isn't what you want - the pattern [(c,b)] only matches lists with exactly one item in. Since you don't care about the list of neighbours you can write:
giveLabel (Node a xs) = a
...where xs will bind to the whole list of neighbours; but actually since you don't even care about that, you can write:
giveLabel (Node a _) = a
...where _ is a useful way of pattern matching against a parameter you aren't going to use.
