I need to write a function checkNode that accepts a graph in form of:

[(1,[2,3]), (2, [3,4])]

as an argument where the first integer in a Tuple is a Node and the list shows all the other Nodes the initial Node is connected to.

checkNode has a second parameter, an Integer.

checkNode [(1,[2,3]), (2, [3,4])] 2 outputs True, because 2 is a node in the graph, whereas

checkNode [(1,[2,3]), (2, [3,4])] 7 outputs False.

Here's what I have so far:

checkNode :: (Eq a, Num a) => [(a, [a])] -> a -> Bool
checkNode [] _ = False
checkNode [(x,_):d] n = if (n == x) then True else checkNode d n

and here's the error:

test2.hs:53:12: error:
    • Couldn't match expected type ‘(a, [a])’
                  with actual type ‘[(a, [a])]’
    • In the pattern: (x, _) : d
      In the pattern: [(x, _) : d]
      In an equation for ‘checkNode’:
          checkNode [(x, _) : d] n = if (n == x) then True else checkNode d n
    • Relevant bindings include
        checkNode :: [(a, [a])] -> a -> Bool (bound at test2.hs:52:1)    
| 53 | checkNode [(x,_):d] n = if (n == x) then True else checkNode d n    |            
                  ^^^^^^^

CodePudding user response：

The solution was to alter the way the function reads the graph input:

checkNode :: (Eq a, Num a) => [(a, [a])] -> a -> Bool
checkNode [] _ = False
checkNode ((x,_):d) n = if (n == x) then True else checkNode d n

namely enclose the graph in the round brackets.

CodePudding user response：

If you want to only check if there is an edge leaving that item (so the query is in the first item of one of the 2-tuples), you can check with:

checkNode :: (Foldable f, Eq a) => f (a, [b]) -> a -> Bool
checkNode graph q = any ((q ==) . fst) graph

this will check if any item in the Foldable (that can be a list) is a 2-tuple with q as first item.

This will not check if there is an incoming edge for a node, so 4 is not a node of the graph [(2, [4])] for example.