and-ing, or-ing?

[chris.danx]

Hi,

How do you "and" two truth states?  In the following "addEdge" function 
It is supposed to add an edge if and only if both vertices exist...  the 
function existsVertex is defined to return true if a vertex exists in a 
given graph.


%% return true if given vertex is in the graph.
%%
existsVertex(N, {graph, []}) ->
     false;
existsVertex(N, {graph, [{N,Y}|Xs]}) ->
     true;
existsVertex(N, {graph, [_|Xs]}) ->
     existsVertex (N, {graph, Xs}).


%% create connection between given vertices
%%
addEdge (N, M, {graph, []}) ->
     nil;
addEdge (N, M, {graph, L}) ->
     bothExist = ???
     if bothExist ->
	    {graph, []};
        true ->
	    {error, {graph, L}}
     end.


It's not complete yet, the return values are just temporary until I 
figure out how to check the vertices exist...


Cheers,
Chris

p.s. I might add a function which returns an existing {vertex, [edge]} 
tuple to make it more efficient, but for now I'm keeping it simple.