Searches a list of pairs for a pair whose first component equals a
specified value, then returns the second component of the pair.
assoc x [(x1,y1),...,(xn,yn)] locates the first (xi,yi) in a left-to-right
scan of the list such that xi equals x. Then yi is returned. The lookup
is done on an eqtype, i.e., the SML implementation must be able to decide equality
for the type of x.
Fails if no matching pair is found. This will always be the case if
the list is empty.
- assoc 2 [(1,4),(3,2),(2,5),(2,6)];
> val it = 5 : int