Notice that patterns are recursive (case (iv) from the previous page). In determining whether two patterns "match", we can therefore use a recursive algorithm, in pseudocode:
fun matches(left, right) =
if left is a constant then
if left = right then true else false
else if left is a variable then true
else if left is a wildcard then true
else if left is a compound constructor then
if right has the same top-level constructor then
if each component of left matches each component of right then
true
else
false
else
false
else (* left may not appear in a pattern! *)
raise some kind of error
Trace the execution of the pattern-matching algorithm over the following value bindings:
val 3 = 3;
val 3 = 4;
val _ = "walrus";
val (3+5, x) = (8, 9);
val x :: _ :: xs = [2,3,4];
val ({baz="hi", foo=seal}, fish::nil) = ({foo="bar", baz="hi"}, [827]);
val [2,3,x] @ nil = 2::3::4::nil;