Hello,

The following code does not type check:

var lst = empty
lst := link(1, lst)
print(1 + lst.first)

It complains that lst has type (List<?-1> % is-empty) and thus has no field named first.

On a related topic, I suspect that Pyret's type checker does not catch property the problem of polymorphic references (see the history of ML). For instance, the following code does type check:

fun head<a>(l :: List<a>) -> a:
  cases (List<a>) l:
    | empty => raise("head")
    | link(x, _) => x
  end
end

var lst = empty

lst := link(1, lst)
print(head(lst))

fun foo(n :: Number) -> Number: n end
lst := link(foo, lst)

print(head(lst)(1))

but it should not. Removing the second assignment leads to a runtime error (trying to apply something which is not a function).