[dreamcompiler]

You're right. The standard leaves the interpretation of type declarations in CL up to the implementation. I was coming from a CCL perspective, and I should have checked the others.

In CCL:

  ? (defun foo (a) (declare (type (integer 0 100) a)) (+ a 10))
  FOO
  ? (foo -1)
  9

It's dangerous to assume the effect type declarations will have in CL. You have to test it.

[lispm]

Even in CCL it depends on the compiler settings:

  ? (declaim (optimize (safety 3)))
  NIL
  ? (defun foo (a) (declare (type (integer 0 100) a)) (+ a 10))
  FOO
  ? (foo "bar")
  > Error: The value "bar" is not of the expected type (MOD 101).
  > While executing: FOO, in process listener(1).
  > Type :POP to abort, :R for a list of available restarts.
  > Type :? for other options.
  1 > 

Type declaration added -> runtime safety increased...

Best read the implementation manual to see how it deals with optimization values and type declarations.