September 22, 1999

Strong Typing

Slide #30

Huh???

        val sort = fn : 'a list -> int list

How is this possible?

One way the impossible can occur is if it never can occur

``Dance with you? Yeah, when Dr. Ruth is elected president.''

``But Dr. Ruth isn't an American citizen.''

``Right!''

``When the function returns, it'll return a list of integers.''

``But it can't possibly return a list of integers.''

``Right!''

In fact, this function has a bug. It runs forever on any nonempty list.

Type Checking found an infinite loop bug!

!!!!!!!!!!