Hi
I understand this has nothing to do with type checking, but why can't the compiler give a warning about this? Or is this by design or because it is impossible to check with more complex recursive data types?
Take a look at Catch from Neil Mitchell: http://www-users.cs.york.ac.uk/~ndm/catch/ .
Using the released version of Catch on the example you gave: Analysing Checking [1/1]: Main: Pattern match failure in function at 5:1-5:10. Partial: "Main.f" Partial: "Main.main" Partial: "main" Answer: 0 This says: the error message you will get is about a pattern match on line 5 (that's where 'f' is in the example program). The list of partial functions, in some kind of call-stack order, is Main.f, then Main.main - i.e. your main function calls f which is partial. Answer 0 means "the necessary precondition for safety is false" - or its not safe at all. If you turn on logging Catch will additionally tell you that the precondition on 'f' is that the data structure must be a 'A' constructed value. Thanks Neil