[Haskell] Eternal Compatibility In Theory

[robert dockins]

>>Is there a way to reliably and automatically check if two versions of
>>a haskell module are interface compatible?
>  
> No, because it would have to check whether the semantics of functions
> is the same, even if they are written differently.

Of course, we cannot expect the computer to examine the semantics.  We 
must rely on people to know when semantics change.

Suppose I want to ask the easier question "do these two text files 
implement haskell modules which are _type_ compatable?", how would I do 
it?  Ie, I want the test to fail if I change the type of some function 
foo, or if I add a method to a class declaration etc.