[Grace-core] Structural Typing vs Specifications

[Andrew P. Black]

On 14 Oct 2011, at 10:14, Kim Bruce wrote:

> I agree with Andrew, though I have additional reasons.  I agree that it is a non-problem in practice.  I've never seen anyone confusing a cowboy drawing a gun and an artist drawing a picture in a program.
> 
> My second reason is that to actually enforce the conformance of pre- and post-conditions is undecidable and would require a theorem-prover to come close to being useful.  While I like Eiffel's requirements to "or" preconditions on to those of the superclasses methods and the "and" postconditions, this is not likely to be of any use with structural types.
> 


I was imagining a mode — maybe the default mode, maybe not — in which pre-and post conditions were checked dynamically.    My point was that the self.gun.hostered message would be an error.   Not an assertion failure, but an error.

	Andrew
  
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