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by chongli 4196 days ago
You don't have to see that the proof is proving what you intend; the type-checker does that for you. This is why some people sometimes refer to type checkers as small theorem provers.

As for your assert example, how do you know append will work as you expect for all possible strings (including null characters or weird unicode edge cases)? With a proof you will know.
1 comments
vertex-four 4196 days ago
> With a proof you will know.

But I could just as easily accidentally write a proof which proves something else, couldn't I? This is complex code expressing complex ideas - a type system would certainly help, but it can't tell me that I'm proving the wrong thing.

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chongli 4195 days ago
But I could just as easily accidentally write a proof which proves something else, couldn't I?

Then your proof would be rejected by the compiler. Remember, the types specify your proposition: i.e. what you are intending to prove. The actual proof itself is the function you implement for that type.

As for whether you're proving the right thing or the wrong thing, a type system is no less helpful than a test suite. The advantage of a type system is that it checks whether your types are consistent within the entire program rather than in merely the specific test you're running.

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vertex-four 4195 days ago
> Then your proof would be rejected by the compiler.

Only for some types of mistake, surely. If that was always the case, we'd have a compiler that could read minds.

> As for whether you're proving the right thing or the wrong thing, a type system is no less helpful than a test suite.

Really? I can write down in my test suite, "assert (add 1 1) == 2". Anyone can come along and look at that and make sure it matches the spec. We can add additional tests for various bounds, and possibly use a quickcheck-like tool in addition, and for 99.99% of use cases be happy and confident that we're at least writing the right thing.

What's the type for that and does it actually have any resemblance to "the add function adds two numbers together", or do I have to read a couple of papers and have a background in university-level math to convince myself that the proof actually does what it says?

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