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Would you then accept a worthless value language if the type language was perfect?

Anyway, this was quite an enlightening comment for me: Agda/Idris/Coq's type language is technically equivalent to its value language.

Given a sufficiently interesting type language, could we statically type the problem I posed in another comment here

https://news.ycombinator.com/item?id=8966065

?



I'm not sure anyone has developed a language with a great type level language and a worthless value level one. But that said, theorem provers (like Coq) essentially consider value-level computation as vestigial: nobody cares about running Coq programs, just checking them.

To encode what you're looking for requires type-level natural numbers and a reasonable notion of type-level equality. These are fairly non-trivial. You can encode this in Haskell, but type leve" equality is a bit hard to work with so you may suffer some pains (I know, I've done it several times!). Agfa and Idris would be where I would look to see this sort of thing materialize.


That depends. Can I get the results directly from the compiler, or must I run the actual program?

About your example, it's not idiomatic Haskell. If you try to code the same exact algithm in Haskell, you'll have the same exact problem (ghc has a warning for it); if you recode it in fmap or list pattern matching that are idiomatic to Haskell, you'll avoid the problem.




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