Add a "never" value to the language

[denismerigoux]

It's useful to have a "never" value in the language :

• as a placeholder for an unimplemented expression ;
• as an assert false marker for pattern matching cases which you know should not happen ;
• etc.

Hence, I propose the addition to the language of a "never" value that has the syntactic kind of an expression. It should be polymorphic in type to placehold anything. At execution type, it should behave like an assertion failure.

Concerning the syntax, I propose : impossible (for French and English). But we should discuss this on the next syntax committee :)

[AltGr]

This would be extremely convenient ; but would certainly shift the balance on how users are constrained into handling all cases. Since they might always resort to defining "dummy" values in any case, this is much better.
(for example, one might write maximum of <list> or if list empty then impossible ; still verbose but explicit on the fact that the case is unhandled)

It may also increase the likeliness of having lazy evaluation diverge from eager evaluation (unless never is made always lazy, but that's probably not a good idea).

Other languages I know of use the following keywords: abort, error, die, panic. Not that these are verbs, which conveys something different compared to never or impossible.
This said, error might actually be good here in my opinion.

[AltGr]

Forgot to mention this, which might be a better choice: fail (or failure)
In French, in any case, I think échec sounds best.

[AltGr]

One half of the Syntax Committee says "go with impossible"