[To be discussed] Allow match guards as boolean expressions

[marzipankaiser]

When coming from other languages, it is surprising that you can't, e.g. use not(x is Cons(_,Red())) in an if/while. Also, splitting up longer conditionals using bindings is not possible when using match guards. Using match guards instead of equality predicates is especially useful on llvm, since there is no infixEq on data types, and no generic equals implemented.

Examples

while(not(asBool(cmd is Some("quit")))) {
     cmd = readCmd()
     //...
}

Proposal

Allow x is p in expression position and desugar it to if(x is p) { true } else { false }.

Considerations and Alternatives

• It might be clearer to add some syntax for this, e.g. asBool(x is p) (better name required)
• Regarding binding, the behaviour is slightly unexpected when changing to booleans. This could be ameliorated by adding a check that the converted pattern does not bind anything. Note that this disallows certain patterns, though (e.g. (x is Some(y) and valid(y)).
• For the usecase of while(not...), it would be possible to add until, e.g.