Right now to understand whether a function is failable or not, you have to read through its body to find uses of ? or fail. That, or run the compiler and see if it fails with an error message pointing out to a failable function.

Considering that the way a function is called depends on if it is failable, I believe it is a good idea to make it easier to tell them apart — by putting some indication into the function signature.

To be more specific, I propose to make the error a part of the return type signature, using ? at the end of the type:

fun failing(name: Text): Null? {
    $command$?
    parse(name)?
    fail 1
}

// Not sure if this is necessary, if the function doesn't have types anyway
fun failing(name): ? {
    $command$?
    parse(name)?
    fail 1
}

Although, there may be a better way to do this.