interpolation in assertion failure message

[CeylonMigrationBot]

[@gavinking] When we implemented assertions, we left one issue for later: how to interpolate expressions into the assertion failure message. Right now, we can write:

"exponent must be non-negative"
assert (power>=0);

Where the string literal is an abbreviated doc annotation. I've defended this solution on the grounds that it lets ceylondoc include the preconditions on parameters in its generated documentation. However, in truth, we're eventually going to need interpolation here. So the question is: what should the syntax be. Clearly, the following is one possibility:

"exponent ``power`` must be non-negative"
assert (power>=0);

Could ceylondoc do something reasonable with this? I don't see why not. But is it just to "weird" to suddenly let you have interpolation in a doc string, all of a sudden, in one special case?

Alternatively, we could let you write something like this:

assert (power>=0) "exponent ``power`` must be non-negative";

or this:

assert (power>=0) => "exponent ``power`` must be non-negative";

The => is not strictly necessary. But for some reason I find it more natural to format across multiple lines. This:

assert (power>=0) 
        "exponent ``power`` must be non-negative";

is to me less natural than this:

assert (power>=0) 
        => "exponent ``power`` must be non-negative";

Whatever, either option would be acceptable to me.

WDYT?

[Migrated from ceylon/ceylon-spec#586]

[CeylonMigrationBot]

[@gavinking] I've so far had zero (0.0) feedback on this issue. Thoughts?

[CeylonMigrationBot]

[@tombentley] I do like

assert (power>=0) => "exponent ``power`` must be non-negative";

assuming ceylondoc is still allowed to document this when it occurs as a precondition.

[CeylonMigrationBot]

[@gavinking]

assuming ceylondoc is still allowed to document this when it occurs as a precondition.

Well, that's the real question: what is ceylondoc going to do with this shit? I suppose it can render a link to the parameter, no?

[CeylonMigrationBot]

[@DiegoCoronel] We had a similar discussion in #3725 , i did not know about this issue.

[CeylonMigrationBot]

[@gavinking] Well, I kinda implemented this in branch 586, but now I'm starting to doubt the usefulness of it. I think it's better to at least slip this feature from M6.

[CeylonMigrationBot]

[@gavinking] Not for 1.0.

[CeylonMigrationBot]

[@ncorai] Now that all validation is done through assert statements, it would be really nice to have interpolation in the message for milestone 1.1. The longer we wait the more the existing code will be littered with not-quite-useful error messages.

[ncorai]

I regret to say that release 1.5 feels as far away today as it did 3 years ago. How about 1.3.1 instead? Pretty please 😃

[tombentley]

Yes it doesn't seem so hard to do and would be really useful.

[bjansen]

Today I converted Java code to Ceylon and needed an interpolated assert message, but unfortunately I had to write that message as a comment instead.

FTR I like the fat arrow used in the initial comment, it kinda looks like what I would write when taking notes on paper:

something something => explanation

[gavinking]

@bjansen

but unfortunately I had to write that message as a comment instead.

Huh? What's wrong with if (...) { throw AssertionError(....); }?

[bjansen]

I hadn't thought of this :)

[gavinking]

😀

[gavinking]

I have implemented support for this in the typechecker. However, the backends need to be modified to support this (which I assume is a pretty trivial change).

@tombentley? @chochos?

[gavinking]

Now available on the 3692 branch.

[gavinking]

I've implemented support in the JVM backend. JS still to go.

[gavinking]

@lucaswerkmeister note that this change will affect ceylon.ast

[gavinking]

Done!

[xkr47]

Seems this was the chosen syntax?

"exponent ``power`` must be non-negative"
assert (power>=0);

[lucaswerkmeister]

Thanks for the heads-up @gavinking! (Formatter works without changes, nice!)

[chochos]

Looks really cool. Reminds me of groovy's "power assert". Just updated the assertion messages in the check module