A trace function

[copumpkin]

There might well be one already, but I looked under the "Assertions and Debugging" section of the stdlib docs and only saw assertions.

Basically, I want a function that is equivalent to

local const(str, x) = x;

But that prints str to stderr as a side effect when forced. Very handy for debugging odd execution flow and the like.

Both Haskell and Nix call this function trace, but I'm open to other ideas.

[sparkprime]

Yes, this is a great idea and something I've had in the back of my mind for a while. You can sort of debug with error right now -- error foo will print out foo, but will also block execution.

One concern is how many implementation hacks this would expose. For example, at the moment, object-level locals are re-evaluated whenever you read a field of the object (as they are desugared into every field). They are not cached like regular thunks. This will give surprising output! However it is probably still useful.

[copumpkin]

Hah, didn't realize that. Yeah, I don't think anyone will be expecting particularly sensible properties around the output, other than it somehow representing what the VM is doing.

[sparkprime]

It is trivial to add a new builtin that prints str and returns x. I'm not saying that would be the best way to provide this feature, but it would give you something to play with if you wanted to do that.

[copumpkin]

I've been meaning to poke around the code for a while. I'll see if I can scrounge together some time one of these days and hack at it, if you don't get sick of waiting for me before that 😄

[sparkprime]

To save some time -- you need to add another builtin to jsonnet_builtin_decl in desugarer.cpp then modify vm.cpp to add the corresponding case to the inner interpreter loop

                 case FRAME_BUILTIN_FORCE_THUNKS: {

and I apologize for the hairiness of that code :)

[sbarzowski]

https://stackoverflow.com/questions/49742536/how-to-printf-in-jsonnet/49760216#49760216
The need for this keeps coming back. it's pretty clear people want to debug jsonnet sometimes :-).

[ahakanbaba]

I will push a PR soon.