Debugging with Kio'

Kio' is the small core that surface Kio lowers to and then validates before backend emission. When a surface construct behaves differently than you expected, emitted Kio' shows the completed, independently checked program that the backend will see.

Use this guide for advanced debugging of elaborator calls, match!, tuple and sum elaborations, do blocks, placeholder lambdas, and backend emission surprises. For interactive navigation through names and types, start with kio repl.

Build the Kio' target

Kio' is a build target, not a diagnostic flag. Add a kio-prime target to the package's package file:

package app;

build {
  cache "out/.kio-cache/";

  target kio-prime {
    out "out/kio-prime/"
  }
}

Then build just that target:

kio build kio-prime

The output directory contains one Kio' text file per completed module. Those files are source, not an implementation AST dump, so you can read them directly. The kio build invocation that emitted them first checked the Lowered surface program, substituted every recorded elaboration and inferred annotation, and validated the assembled package again as standalone Kio'.

Read the lowered form

Kio' has no surface conveniences. The most useful thing to know is what disappears:

Surface Kio	What to look for in Kio'
Elaborator calls such as fit!, widen_sum!, narrow_prod!, one_prod!	Explicit intrinsic trees; the elaborator call itself is gone
Tuple literals and product projection	__pair__, __fst__, __snd__
Sum construction and branching	__left__, __right__, __either__
if / else	__if_then_else__
match!	A decision tree built from intrinsics and let-bound handlers
do blocks	Nested calls to the receiver's bind
Placeholder lambdas	Explicit lambdas with generated parameters
labels, literal aliases, operators, equiv	Lowered or removed before the Kio' boundary

If the Kio' is surprising, the issue is earlier than the backend: resolution, typechecking, elaboration, or lowering. If the Kio' matches your expectation but a backend's output does not, the issue is in backend emission or host integration.

A practical loop

1. Reproduce the behavior with kio check or a small equiv claim.
2. Build kio-prime.
3. Inspect the module that contains the surprising expression.
4. Search for the lowered helper, intrinsic tree, or generated lambda.
5. Compare that shape with the relevant surface construct in specs/language.md and the Kio' core in specs/prime.md.

For reusable compile-time claims, move the reduced expression pair into Testing with equiv. For name lookup and type inspection, use kio repl before dropping to Kio'.