Debugging

MeTTa LSP has two complementary debugging views.

The Trace command shows the grapher reduction states for a query:

metta-lsp trace examples/13-trace.metta "(sum-to 3)" --max 8

The VS Code command MeTTa: Why Did This Reduce? uses the @metta-ts/debug engine trace. Put the cursor on a runnable ! query or bare call, then run the command. If the cursor is not on a runnable form, the server explains the file's last runnable query.

The output channel reports:

• result atoms,
• grounded reducer counts, such as top-k-by-atom: 1,
• higher-order specialization, such as twice -> twice$inc,
• stack-overflow cut points,
• total reduction count.

For a small queue reducer:

(: Score (-> Expression Number))
(= (Score (item $name $score)) $score)
(= (Score ()) -99999.0)
(: LimitSize (-> Expression Number Expression))
(= (LimitSize $L $size)
   (top-k-by-atom Score $size $L))
!(LimitSize ((item a 1) (item b 3) (item c 2)) 2)

why reports the result ((item b 3)), the grounded reducer top-k-by-atom: 1, and the engine reduction count.

Debug Adapter

The VS Code debug adapter still steps frames with the grapher reducer, so Next moves through expression states. It also exposes a Trace scope from the engine trace. The scope contains reductions, events, grounded:<op> variables, specialized[N] entries, and overflow[N] cut points.

When the engine reports a native stack overflow, Continue stops at the overflow cut-point atom and prints stack overflow cut-point: ... in the Debug Console. The Reduction scope still shows the current expression and step count.