Thin Tcl/Tk binding (import tk)

• Type: feature (Track B — library; lib/pcl/tk.pas). Enables the NilPy IDE demo ([[feature-demo-nilpy-ide]], Track E).
• Status: done
• Opened: 2026-07-17.

Approach

The whole GUI is Tcl/Tk command strings through TkEval — the CPython-tkinter model minus tkinter's weight. lib/pcl/tk.pas binds the system libtcl8.6/libtk8.6 sonames with hand external clauses (no -dev headers, no C-import-registry change). Thin by construction: Tk does all widget/layout/event work.

v1 — LANDED (commits f2dd8206, tk.pas StrPas fix)

• TkInit / TkEval / TkMainLoop over the Tcl/Tk C API. DT_NEEDED pulls libtcl/libtk; runs under xvfb.
• examples/tk/hello.npy: import tk from Nil-Python opens a window + runs the event loop. Proven end-to-end.
• Event model settled — POLL, no C trampoline. A widget writes a Tcl variable (-command {set action run}); NilPy polls it with TkEval("update") + TkEval("set action") in its own loop. Proven working. This keeps the wrapper thin — no Tcl_CreateObjCommand/callback plumbing, pure TkEval.
• Found + filed along the way: [[bug-pascal-ansistring-cast-of-cdecl-call-result]] (silent, the StrPas fix), [[feature-nilpy-break-continue]] (NilPy loop-control gap).

Remaining

1. tkinter-shaped convenience surface — thin helpers so common Python tk snippets run (e.g. Text, Button, pack, bind, mainloop mapping to one TkEval each). Stays thin: name→command mapping, not a widget reimplementation. Where NilPy's v1 subset limits compat (kwargs, str+str, callbacks), that is a NilPy limit, not the wrapper.
2. ttk themed widgets — one TkEval("ttk::style ...") away; closes most of the dated-look gap for free.
3. Gate smoke — an xvfb-wrapped hello smoke wired into test-nilpy/lib-test (testmgr already models the xvfb resource). Auto-close via after so it terminates.

Acceptance

• v1: import tk opens a window from NilPy and runs the loop (DONE).
• Surface: a representative tkinter-style snippet (label + button + poll loop) runs.
• A gated xvfb smoke; make test-nilpy green.

Non-goals

• Not full tkinter parity — the common subset, thin.
• Not a C-callback event system — the poll model is the deliberate thin choice.

Log

• 2026-07-20 (Track B) — Item 3 (gate smoke) landed. examples/tk/hello.npy now runs under xvfb-run as part of make lib-test, asserting its exact output. It SKIPs cleanly when xvfb-run or the system libtk8.6 is missing — an absent GUI stack on a build host is not a code defect, and reddening the Track B gate over one would just train people to ignore it.

  It cannot hang the suite: the .npy closes itself via after 400 {destroy .}, so termination does not depend on anything the harness does.

  Wired into lib-test rather than test-nilpy on purpose — test-nilpy builds with $(COMPILER) (a fresh compiler), and Track B builds with $(PXX_STABLE) and never rebuilds. Same coverage, correct lane.

  Items 1 (tkinter-shaped convenience surface) and 2 (ttk themed widgets) remain open; both are additive wrapper work on top of TkEval with no blocker.

• 2026-07-20 (Track B) — Items 1 and 2 landed; the ticket is complete.

  Item 1, tkinter-shaped surface (lib/pcl/tk.pas): TkTitle, TkLabel, TkButton, TkEntry, TkText, TkFrame, TkPack, TkGrid, TkBind, TkDestroy, TkAfter, plus TkGetText/TkSetText. Each is exactly one TkEval with arguments interpolated — a name-to-command mapping, no widget objects, no state that can drift out of sync with Tk. Where a helper is missing, TkEval still does the job in one more line, and that escape hatch is what keeps the wrapper from growing into a widget layer.

  TkGetText/TkSetText are the one place with logic: they read winfo class and pick entry get vs text get 1.0 end-1c vs cget -text, so callers do not branch on widget kind. That is convenience, not abstraction — it maps to the same three commands a human would type.

  Item 2, ttk: TkThemeUse, TkThemeNames, TtkLabel, TtkButton, TtkEntry, TtkFrame. As predicted, one command prefix.

  Values are interpolated in Tcl {braces}, so a caption containing spaces or brackets stays a single argument. Known limit: a caption containing an unbalanced brace still breaks. That is inherent to a thin string-passing wrapper, and it is documented at the declaration rather than papered over — starting to escape Tcl here would be the first step toward the widget layer this deliberately is not.

  examples/tk/widgets.npy demonstrates the surface and is gated headless alongside hello.npy in make lib-test, asserting the round-tripped text of an entry, a text widget and a label (all three read back through TkGetText).

• 2026-07-20 — resolved, commit HEAD.