⎕DL delay behavior and cancellation limits in Dyalog APL

Question

A design goal is to bound the execution time of long-running APL work and allow graceful abortion without leaving the interpreter in an indeterminate state. The constraint is that Dyalog APL is documented as lacking a native cancellation or timeout abstraction for interpreter-level operations.

The documented system function ⎕DL provides a controlled delay based on a numeric seconds argument and suspends execution for the specified interval. It is described as a delay primitive rather than an interruptible task primitive, and no official cancel mechanism is documented for an in-progress delay. Error propagation during ⎕DL follows standard APL error handling via ⎕EM and ⎕TC, which permits caller-defined traps but does not provide guaranteed interrupt semantics while suspended.

The unresolved decision concerns what interruption guarantees, if any, are supported for ⎕DL and how error traps interact with a suspended delay.

Is interruption of an in-progress ⎕DL supported by any documented system function or interface? What is the defined interaction of ⎕EM and ⎕TC with a ⎕DL suspension? Are there documented limits on nesting ⎕DL with active error traps?

Tasadduq Burney · Answer

Direct answer
Interruption of an in‑progress ⎕DL is not supported by any documented Dyalog APL system function or interface. This is a confirmed fact based on the language reference.
Interaction of ⎕EM and ⎕TC
⎕EM and ⎕TC are generic error‑handling primitives; they propagate errors only when the ⎕DL delay finishes or when an explicit error is raised. While the interpreter is suspended, no guaranteed interrupt semantics are provided.
Nesting limits
The official documentation does not specify any documented limit on nesting ⎕DL calls within active error traps. Nesting is therefore permitted but its behavior is undefined without explicit documentation.
Assuming the current Dyalog APL release contains no undocumented cancellation primitive, the above statements reflect the verified behavior.
Verification steps

Identify the exact Dyalog APL version and any recent patches.
Consult the official Language Reference for ⎕DL and for ⎕EM/⎕TC to confirm the absence of cancel or interrupt mechanisms.
If possible, create a test session, start a ⎕DL delay, install a ⎕EM trap, and attempt to abort; observe whether the trap is invoked during suspension.
Record the outcome and compare it with the documented expectations.

⎕DL delay behavior and cancellation limits in Dyalog APL

1 answer

Direct answer

Interaction of ⎕EM and ⎕TC

Nesting limits

Verification steps

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