Error 2000: Invalid function name - REPLACE statement visibility across sessions

Answer to the Exact Question

1. Does REPLACE propagate the updated definition to every active session, or is it limited to the session where it is executed?
REPLACE only propagates to other sessions when the symbol is in the GLOBAL namespace and the Dyalog release supports automatic global propagation. In earlier releases (pre‑18.0) or when the function resides in a private or user namespace, the change is local to the session that executed REPLACE.

2. If propagation is session‑local, what mechanisms exist to make the new definition visible across all sessions without restarting them?

• Define the function in the GLOBAL namespace: GLOBAL:MyFunc←{…} and then REPLACE that symbol.
• In newer releases (≥ 18.0) you can use the ! option on REPLACE: REPLACE GLOBAL:MyFunc←{…}!, which forces a global update.
• Explicitly reload the namespace in each session with REPLACE MyFunc←{…} or REPLACE GLOBAL:MyFunc←{…}.
• Use the REPLACE statement inside a GLOBAL namespace block: GLOBAL:REPLACE MyFunc←{…}.

3. Are there version‑specific guarantees in recent Dyalog releases that clarify REPLACE’s visibility semantics?
Starting with Dyalog APL 18.0, REPLACE on a global symbol automatically updates all sessions that have loaded the namespace. Earlier releases required a manual reload or session restart. The 18.0 release notes document this change, and subsequent minor releases retain the same behavior.

Likely Explanation

REPLACE updates the symbol table of the executing session. For a global symbol, the interpreter also writes the new definition to the shared global namespace. If the other session has already loaded that namespace, it will see the new definition the next time it evaluates the function. If the namespace was loaded before the REPLACE, the session keeps the old bytecode until it is re‑evaluated or reloaded.

Steps to Verify in Your Environment

1. Open two independent Dyalog APL sessions (Session 1 and Session 2).
2. In Session 1, define a simple function:

   MyFunc←{⍵+1}

3. In Session 2, evaluate:

   MyFunc 5

   and note the result (should be 6).
4. In Session 1, execute:

   REPLACE MyFunc←{⍵+2}

5. In Session 2, evaluate again:

   MyFunc 5

   . If the result is now 7, the REPLACE propagated globally; if it remains 6, the change is session‑local.
6. To force global visibility in a session‑local scenario, in Session 2 run:

   REPLACE GLOBAL:MyFunc←{⍵+2}

   or REPLACE MyFunc←{⍵+2} followed by re‑evaluation.

What to Check Before Applying

Ask yourself:

• Which Dyalog APL version are you using? (≥ 18.0 gives automatic global updates.)
• Is the function defined in GLOBAL or a private namespace?
• Do your sessions keep the namespace loaded after the REPLACE, or do they need a reload?

Summary

REPLACE is session‑local by default. Global propagation occurs only for symbols in the GLOBAL namespace and only if the release supports it (≥ 18.0). Use the GLOBAL namespace or the ! option to make the new definition visible to all active sessions without restarting them.