APL ⎕IO index origin portability across implementations and saved workspaces

Answer the Question First

1. Most reliable initialization method: a profile file that the interpreter reads before the first REPL prompt. For each major APL implementation the file names and paths are:

• Dyalog – .aplrc (or aplrc in the installation directory)
• GNU APL – .gaplrc in the user’s home directory
• NARS2000 – .narsrc in the user’s home directory

Placing ⎕IO←1 in these files guarantees that the origin is set before any user script, module, or interactive session starts. Even when a workspace is loaded, the interpreter will have already applied the profile setting.

2. Fallback if the profile cannot be trusted: a wrapper shell script that launches the interpreter with a command‑line option such as -c '⎕IO←1' (Dyalog, GNU APL) or -i '⎕IO←1' (NARS2000). This forces the origin before any code runs, independent of the user’s profile configuration.

Load‑Time Origin Check

Insert a tiny library module that is imported as the first line of every source file (or the main program). The module contains a single guard that aborts if the origin is not the expected value:

⎕IO←1            ⍝ Expected origin for the project
⍝ Guard – abort if the environment is mis‑configured
if ⎕IO≠1:⎕SIGNAL 101,'Unexpected index origin: ⎕IO='⍕⎕IO

Because the guard executes immediately upon import, any program that starts with the wrong origin will terminate with a clear error message before other code runs, preventing silent mixing of 0‑based and 1‑based indexing.

Practical Steps for a Project

1. Decide on the target origin (commonly 1 for traditional APL code).
2. Add ⎕IO←1 to each implementation’s profile file. Document the file locations for team members.
3. Create a module named checkIO.apl containing the guard shown above.
4. Make every source file begin with (or if using ⎕CY for dynamic import).
5. Optionally provide a wrapper script run-project.sh that calls the interpreter with -c '⎕IO←1' to enforce the origin even if a user’s profile is missing or overridden.

With this pattern, the origin is guaranteed to be set before any user code, and any deviation causes an immediate, descriptive failure.

Assumptions and Caveats

• The profile file names and locations are standard for the major distributions; custom installations may use different names.
• Command‑line flags -c (Dyalog, GNU APL) and -i (NARS2000) are supported in the versions used by the team. If not, the wrapper should document the alternative.
• Latent expressions such as ⎕LX'⎕IO←1' are intentionally omitted because they are evaluated lazily and may be too late for early initialization.

Conclusion

Using a profile file (or, as a safety net, a wrapper script) guarantees that ⎕IO is set before any user code runs across Dyalog, GNU APL, and NARS2000. A minimal load‑time guard module ensures that any mis‑configured environment fails fast with a clear message, preserving consistent indexing behaviour across all implementations.