In scripting languages one often wants to run the overall script like a standalone program, with a top 'main' function, yet also allow the script to be read into the REPL for editing, testing interactively and so on.
In Python a common way to do this is by checking the __main__ variable to determine execution scope.
GNU APL has the --script switch to enable invoking a .APL file and immediately executing it. In standard UNIX fashion the -- switch is also useful to divide GNU APL arguments (those for the interpreter itself) from those intended for the script. Using these two bits of information one can do the same thing in APL:
#!/usr/bin/env apl --script --
⍝⍝ Whatever the script should do by default when run from shell
⎕←'This is an APL script'
⍝ Parse args & run 'main' if not in REPL mode
⍝ in REPL mode, ⎕ARG is the script path itself
⍝ (Recalling that by default, ⎕IO = 1)
args←(⎕ARG⍳⊂"--")↓⎕ARG ⍝ Drop all args up to and including "--"
→(~(⊂'--script')∊⎕ARG)/0 ⍝ If "--script" not present, quit to allow REPL
⍝⍝ ⎕←"[",args,"]" ⍝ (just debugging to see args)
scriptFunction ⍎∊args ⍝ in --script mode, so run default func
checkMode ⍝⍝ check the ⎕ARG vector for --script mode