Was ∆ used in APL as a substitute for space because ECMA-17/ISO 2047 specified △ as graphical representation for space?

Question

Wikipedia on naming conventions in programming states (without source):

In APL dialects, the delta (Δ) is used between words, e.g. PERFΔSQUARE (…)

This is an unusual choice, but I notice that ECMA-17 says:

(…) in certain applications there is a need for a graphical representation of the control characters which are normally non-printing characters.

(…)

Code position	Character	Pictorial repr.	Alphanumeric repr.
2/0	SPACE	△	SP
7/15	DELETE	▨	DT

Both the standard and the first modern APL implementation became available in '68, but the standard described existing usage, and it would make sense to use an existing substitute character for internal "spaces" in names, since space and juxtaposition has syntactic significance in APL.

Is there any indication that APL's usage of ∆ for separating words in identifiers originates in common usage as a control picture for space? Alternatively, is there any indication of another origin of APL's unusual choice?

Answer 1

Some observations, not an entire answer.

In APL\360, only upper case letters, underlined upper case letters, and digits were allowed in user-defined names.

(There were a couple of exceptions: TΔfunction and SΔfunction, for program control, as pointed out by @Adam in comments. This does not seem to imply general use of delta in names)

By the time of APL2 (page 12) delta and underlined delta have been added, along with underscore and the negative symbol, and possibly lower case letters.

The presence of both underscore and delta suggest to me that this wasn't the 'unusual choice' of the OP's wording, since the usual choice -- underscore -- is catered for. That doesn't give the reason for including delta, but (opinion) I am inclined to not believe that its due to its use as a conventional space indication.

Indeed, the use of delta in actual programs seems contrary to its use of a conventional space mark. For all other space indications, we're supposed to understand that the mark in the manual is not what you see on the equipment.

Lastly, nothing in the linked APL2 manual suggests that delta is intended as a separator; it's just a valid character in names, of equal standing with the 'normal' letters. You can start a name with delta, or even use it as a name by itself.