Del

∇

The glyph del is a triangular symbol resembling an inverted Greek delta and is represented by the Unicode character ∇, "nabla". Its uses include:

• In APL\360 and most other APLs, it is used to enter and exit the line editor which provides a function definition mode.
• In Dyalog APL, NARS2000, ngn/apl, and dzaima/APL ∇ represents the current dfn, or the function derived from the current dop, such that anonymous functions and operators may recurse.
• In Dyalog APL, ∇ is used to delimit tradfn members of scripted objects.
• In APL2, APLX, and APL*PLUS, a variant glyph ⍫, consisting of a Del ∇ overstruck with a Tilde ~, is used instead of the closing ∇ to "lock" the function definition. This prevents viewing or modifying the function's definition while still allowing the function to run. Dyalog APL instead offers the Lock Definition (⎕LOCK) system function which provides a more fine-grained management of locked behaviour.
• In APL*PLUS, ⍝∇ marks a special "public comment" which can be retrieved with a dedicated system function even when the function is locked. This is also used to distinguish internal documentation from other comments.

It should be noted that Del (∇) has no connection to Delta (∆) other than visual similarity. The latter is treated as a normal identifier character in all respects.