APL Functional Symbol I-Beam

It gives APL access to system-related services. A numeric selector to ⌶’s left chooses a service; its other arguments provide the data. Dyalog 21.0 documents services for running an expression, serializing an array, and controlling the interpreter.

The name describes the glyph’s shape: it resembles an I-beam, a structural steel beam with flanges at top and bottom. In APL code, ⌶ is a route from array calculations to the interpreter or the larger system around it.

Why was the APL I-beam created?

It gave system programmers a route to the host computer. Kenneth Iverson began developing APL as a mathematical notation in 1957; his book A Programming Language appeared in 1962. APL/360, IBM’s System/360 version, was running inside IBM by 1966. System programmers first used the two-argument form, called dyadic, to execute machine instructions; the later one-argument form, called monadic, let other users request details such as available workspace and time of day.

Later APL dialects moved some environment tasks into system functions and variables, while I-beam calls stayed tied to particular interpreters. Dyalog 21.0 distinguishes withdrawn services from operations its interpreter does not support, so old calls can break when code is ported.