APL (codepage)

The APL code page is an EBCDIC-based code page used specifically to write programs written in the APL programming language.

Due to its origins on IBM Selectric-based teleprinters, APL symbols have traditionally been represented on the wire using a unique, non-standard character set. In the beginning, there were few terminal devices which could reproduce them—the most popular ones being the IBM 2741 and IBM 1050 fitted with a specific APL print head. Over time, with the universal use of high-quality graphic display, printing devices and Unicode support, the APL character font problem has largely been eliminated.

Most APL symbols are present in Unicode, in the Miscellaneous Technical range, although some APL products may not yet feature Unicode, and some APL symbols may be unused or unavailable in a given vendor's implementation. Missing from Unicode are the traditional underscored alphabetic characters; their usage has been eliminated or deprecated in most APL implementations.

Nowadays Unicode allows APL to be stored in text files, published in print and on the web, and shared through email and instant messaging. Entering APL characters still requires the use of either a specific input method editor or keyboard mapping, or of a specific touch interface. APL keyboard mappings are available for free for the most common operating systems, or can be obtained by adding the Unicode APL symbols to existing keyboard map.

Note the mnemonics associating an APL character with a letter: ?(question mark) on Q, ⋆(power) on P, ρ(rho) on R, ⊥(base value) on B, ⊤(eNcode) on N, ∣(modulus) on M and so on. This makes it easier for an English-language speaker to type APL on a non-APL keyboard, providing one has visual feedback on one's screen. Also, decals have been produced for attachment to standard keyboards, either on the front of the keys or on the top of them.

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