**APL-20 PANDAMONIUM REIGNS!**

**APLSF** (System Function) is an implementation of the APL programming language for TOPS-20 that is available on TWENEX.

===== In and Out =====

To start the APL interpreter, type the following from EXEC:

  @APLSF
  
At the ''terminal..'' prompt, enter ''tty'' for a non-APL keyboard.

Leave APLSF by entering one of the following commands (APLSF converts all alphabetic input to upper-case):

|'')MON''|Terminate APLSF and return to EXEC.|
|'')OFF''|Terminate APLSF and log out of TWENEX.|
|'')CONTINUE''|Save active workspace to file ''DSK:CONTIN.APL'', terminate APLSF, log-out from TWENEX. The saved workspace will be loaded automatically the next time you run APLSF.|

To interrupt APLSF during execution, press ''Ctrl-C'' once or twice. 

Press ''Ctrl-C'' five (5) times to suspend APLSF and return to EXEC. Resume APLSF with command ''@REENTER'' or ''@CONTINUE''.


===== APL Character Set =====

Although APL characters cannot be displayed or input when the terminal is in TTY mode, APLSF provides ASCII character string equivalents for all the APL characters as well as "escape mode" (single character preceded by "@") keys that correspond to standard APL keyboard mappings. ASCII and escape mode sequences can be intermixed.

^ APL \\ Character ^ Name ^ ASCII ^ Escape \\ Mode ^ ^ APL \\ Character ^ Name ^ ASCII ^
| ∧ | and | & | | | ⍒ | Grade Down | .GD |
| <- | assignment | _ | | | ⍋ | Grade Up | .GU |
| ÷ | divide | % | | | ⌶ | I-Beam (histogram) | .IB |
| ⋆ | exponentiate | * | @E | | ⍟ | LoGarithm | .LG |
| × | multiply | # | | | ⍲ | NaNd | .NN |
| ? | question (roll and deal) | ? | @Q | | ⍱ | NoR | .NR |
| ' | quote string | ' | @K | | ⍀ | back expansion | .CB |
| ↑ | take | %%^%% | @Y | | ⊖ | (Circle) Rotate | .CR |
| ∣ | residue (ABsolute value) | .AB | @M | | ⌿ | back scan | .CS |
| α | ALpha | .AL | @A | | ⌹ | Divide Quad | .DQ |
| ⎕ | quad (BoX) | .BX | @L | | | Input Quad | .IQ |
| ⌈ | CEiling (maximum) | .CE | @S | | | Output Quad | .OQ |
| ↓ | drop (Down Arrow) | .DA | @U | | | OUt | .OU |
| ¨ | Dieresis | .DD | | | ⍫ | Protected Del | .PD |
| ⊥ | DEcode | .DE | @B | | | Quad Del | .QD |
| ∇ | DeL | .DL | @G | | ⍞ | Quad Quote | .QQ |
| ◊ | DiaMond | .DM | | | ⌽ | ReVersal | .RV |
| ∩ | Down Under | .DU | @C | | ⍉ | TRanspose | .TR |
| ⊤ | ENcode | .EN | @N | | ⍎ | eXecute | .XQ |
| ∈ | EPsilon | .EP | @E | | ⍕ | ForMat | .FM |
| ⌊ | FLoor | .FL | @D | | ⍝ | Comment (lamp) | " |
| ≥ | Greater then or Equal to | .GE | | | __A__-__Z__ | underscored alphabetics | .ZA-.ZZ |
| -> | GO to (branch) | .GO | | | ⍙ | underscored lower del | .Z@ |
| ⍳ | IOta | .IO | @I |
| { | Left curly Brace | .LB | |
| ∆ | delta (Lower Del) | .LD | @H |
| ≤ | Less than or Equal to | .LE | |
| ⊢ | Left tacK | .LK | |
| ○ | circle (Large O) | .LO | @O |
| ⊃ | Left Union | .LU | @X |
| ≠ | Not Equal to | .NE | |
| − | NeGation | .NG | |
| ∼ | NoT | .NT | @T |
| ⍵ | OMega | .OM | @W |
| ∨ | OR | .OR | |
| } | Right curly Brace | .RB | |
| ⍴ | RhO | .RO | @R |
| ⊣ | Right tacK | .RK | |
| ⊂ | Right Union | .RU | @Z |
| ∘ | jot (Small O) | .SO | @J |
| _ | UnderScore | .US | @F |
| ∪ | Up Union | .UU | @V |

===== Workspace Commands =====

|'')SAVE [//wsname//]''|Save workspace in file ''DSK://wsname//.APL''.|
|'')LOAD //wsname//''|Load saved workspace from file ''DSK://wsname//.APL''.|
|'')DROP //wsname//''|Delete saved workspace file ''DSK://wsname//.APL''.|
|'')CLEAR''|Clear active workspace.|
|'')FNS [//letter//]''|List functions in workspace (starting alphabetic list from //letter//).|
|'')VARS [//letter//]''|List global variables in workspace (starting alphabetic list from //letter//).|
|'')ERASE //name// ...''|Erase global variable or function //name// from workspace.|

===== Function Editor =====

To define a new function and invoke the editor, enter one of the following commands:

|''.DL[//r//_]//a1 f a2//[;//v//]...''|Define a dyadic (two-argument infix) function|
|''.DL[//r//_]//f a1//[;//v//]...''|Define a monadic (one-argument prefix) function|
|''.DL[//r//_]//f//[;//v//]...''|Define a niladic (no-argument) function|

Where:
|//f//|Function name|
|//a1, a2//|Dummy arguments|
|//r//|Dummy variable for return value|
|//v//|Local variable|

To edit an existing function, enter the command ''.DL//f//''.

The editor will start and prompt with the current line number, for example ''[1]''.

==== Editor commands ====

|''//text//''|Add //text// at current line.|
|''[//n//]//text//''|Replace line //n// with //text//. Edit the function header as line 0.|
|''[//n.m//]//text//''|Insert //text// as new line after line //n//. (Lines are automatically renumbered after each editing session.)|
|''[.LD//n//]''|Delete line //n//.|
|''[.BX]''|List function source.|
|''[//n//.BX]''|List line //n//.|
|''[.BX//n//]''|List from line //n// to the end of the function.|
|''.DL''|Save function and return from editor.|

Multiple commands can be entered on a single line. For an example, you can open the editor on existing function ''FUN1'', list its source, and immediately exit the editor by entering the following command in the APL interpreter:

     .DLFUN1[.BX].DL

===== References =====

  * [[http://bitsavers.trailing-edge.com/pdf/dec/pdp10/TOPS10_softwareNotebooks/vol06/AA-H200A-TK_APLSF_Language_Manual_Aug79.pdf|APLSF Language Manual]]. Digital Equipment Corporation, 1979. 
  * [[http://www.softwarepreservation.org/projects/apl/Manuals/APL1130Primer|APL\1130 Primer]]. International Business Machines Corporation, 1968.