_   _  ____ _   _          _
 | \ | |/ ___| \ | |        | |
 |  \| | |  _|  \| |      __| |__
 | |\  | |_| | |\  |     / _   _ \
 |_| \_|\____|_| \_|    / / | | \ \
    _    ____  _        | | | | | |
   / \  |  _ \| |       \ \_| |_/ /
  / _ \ | |_) | |        \__   __/
 / ___ \|  __/| |___    ____| |____
/_/   \_\_|   |_____|  |___________|

An APL compiler written in CoffeeScript
Runs on node.js or in a browser
Uses Jison for parsing
Literate source code

In-browser demo

Mobile demo (still a web page, but intended for small touchscreens)

Wtf is this?

APL is an ancient array-oriented weird-looking elegant programming language.

• ancient: It was conceived in the 1960s based on a Harvard professor's mathematical notation, which he published in a book titled "A Programming Language", hence the name.

• array-oriented: Every variable is viewed as a multi-dimensional array; in particular, scalars are 0-dimensional arrays. When a function is applied on an array, it acts on all items simultaneously.

• weird-looking: APL uses non-ASCII characters for most of its built-in functions. When it was invented, ASCII hadn't yet been established as a standard anyway.

• elegant: APL code tends to be very concise and expressive. Many well-known algorithms can literally fit into several characters. What is more, code is agnostic about the number of dimensions, so it often works without modification for higher-dimensional inputs.

This project is an attempt to breathe back life into APL for a modern execution environment, namely the ubiquitous JavaScript.

A taste of classic APL

1 2 3 + 4 5 6  ⍝ returns 5 7 9; the array 1 2 3 added to 4 5 6, item by item
7 + 4 5 6      ⍝ returns 11 12 13; the scalar 7 is extended to match the length of 4 5 6
¯1             ⍝ the high minus (¯) is used for negative numbers
2j3 ÷ 4j5      ⍝ complex numbers; AjB stands for A+iB in the usual math notation
2 × 3 + 4      ⍝ = 2 × (3 + 4); all functions have the same precedence and are right-associative
⍳ 5            ⍝ the iota, or index generator; returns 0 1 2 3 4
3 ≤ 4          ⍝ returns 1; integers 0 and 1 are used as booleans
3 ≤ ⍳ 5        ⍝ returns 0 0 0 1 1; the 3 is compared against each item in ⍳5
2 3 ⍴ ⍳ 6      ⍝ a matrix with 0 1 2 in the first row and 3 4 5 in the second
6 ? 49         ⍝ randomly select 6 distinct numbers between 0 and 48
+/ 3 5 8       ⍝ 16, slash is the "reduce" operator, so plus-slash means "sum"
+/[k] A        ⍝ summation along the k-th axis
A +.× B        ⍝ matrix multiplication; . is an operator, it gives the inner product of two functions
⌊3.14          ⍝ 3; functions have double meaning; e.g. with only one (right) arg, ⌊ means "floor"
7⌊5            ⍝ 5; with 2 args it means "minimum"; 1-arg is said to be "monadic", 2-arg "dyadic"

⍝ Lambda expressions
f ← {⍺+2×⍵}    ⍝ ⍺ and ⍵ are the left and right formal parameters
5 f 3          ⍝ would return 11

⍝ Map, filter, reduce
a ← 1 2 3 4
{1+3×⍵} a      ⍝ map; simply apply the function on the array; returns 4 7 10 13
({⍵>2} a) / a  ⍝ filter; returns 3 4; note that here / is used as a function, not operator
{⍺×⍵} / a      ⍝ reduce; returns 24; here / is an operator, it takes a function on the left
×/a            ⍝ same as {⍺×⍵}/a
{(⍺×t)+⍵} / a  ⍝ evaluate polynomial with coefficients a at point t

⍝ Head and tail
a ← 5 6 7 8
1 ↑ a          ⍝ returns 5; pronounced "one take of a"
1 ↓ a          ⍝ returns 6 7 8; pronounced "one drop of a"

Some unorthodox additions

⍝ The index origin is fixed at 0
⎕IO                    ⍝ returns 0
⎕IO ← 1                ⍝ gives an error

⍝ Embedded JavaScript:
3 + «Math.sqrt(25)»    ⍝ returns 8

⍝ Computed variables are syntactically indistinguishable from other variables:
r←3                    ⍝ radius
get_c←{○ r×2}          ⍝ circumference
get_S←{○ r⋆2}          ⍝ surface
⌊ r c S                ⍝ gives 3 18 28  ("⌊" is the floor function)
r←5
⌊ r c S                ⍝ gives 5 31 78

Editor support

Vim keymap and syntax