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

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