Start here: X_eTaL in fifteen minutes
from typing ASCII to importing a library, run as you read

An underscore after the first letter makes a name a function: r_ev is drawn as rev with the r underlined, and it reverses.

r̲ev "stressed"

⍝ typed:
⍝   r_ev "stressed"

desserts

The underline is the whole difference between a value and a function: rev would be an ordinary variable. The editor and the REPL draw each line as you type it, so you always see which is which.

Arithmetic works on whole arrays. A single number on one side meets every item on the other.

1 2 3 + 10

⍝ typed:
⍝   1 2 3 + 10

11 12 13

r_ange 6 is 1 to 6, and r_eshape makes a matrix of it: the shape goes on the left.

m ← 2 3 r̲eshape r̲ange 6
m

⍝ typed:
⍝   m := 2 3 r_eshape r_ange 6
⍝   m

1 2 3
4 5 6

:= binds a name; it is drawn as an arrow.

A function passed to another function is quoted. '+ r_/ is "reduce by plus": the sum. On a matrix it adds the rows together, giving one sum per column.

'+ r̲/ 1 2 3 4
'+ r̲/ m

⍝ typed:
⍝   '+ r_/ 1 2 3 4
⍝   '+ r_/ m

10
5 7 9

Your own functions are lambdas in braces, _r standing for the right argument (drawn as APL's omega). A name of yours carries the u: prefix, drawn as a small superscript.

ᵘs̲quare ← { ⍵ × ⍵ }
ᵘs̲quare 1 2 3
'ᵘs̲quare e̲ach 4 5

⍝ typed:
⍝   u:s_quare := { _r * _r }
⍝   u:s_quare 1 2 3
⍝   'u:s_quare e_ach 4 5

1 4 9
16 25

A function works along the first axis unless a subscript names another. '+ r_/_2 m adds along axis 2, within each row; 1 o_-_2 m rotates each row by one.

'+ r̲/₂ m
1 o̲-₂ m

⍝ typed:
⍝   '+ r_/_2 m
⍝   1 o_-_2 m

6 15
2 3 1
5 6 4

Functions in square brackets make a new function without naming its argument. Three are a fork: ['+ r_/ / t_ally] x is the sum of x divided by its count, the mean. Two are an atop: [n_eg a_bs] x is n_eg (a_bs x).

ᵘm̲ean ← ['+ r̲/ ÷ t̲ally]
ᵘm̲ean 1 2 3 4
[n̲eg a̲bs] -5

⍝ typed:
⍝   u:m_ean := ['+ r_/ / t_ally]
⍝   u:m_ean 1 2 3 4
⍝   [n_eg a_bs] -5

2.5
-5

u_se< imports a library under a prefix of your choice; its names then carry that prefix. Stats is one of the libraries built in.

ˢ⁼u̲se< "Stats"
ˢs̲d 2 4 4 4 5 5 7 9

⍝ typed:
⍝   "s:" u_se< "Stats"
⍝   s:s_d 2 4 4 4 5 5 7 9

2.0

• The language tour, docs/literate/tour.org (and just tour): every feature, a line or two each.
• The reference, docs/reference.md: every built-in with examples.
• What works today, docs/status.md.
• Trains beside the combinators, docs/literate/trains.org; classic APL programs, docs/literate/classics.org.
• The live demo: the same language in the browser, with the demos and the classics one click away.