APL

unimplemented! (keyword){:type "keyword", :affiliated {}, :key "PROPERTY", :value "header-args :session *new*", :position {:start {:line 6, :column 1, :offset 109}, :end {:line 7, :column 1, :offset 148}}}

APL is an array programming language with a shitty proprietary implementation. This Babel file took like 6 miserable hours to get working }:3.

Unlike J, APL makes heavy use of non-ASCII glyphs. 

×/⍳6 ⍝ equivalent to the expression !6
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "720\n", :position {:start {:line 19, :column 1, :offset 556}, :end {:line 20, :column 2, :offset 563}}}

Basics

Arithmetic

1+2
10-3
2×3
10÷5
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Assignment

← is called gets, and assigns a value to a variable. 

x←2×3
x
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "6\n", :position {:start {:line 55, :column 1, :offset 822}, :end {:line 56, :column 2, :offset 827}}}

⊢ (same) trivially returns its argument, like the identity function. It can be useful in interactive sessions to print the value of expressions that otherwise wouldn't return anything, e.g. assignments. 

⊢x←2×3
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "6\n", :position {:start {:line 65, :column 1, :offset 1090}, :end {:line 66, :column 2, :offset 1095}}}

Assigning to ⎕ (quad) will print the assigned expression. 

⎕←2×3
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "6\n", :position {:start {:line 75, :column 1, :offset 1212}, :end {:line 76, :column 2, :offset 1217}}}

Like other languages, _ is conventially used as an asignee to discard a value. 

(a _ b) ← 1 2 3 ⍝ Discard second value.
a b
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "1 3\n", :position {:start {:line 86, :column 1, :offset 1391}, :end {:line 87, :column 2, :offset 1398}}}

Indexing

APL has many ways to index arrays.

Index origin

Most languages origin their arrays at 0. Few origin at 1. APL lets you choose }:). 

x←2 4 6 8
⎕IO←1 ⍝ Set the 'index origin' to 1.
x[1]
⎕IO←0 ⍝ Set the 'index origin' to 0.
x[1]
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C-style bracket syntax

This syntax is immediately recognisable to users of C descendents. 

x←0 2 4 6
x[1]
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "2\n", :position {:start {:line 123, :column 1, :offset 1877}, :end {:line 124, :column 2, :offset 1882}}}

You can index an array by an array to request several indices: 

⎕IO←0
x←0 2 4 6
x[0 2]
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "0 4\n", :position {:start {:line 135, :column 1, :offset 2017}, :end {:line 136, :column 2, :offset 2024}}}

Note that this syntax is considered unidiomatic, due to its stand-out syntax (neither a monad nor a dyad) and the fact that it does not work in tacit functions.

Functional indexing (squad)

No, that isn't a missing character — it just looks like that. The glyph ⌷ (pronounced squad, for "squashed quad") is seen by many as an improvement on the C-style bracket syntax. Unlike bracket indexing, squad is a normal dyadic function. 

1⌷1 2 3 4
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "2\n", :position {:start {:line 149, :column 1, :offset 2523}, :end {:line 150, :column 2, :offset 2528}}}

Like the bracket syntax, one can request multiple indices. In dissimilarity, however, the indexing array must be wrapped: 

(⊂1 3)⌷2 4 6 8 10
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "4 8\n", :position {:start {:line 159, :column 1, :offset 2717}, :end {:line 160, :column 2, :offset 2724}}}

Terminology

See J's terminology.

Dfns (anonymous functions)

Dfns are a Dyalog extension, not fully implemented in GNU APL. A dfn is written inside a pair of braces. Inside these braces, some additional identifiers are valid: ⍺ (left arg), ⍵ (right arg), ∇ (self-reference), ⋄ or newline (statement separator), and : (guard).

⍺ and ⍵

As previously mentioned, ⍺ and ⍵ refer to the dfn's left and right arguments. Without a left argument, a function is monadic. 

square←{⍵×⍵}
square 4
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "16\n", :position {:start {:line 180, :column 1, :offset 3332}, :end {:line 181, :column 2, :offset 3338}}}

It shouldn't be a surprise that dyads are written infix, but it caught me. }:) 

squareAndSum←{(⍵×⍵)+⍺×⍺}
2 squareAndSum 3
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "13\n", :position {:start {:line 191, :column 1, :offset 3508}, :end {:line 192, :column 2, :offset 3514}}}

Guarded expressions

A guarded expression is essentially just a conditional expression. APL's guards bear syntactic resemblance to C's ternary operator: the expression C:T⋄F evaluates to T when C is truthy, and F otherwise. 

isThree←{⍵=3:'is three'⋄'is not three'}
isThree 3
isThree 300
unimplemented! (drawer){:type "drawer", :affiliated {:results ""}, :name "RESULTS", :contents-begin 3871, :contents-end 3901, :position {:start {:line 205, :column 1, :offset 3861}, :end {:line 210, :column 6, :offset 3906}}, :children ([:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "fixed-width"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"fixed-width\",\n :affiliated {},\n :value \"is three\\n\\nis not three\\n\",\n :position\n {:start {:line 206, :column 1, :offset 3871},\n :end {:line 209, :column 2, :offset 3900}}}\n"]])}

Self-reference (recursion)

∇ stands as a reference to the enclosing dfn itself. 

factorial←{⍵>0:⍵×∇⍵-1⋄1}
factorial 6
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "720\n", :position {:start {:line 222, :column 1, :offset 4079}, :end {:line 223, :column 2, :offset 4086}}}

Function/operator reference

The names listed are not those commonly-accepted. This is for my own personal recollection. }:3

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" (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 4672,\n :contents-end 4715,\n :position\n {:start {:line 239, :column 1, :offset 4671},\n :end {:line 239, :column 45, :offset 4715}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4673,\\n :contents-end 4674,\\n :position\\n {:start {:line 239, :column 2, :offset 4672},\\n :end {:line 239, :column 11, :offset 4681}},\\n :children (\\\"⊃\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4682,\\n :contents-end 4686,\\n :position\\n {:start {:line 239, :column 11, :offset 4681},\\n :end {:line 239, :column 28, :offset 4698}},\\n :children (\\\"head\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4714,\\n :contents-end 4714,\\n :position\\n {:start {:line 239, :column 28, :offset 4698},\\n :end {:line 239, :column 45, :offset 4715}},\\n :children nil}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 4717,\n :contents-end 4760,\n :position\n {:start {:line 240, :column 1, :offset 4716},\n :end {:line 240, :column 45, :offset 4760}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4718,\\n :contents-end 4719,\\n :position\\n {:start {:line 240, :column 2, :offset 4717},\\n :end {:line 240, :column 11, :offset 4726}},\\n :children (\\\"⊂\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4727,\\n :contents-end 4739,\\n :position\\n {:start {:line 240, :column 11, :offset 4726},\\n :end {:line 240, :column 28, :offset 4743}},\\n :children (\\\"wrap/enclose\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4759,\\n :contents-end 4759,\\n :position\\n {:start {:line 240, :column 28, :offset 4743},\\n :end {:line 240, :column 45, :offset 4760}},\\n :children nil}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 4762,\n :contents-end 4807,\n :position\n {:start {:line 241, :column 1, :offset 4761},\n :end {:line 241, :column 47, :offset 4807}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4763,\\n :contents-end 4765,\\n :position\\n {:start {:line 241, :column 2, :offset 4762},\\n :end {:line 241, :column 11, :offset 4771}},\\n :children (\\\"∘.\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4787,\\n :contents-end 4787,\\n :position\\n {:start {:line 241, :column 11, :offset 4771},\\n :end {:line 241, :column 28, :offset 4788}},\\n :children nil}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4789,\\n :contents-end 4802,\\n :position\\n {:start {:line 241, :column 28, :offset 4788},\\n :end {:line 241, :column 47, :offset 4807}},\\n :children ([:code \\\"(<*>) @List\\\"])}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 4809,\n :contents-end 4852,\n :position\n {:start {:line 242, :column 1, :offset 4808},\n :end {:line 242, :column 45, :offset 4852}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4810,\\n :contents-end 4816,\\n :position\\n {:start {:line 242, :column 2, :offset 4809},\\n :end {:line 242, :column 11, :offset 4818}},\\n :children (\\\"«pipe»\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4834,\\n :contents-end 4834,\\n :position\\n {:start {:line 242, :column 11, :offset 4818},\\n :end {:line 242, :column 28, :offset 4835}},\\n :children nil}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4836,\\n :contents-end 4847,\\n :position\\n {:start {:line 242, :column 28, :offset 4835},\\n :end {:line 242, :column 45, :offset 4852}},\\n :children (\\\"residue/mod\\\")}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 4854,\n :contents-end 4897,\n :position\n {:start {:line 243, :column 1, :offset 4853},\n :end {:line 243, :column 45, :offset 4897}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4855,\\n :contents-end 4856,\\n :position\\n {:start {:line 243, :column 2, :offset 4854},\\n :end {:line 243, :column 11, :offset 4863}},\\n :children (\\\"⌊\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4864,\\n :contents-end 4869,\\n :position\\n {:start {:line 243, :column 11, :offset 4863},\\n :end {:line 243, :column 28, :offset 4880}},\\n :children (\\\"floor\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4881,\\n :contents-end 4884,\\n :position\\n {:start {:line 243, :column 28, :offset 4880},\\n :end {:line 243, :column 45, :offset 4897}},\\n :children (\\\"min\\\")}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 4899,\n :contents-end 4942,\n :position\n {:start {:line 244, :column 1, :offset 4898},\n :end {:line 244, :column 45, :offset 4942}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4900,\\n :contents-end 4901,\\n :position\\n {:start {:line 244, :column 2, :offset 4899},\\n :end {:line 244, :column 11, :offset 4908}},\\n :children (\\\"⌈\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4909,\\n :contents-end 4913,\\n :position\\n {:start {:line 244, :column 11, :offset 4908},\\n :end {:line 244, :column 28, :offset 4925}},\\n :children (\\\"ceil\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4926,\\n :contents-end 4929,\\n :position\\n {:start {:line 244, :column 28, :offset 4925},\\n :end {:line 244, :column 45, :offset 4942}},\\n :children (\\\"max\\\")}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 4944,\n :contents-end 4987,\n :position\n {:start {:line 245, :column 1, :offset 4943},\n :end {:line 245, :column 45, :offset 4987}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4945,\\n :contents-end 4946,\\n :position\\n {:start {:line 245, :column 2, :offset 4944},\\n :end {:line 245, :column 11, :offset 4953}},\\n :children (\\\"~\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4954,\\n :contents-end 4957,\\n :position\\n {:start {:line 245, :column 11, :offset 4953},\\n :end {:line 245, :column 28, :offset 4970}},\\n :children (\\\"not\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4986,\\n :contents-end 4986,\\n :position\\n {:start {:line 245, :column 28, :offset 4970},\\n :end {:line 245, :column 45, :offset 4987}},\\n :children nil}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 4989,\n :contents-end 5032,\n :position\n {:start {:line 246, :column 1, :offset 4988},\n :end {:line 246, :column 45, :offset 5032}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 4990,\\n :contents-end 4991,\\n :position\\n {:start {:line 246, :column 2, :offset 4989},\\n :end {:line 246, :column 11, :offset 4998}},\\n :children (\\\"⌷\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5014,\\n :contents-end 5014,\\n :position\\n {:start {:line 246, :column 11, :offset 4998},\\n :end {:line 246, :column 28, :offset 5015}},\\n :children nil}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5016,\\n :contents-end 5021,\\n :position\\n {:start {:line 246, :column 28, :offset 5015},\\n :end {:line 246, :column 45, :offset 5032}},\\n :children (\\\"index\\\")}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 5034,\n :contents-end 5077,\n :position\n {:start {:line 247, :column 1, :offset 5033},\n :end {:line 247, :column 45, :offset 5077}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5035,\\n :contents-end 5036,\\n :position\\n {:start {:line 247, :column 2, :offset 5034},\\n :end {:line 247, :column 11, :offset 5043}},\\n :children (\\\"↑\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5059,\\n :contents-end 5059,\\n :position\\n {:start {:line 247, :column 11, :offset 5043},\\n :end {:line 247, :column 28, :offset 5060}},\\n :children nil}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5061,\\n :contents-end 5065,\\n :position\\n {:start {:line 247, :column 28, :offset 5060},\\n :end {:line 247, :column 45, :offset 5077}},\\n :children (\\\"take\\\")}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 5079,\n :contents-end 5122,\n :position\n {:start {:line 248, :column 1, :offset 5078},\n :end {:line 248, :column 45, :offset 5122}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5080,\\n :contents-end 5081,\\n :position\\n {:start {:line 248, :column 2, :offset 5079},\\n :end {:line 248, :column 11, :offset 5088}},\\n :children (\\\"¨\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5104,\\n :contents-end 5104,\\n :position\\n {:start {:line 248, :column 11, :offset 5088},\\n :end {:line 248, :column 28, :offset 5105}},\\n :children nil}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5121,\\n :contents-end 5121,\\n :position\\n {:start {:line 248, :column 28, :offset 5105},\\n :end {:line 248, :column 45, :offset 5122}},\\n :children nil}\\n\"]])}\n"]] [:details [:summary {:style {:font-family "IBM Plex Sans"}} ("unimplemented!" " (" [:code "table-row"] ")")] [:samp {:style {:overflow "scroll", :display "block", :white-space "pre"}} "{:type \"table-row\",\n :row-type \"standard\",\n :contents-begin 5124,\n :contents-end 5169,\n :position\n {:start {:line 249, :column 1, :offset 5123},\n :end {:line 249, :column 47, :offset 5169}},\n :children\n ([:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5125,\\n :contents-end 5126,\\n :position\\n {:start {:line 249, :column 2, :offset 5124},\\n :end {:line 249, :column 11, :offset 5133}},\\n :children (\\\"⍨\\\")}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5149,\\n :contents-end 5149,\\n :position\\n {:start {:line 249, :column 11, :offset 5133},\\n :end {:line 249, :column 28, :offset 5150}},\\n :children nil}\\n\"]]\n [:details\n [:summary\n {:style {:font-family \"IBM Plex Sans\"}}\n (\"unimplemented!\" \" (\" [:code \"table-cell\"] \")\")]\n [:samp\n {:style {:overflow \"scroll\", :display \"block\", :white-space \"pre\"}}\n \"{:type \\\"table-cell\\\",\\n :contents-begin 5151,\\n :contents-end 5166,\\n :position\\n {:start {:line 249, :column 28, :offset 5150},\\n :end {:line 249, :column 47, :offset 5169}},\\n :children ([:code \\\"join @((->)w)\\\"])}\\n\"]])}\n"]])}

Folds

⌿ folds over columns, while / folds over rows.

Filters

/, when the left argument is a boolean array, uses it similarly to a bitmask: 

1 1 0 1 0/1 2 3 4 5
unimplemented! (fixed-width){:type "fixed-width", :affiliated {:results ""}, :value "1 2 4\n", :position {:start {:line 264, :column 1, :offset 5392}, :end {:line 265, :column 2, :offset 5401}}}

Iteration / corecursion

The power operator

The power operator repeatedly applies a given step function.

Examples

Nth Fibonacci number

This snippet uses corecursion on the array 0 1 to find the nth Fibonacci number. Each step builds another two-element array with the previous value and the sum:

0 1 → 1 1 → 1 2 → 2 3 → ... 

⎕IO←0
fib←{⊃⊃¯1↑{(1⌷⍵)(+/⍵)}\(⌈⍵+1)/⊂0 1}
fib¨⍳10 ⍝ First ten Fibonacci numbers.
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This version is a bit more intuitive, I think. Starting with 0 1, each step sums the previous array's last two elements and appends the sum to the array. That step function is run n times using the power operator. The penultimate element of the final array is returned. 

fib←{⊃¯2↑({⍵,+/¯2↑⍵}⍣⍵)0 1}
fib 5
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Prime factorisation

Not a terribly smart algorith, but it works. 

f←{⍺←2⋄(⍺×⍺)>⊃⍵:⍵⋄0=⍺|⊃⍵:⍺∇((⊃⍵)÷⍺),⍺,1↓⍵⋄({2=⍵:3⋄⍵+2}⍺)∇⍵}
f 720
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References