Get the code: learnbqn.bqn

BQN is a modern array language (similar to APL) that aims to eliminate burdensome aspects of the APL tradition.

It is recommended to try these code examples out in a REPL. The online REPL is recommended for quick start, since it comes with keyboard and easy to access help. You can try building CBQN for a local install, but it will need keyboard setup.

```
# This is a comment.
# The characters ',' and `⋄` are statement separators.
##################
# Main datatypes #
##################
# Numbers
1,2,3,4
¯1,¯2,¯3 # Negative numbers are written with a high minus
π,∞,¯π,¯∞ # Pi and Infinity are defined constants
1_234_456 # You can add underscores in between numbers
# This does not change their value
1.3E4 # Scientific notation is supported
# Characters
'a','⥊'
'
' # Yes, you can put *any* character in a character literal
@ # Null character ('\0' in C)
# Arrays
1‿2‿3 # Stranding, good for simple lists
⟨1,2,3⟩ # General list notation
⟨1‿2,2‿3⟩ # Both can be mixed
[1‿2,2‿3] # Array notation
# An array is multidimensional, as opposed to containing sublists.
# It must be rectangular in shape (a grid structure rather than a tree structure)
[1‿2‿3,4‿5] # This is hence invalid
# May be familiar coming from Numpy, MATLAB and similar languages.
"asdf" # Character array (String)
"newline
separated" # Allows newlines
"quo""tes" # Escape a double quote by typing it twice
# Functions
1{𝕨+𝕩}3 # All functions are infix
# 𝕨 is left argument, 𝕩 is right argument
{-𝕩}5 # 𝕨 can be omitted
1+3 # Same as the above
{𝕊𝕩} # 𝕊 is a recursive call
# (this function will loop forever)
{𝕨 𝕊 𝕩: 𝕨+𝕩} # Functions can have headers (too many cases to discuss here)
# Headers can define arity
{𝕊 a‿b: a}1‿2 # and also do basic pattern matching
# (returns 1)
# Modifiers (higher order functions)
{𝕗,𝔽,𝕘,𝔾} # 𝔽 and 𝔾 are the operands as callable functions
# 𝕗 and 𝕘 are the operands as values
{𝔽𝕩} # 1-modifiers use 𝔽/𝕗 ONLY
˜,˘,¨,⁼,⌜ # primitive 1-modifiers are superscripts
{𝕨𝔽𝔾𝕩} # 2-modifiers MUST use both 𝔽/𝕗 and 𝔾/𝕘 in body or header
⊸,∘,○,⟜ # primitive 2-modifiers all have circles
+{⟨𝕗⟩} # returns ⟨ + ⟩
1-{𝔽 𝕨 𝔾 𝕩 }×2 # returns ¯2 (operators are *also* infix)
# (same as 1 -○× 2)
# Trains (Special form of function composition)
(+´÷≠) # Average (but how?)
# The above train is an F G H train, where
# (F G H) 𝕩 → (F 𝕩) G (H 𝕩)
# F ← +´, G ← ÷, H ← ≠
# In explicit form, this is
{(+´𝕩)÷≠𝕩}
# The second pattern is (f g) 𝕩 → f g 𝕩.
# longer trains are complex arrangements of these patterns, involving constants and Nothing (·).
# Read more about trains at https://mlochbaum.github.io/BQN/doc/train.html
# Evaluation order:
# BQN evaluates functions right to left with no precedence rules governing *functions*. Functions are what
# one would call operators in a mainstream language.
1÷2+3 # 1÷(2+3) = 0.2
(1÷2)+3 # ((1÷2)+3) = 1.5
# Modifiers:
# Modifiers are higher order functions, and bind tighter than functions. Modifiers execute left to right.
# Modifiers can take non-function arguments e.g. Constant (`˙`)
+
1+˜2+○-∘×3 # 1(+˜)(2((+○-)∘×)3)
# Variables
# Since the case of a variable matters to determine what it means, BQN variables are *case insensitive*
# The case that a variable is written in can change the way it is interpreted by BQN.
# Eg. `F` refers to a value as a callable function, whereas `f` refers to the same variable as just a value.
# Variable assignment is done with `←`. Variables have naming conventions based on their value:
subject ← 1‿2‿3 # Arrays, single values, namespaces come under this
# name must start with with a lowercase letter
Function ← {𝕨+𝕩} # Primitive and user defined functions come under this, both monadic and dyadic
# Starts with an uppercase letter
_1modifier ← {𝕨𝔽𝕩} # Starts with an underscore
_2modifier_ ← {𝔽𝕨𝔾𝕩} # Starts and ends with an underscore
# Variable modification is done with `↩`. An existing name cannot be reassigned with `←`.
Func ↩ {"Hello"∾𝕩}
array_or_atom +↩ 2 # You can use a dyadic function for modification
#≡ 3‿4‿5
array_or_atom -↩ # Or a monadic function.
#≡ ¯3‿¯4‿¯5
# Due to all functions being infix, you can use your own functions for modification as well:
array_or_atom {2⋆𝕩}↩ #≡ ⟨ 0.125, 0.0625, 0.03125 ⟩
##################
# BQN Primitives #
##################
# All of BQN's base primitives are a single character long. Refer to https://mlochbaum.github.io/BQN/help/index.html for
# examples.
# Here we will look at a few primitives from each section. You will want to consult the docs for detailed explanations.
# Primitive Functions
# All BQN functions are variadic, and can take one or two arguments. The base functions have both monadic and dyadic overloads.
# Usually the two overloads for a function are related.
## Arithmetic Functions
+, -, ×, ÷ # Add, Subtract, Signum/Multiply, Reciprocal/Divide , '*' does NOT do multiplication
# ⌊∘÷ does floor division
√, ⋆ # Square root/Nth root, e^x/Power
# All Arithmetic functions vectorize:
1 + 2‿3‿4 #≡ 3‿4‿5
1‿2‿3 + 2‿3‿4 #≡ 3‿5‿7
# Character arithmetic(+ and - only):
"abc"+3 #≡ "def"
'a'-'d' #≡ ¯3
## Logic Functions
∧, ∨, ¬ # For Booleans, return 1 or 0
≤, <, >, ≥, = # Vectorizing comparisons
≡, ≢ # Nonvectorizing comparisons
## Array manipulation Functions
↕ # Make a range
∾, ≍, ⋈ # Joining arrays together
a←1‿2‿3,b←4‿5 # Let us take a and b.
a∾b #≡ 1‿2‿3‿4‿5
a≍b # Same as previous, since a and b are not multidimensional
# Adds an extra dimension, similar to a ⋈ for multidimensional arrays.
a⋈b #≡ ⟨1‿2‿3, 4‿5⟩
⊑, ⊏ # Indexing
1⊑1‿2‿3 #≡ 2 (BQN is 0-indexed)
1‿2⊏1‿2‿3 #≡ 2‿3 (for multiple indices)
↑, ↓ # Getting a prefix, suffix of an array.
# together they can be used for slicing
⥊ # Reshape/repeat items to create a new array
# Primitive 1-Modifiers
## Looping combinators
¨, ˘, ⌜ # Mapping/Zipping
´, ˝ # Fold from right
` # Scan from left
## General combinators
˜ # duplicate argument/swap args - Very useful!
˙ # Create constant function
1 -˜ 2 #≡ 2 - 1
+˜ 2 #≡ 2 + 2
# Primitive 2-modifiers
## Control Flow
◶ # Choose from a list of funcs
⍟ # Repeat n times
## General Combinators
⊸, ⟜ # hook, hookf
∘, ○ # simple function composition
##########
# Blocks #
##########
# Code delimited by {}
# Lexically scoped
# For more info: https://mlochbaum.github.io/BQN/doc/block.html
# Can have headers, which are ways to explicitly define what a block should be.
# A block without headers is automatically inferred from its special variables (𝕨, 𝕩, ...).
# Function blocks
# Implicit variables(Capitals are functions):
# - 𝕨, 𝕎 left argument
# - 𝕩, 𝕏 right argument
# - 𝕤, 𝕊 represent the block itself
# Optional: one or more headers that trigger based on
# - pattern match (':') o
# - condition ('?') (similar to if-then-else)
{ # A factorial using headers:
𝕊 0: 1;
𝕊 𝕩: 𝕩×𝕊 𝕩-1
}
{ # Factorial with predicates
𝕩<2 ? 1; # Similar to an if-else pattern.
𝕩×𝕊 𝕩-1
}
# Modifier blocks
# create 1-modifiers and 2-modifiers, which have separate types
# Implicit variables(Capitals are functions):
# - has 𝕨 and 𝕩 if needed
# - 𝕗, 𝔽 left operand
# - 𝕘, 𝔾 right operand (only in 2-modifiers)
# - 𝕣 represents the block itself* (requires underscores as per convention)
# Same header rules as functions.
{ 𝕨=0 ? 𝔽 𝕩; 𝔾 𝕩 } # execute 𝔽 or 𝔾 based on whether left argument is 0.
# Namespace blocks
# Create immutable namespaces with fields
# Require exports (`⇐`) for accessible fields.
# Use '.' for field access
n←{
A←+
b⇐4
}
n.b #≡ 4
n.a # ERROR
# Immediate Blocks
# No arguments taken
# Run the code inside and return the last statement
# Often responsible for strange errors.
# Can be mistaken for other blocks easily
# Good for avoiding scoping issues
{
1‿2‿3
}
{+} # Trick for returning a function as a value
####################
# Basic constructs #
####################
# Functional programming
# `¨` is used for mapping, as discussed before:
{𝕩∾2}¨1‿2‿3 #≡ ⟨1‿2,2‿2,3‿2⟩
# ⋈¨ is a plain zip, which produces pairs.
# `¨` acts as a zipWith when used with two arguments:
1‿2‿3 {⟨𝕩+2,2⥊𝕨⟩} 4‿5‿6 #≡ ⟨⟨6,1‿1⟩,⟨7,2‿2⟩,⟨8,3‿3⟩⟩
# `/` is replicate, which serves several purposes *including* filtering.
# elements in 𝕩 are repeated by the corresponding number in 𝕨.
1‿2‿3‿0/4‿5‿6‿7 #≡ 4‿5‿5‿6‿6‿6
# a simple filter idiom is F⊸/:
{2|𝕩}⊸/67‿42‿83 # keep the odd elements
#≡ 67‿83
# Conditionals
# There are two main ways to define a conditional.
## Predicate headers
{
𝕩 > 2: "greater than 2";
𝕩 < 2: "lesser than 2";
"equal to 2"
}
## Choose (function-based)
# - 2-modifier
# - 𝔾: list of functions that serve as bodies
# - 𝔽: condition function that specifies which function from 𝔾 to select
# The same conditional as above would be:
{⊑/⟨𝕩>2, 𝕩<2, 𝕩=2⟩}◶⟨
{𝕊: "greater than 2"}
{𝕊: "lesser than 2"}
{𝕊: "equal to 2"}
⟩
## Some helpers for conditionals
If ← {𝕏⍟𝕎@}´ # Used as If ⟨Condition, Block⟩
IfElse ← {c‿T‿F: c◶F‿T@} # Used as IfElse ⟨Condition, Block, ElseBlock⟩
# Looping
# The primary form of unbounded looping is recursion (performed with 𝕊).
# BQN does not eliminate tail calls, but the while idiom can be used to work around this:
While ← {𝕩{𝔽⍟𝔾∘𝔽_𝕣_𝔾∘𝔽⍟𝔾𝕩}𝕨@}´ # While 1‿{... to run forever
DoWhile ← {𝕏@ ⋄ While 𝕨‿𝕩}´
# A For loop can be done with ¨, functions need not be pure.
```

Got a suggestion? A correction, perhaps? Open an Issue on the GitHub Repo, or make a pull request yourself!

Originally contributed by Raghu Ranganathan, and updated by 2 contributors.