BQN primitives

Primitives are the basic functions and modifiers built into the language, written with individual glyphs (more about the concept here). The role of a primitive when written always matches its type (but you can use its value in other roles by assigning it, or other methods).

Primitives have no side effects other than errors, and can't perform infinite computations, except when a primitive modifier calls an operand function that does one of these things (and this can only happen when arguments are passed, as primitive modifiers are always deferred). Side effects here include both writing state such as variables or printed output, and reading any outside state, so that a function without them always returns the same result if passed the same arguments. Since trains and list notation have the same nice properties, tacit code written entirely with primitives, trains, and lists always describes finite, self-contained computations.

Recursion is the primary way to perform potentially infinite computations in BQN, and it can be packaged into control structures like While for ease of use. A given BQN implementation might also provide system values for "impure" tasks like file access or other I/O.


A function call with one argument (prefix) is called "monadic" and one with two arguments (infix) is "dyadic".

Glyph Monadic Dyadic
+ Conjugate Add
- Negate Subtract
× Sign Multiply
÷ Reciprocal Divide
Exponential Power
Square Root Root
Floor Minimum
Ceiling Maximum
Sort Up And
Sort Down Or
¬ Not Span
| Absolute Value Modulus
Less Than or Equal to
< Enclose Less Than
> Merge Greater Than
Greater Than or Equal to
= Rank Equals
Length Not Equals
Depth Match
Shape Not Match
Identity Left
Identity Right
Deshape Reshape
Join Join to
Solo Couple
Enlist Pair
Prefixes Take
Suffixes Drop
Range Windows
» Nudge Shift Before
« Nudge Back Shift After
Reverse Rotate
Transpose Reorder Axes
/ Indices Replicate
Grade Up Bins Up
Grade Down Bins Down
First Cell Select
First Pick
Classify Index of
Occurrence Count Progressive Index of
Mark Firsts Member of
Deduplicate Find
Group Indices Group
! Assert Assert with Message


Atop 𝔽𝔾 𝕩 𝔽 𝔾 𝕩 𝕨 𝔽𝔾 𝕩 𝔽 𝔾 𝕨 𝕩 Over 𝔽𝔾 𝕩 𝔽 𝔾 𝕩 𝕨 𝔽𝔾 𝕩 𝔽 𝔾 𝔾 𝕨 𝕩 Constant 𝕗˙ 𝕩 𝕗 𝕩 𝕨 𝕗˙ 𝕩 𝕗 𝕨 𝕩 Before 𝔽𝔾 𝕩 𝔾 𝔽 𝕩 𝕨 𝔽𝔾 𝕩 𝔾 𝔽 𝕨 𝕩 After 𝔽𝔾 𝕩 𝔽 𝔾 𝕩 𝕨 𝔽𝔾 𝕩 𝔽 𝔾 𝕨 𝕩 Self/Swap 𝔽˜ 𝕩 𝔽 𝕩 𝕨 𝔽˜ 𝕩 𝔽 𝕨 𝕩

Combinators only control the application of functions, which are passed as operands. A data value such as a number or array can also be an operand and, as always, applies as a constant function.

Glyph Name(s) Definition Description
˙ Constant {𝕩𝕗} Return a function that returns the operand
˜ Self/Swap {𝕩𝔽𝕨𝕩} Duplicate one argument or exchange two
Atop {𝔽𝕨𝔾𝕩} Apply 𝔾 to both arguments and 𝔽 to the result
Over {(𝔾𝕨)𝔽𝔾𝕩} Apply 𝔾 to each argument and 𝔽 to the results
Before/Bind {(𝔽𝕨𝕩)𝔾𝕩} 𝔾's left argument comes from 𝔽
After/Bind {(𝕨𝕩)𝔽𝔾𝕩} 𝔽's right argument comes from 𝔾
Valences {𝔽𝕩;𝕨𝔾𝕩} Apply 𝔽 if there's one argument but 𝔾 if there are two
Choose {f(𝕨𝔽𝕩)𝕘 𝕨F𝕩} Select one of the functions in list 𝕘 based on 𝔽
Under {𝔾𝔽𝔾} OR {(𝔾𝕩)𝕨𝔽𝔾𝕩𝕩} Apply 𝔽 over 𝔾, then undo 𝔾
Catch {𝕨𝔽𝕩… 𝕨𝔾𝕩} Apply 𝔽, but if it fails catch the error and apply 𝔾

The last three are combinators in spirit but go beyond the actual strict definition: Choose calls the function , Under has an "undo" step at the end, and Catch traps an error. The second definition for Under and the one for Catch are written in pseudo-BQN because they can't be expressed otherwise.

Other modifiers control array traversal and iteration. In three cases a simpler 1-modifier is paired with a generalized 2-modifier: for each of these the 1-modifier happens to be the same as the 2-modifier with a right operand of ¯1.

1-Modifier Name 2-Modifier Name
˘ Cells Rank
¨ Each Depth
Undo Repeat
´ Fold
˝ Insert
` Scan