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BQN–Dyalog APL dictionary

A few tables to help users of Dyalog APL (or similar) get started quickly on BQN. For a higher-level comparison, check Why BQN?. Here we assume ⎕ML is 1 for Dyalog.

Terminology

Array model

BQN uses the based array model, so that a Dyalog simple scalar corresponds to many BQN values: an atom, its enclose, and so on.

BQN shares the terms "cell" and "major cell" with Dyalog, and uses "element" (which may mean different things to different Dyalog users) not for a 0-cell but for the value it contains.

Roles

Dyalog uses value types (array, function, and so on) to determine syntax while BQN uses a separate concept called syntactic roles. See context-free grammar.

Syntax

BQN comments are written with #, not ⍝. BQN strings use double quotes "" while single quotes '' enclose a character.

BQN's block functions use {}, like Dyalog's dfns. The names for inputs and self-reference are different:

Blocks don't have guards exactly, but headers and predicates support some similar functionality (first-class functions can also be used for control structures). Headers can also be used to make a block more explicit about its inputs, more like a tradfn.

The assignment arrow ← defines a new variable in a block, while ↩ modifies an existing one.

BQN uses the ligature character ‿ for stranding, instead of plain juxtaposition. It also has a list notation using ⟨⟩, and [] for higher-rank arrays.

For reading

Glyphs +-×÷⌊⌈|⊣⊢⍉ have nearly the same meaning in BQN as APL. The other primitive functions (except !, Assert) are translated loosely to Dyalog APL below.

Modifiers are a little harder. Many have equivalents in some cases, but Dyalog sometimes chooses different functionality based on whether the operand is an array. In BQN an array is always treated as a constant function.

Some other BQN modifiers have been proposed as future Dyalog extensions:

For writing

The tables below give approximate implementations of Dyalog primitives for the ones that aren't the same. First- and last-axis pairs are also mostly omitted. BQN just has the first-axis form, and you can get the last-axis form with ⎉1.

The form F⍣G (Power with a function right operand; Power limit) must be implemented with recursion instead of primitives because it performs unbounded iteration. The modifier _while_ ← {𝔽⍟𝔾∘𝔽_𝕣_𝔾∘𝔽⍟𝔾𝕩} provides similar functionality without risk of stack overflow. It's discussed here and called as Fn _while_ Cond arg.