[]

[] is Coalton’s reader syntax for collection builders and collection comprehensions.

Syntax

;; finite collection
[]
[⟨expr⟩ ...]

;; collection comprehension
[⟨expr⟩ ⟨clause⟩...]

Semantics

• [...] does not denote a specific, single type. It is generic across several collection-like types.
• [] is an empty collection builder.
• Collection builders and comprehensions must be finite.
• Elements must all have the same type.
• Without a more specific expected type, collection builders default to Seq :t.
• Collection builders, especially empty ones, often need the to fix the intended element type.

Defaulting also applies when a collection is consumed immediately. For example, (fold + 0 [1 2 3]) builds a Seq Integer and returns 6. Choosing Seq leaves element types to ordinary inference; a surrounding type annotation or function signature can still require a different collection type.

Options

• :for ⟨var⟩ :below ⟨ufix⟩ iterates from 0 up to but not including the bound.
• :for ⟨var⟩ :in ⟨iter⟩ iterates over an iterator.
• :with ⟨var⟩ = ⟨expr⟩ introduces a lexical binding for the remaining clauses.
• :when ⟨expr⟩ guards emitted entries.
• New types can opt-in to this syntax by defining instance of FromItemizedCollection and FromCollectionComprehension.

Example

(define evens
  [x :for x :below 10 :when (even? x)])

(define xs
  (the (List F32) [1 2 3]))