[=>]

[=>] is Coalton’s reader syntax for association builders and association comprehensions.

Syntax

;; finite association
[=>]
[⟨key-expr⟩ => ⟨value-expr⟩ ...]

;; association comprehension
[⟨key-expr⟩ => ⟨value-expr⟩ ⟨clause⟩...]

Semantics

• [... => ...] does not denote a specific, single type. It is generic across several association-like types.
• [=>] is an empty association builder.
• Any other bracket form containing => is an association builder or association comprehension.
• Association builders and comprehensions must be finite.
• Keys must all have the same type, and values must all have the same type.
• Without a more specific expected type, association builders default to Seq (Tuple :key :value).
• Association builders, especially empty ones, often need the to fix the intended key and value types.

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 FromItemizedAssociation and FromAssociationComprehension.

Example

(define squares
  [x => (* x x) :for x :below 5])

(define empty-pairs
  (the (HashMap String Integer) [=>]))