1 Getting Started
2 Singleton Types
3 Variant Types

Types: Idioms for Typed Scheme

by Dave Herman (dherman at ccs dot neu dot edu)

This library provides some idioms for Typed Scheme.

    1 Getting Started

    2 Singleton Types

    3 Variant Types

1 Getting Started

To use this library, simply require its main module:

 (require (planet dherman/types:1))

2 Singleton Types

A singleton type is a type that corresponds to a single value. The following form defines a new distinct value and simultaneously binds a variable to that value and a type variable to its singleton type.

(define-constant [const-id : type-id] ...+)

The following example defines a new constant nil and defines its corresponding singleton type Nil:

  (define-constant [nil : Nil])

3 Variant Types

A variant type or disjoint union type is a type defined as the union of a fixed set of disjoint subtypes. This is analogous to ML’s datatype and Haskell’s data. Each variant is either a constant or a structure type.

(define-datatype type-id variant-clause ...)






[variant-id #:constant const-id]





[variant-id ([field-id : field-type] ...)]

The following example defines an expression grammar with one constant variant and three structure variants:

  (define-datatype Expr

    [Nil #:constant nil]

    [Var ([name : Symbol])]

    [Abs ([var : Symbol] [body : Expr])]

    [App ([rator : Expr] [rand : Expr])])