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Improve the guide section on recursive types

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Alexis King 2015-03-27 11:29:55 -07:00
parent e3e73a8286
commit ffe1285828
1 changed files with 18 additions and 7 deletions
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typed-racket-doc/typed-racket/scribblings/guide/types.scrbl
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@ -119,16 +119,27 @@ unions are flattened.
@section{Recursive Types}
@deftech{Recursive types} can refer to themselves. This allows a type
to describe an infinite family of data. For example, this is the type
of binary trees of numbers.
@deftech{Recursive types} are types whose definitions refer to
themselves. This allows a type to describe an infinite family
of data. For example, this is the type of binary trees of numbers.
@racketblock[
(define-type BinaryTree (Rec BT (U Number (Pair BT BT))))]
(define-type BinaryTree (U Number (Pair BinaryTree BinaryTree)))]
The @racket[Rec] type constructor specifies that the type @racket[BT]
refers to the whole binary tree type within the body of the
@racket[Rec] form.
Types can also be @emph{mutually recursive}. For example, the above
type defintion could also be written like this.
@racketblock[
(define-type BinaryTree (U BinaryTreeLeaf BinaryTreeNode))
(define-type BinaryTreeLeaf Number)
(define-type BinaryTreeNode (Pair BinaryTree BinaryTree))]
Of course, types which directly refer to themselves are not
permitted. For example, both of these definitions are illegal.
@interaction[#:eval the-eval
(define-type BinaryTree BinaryTree)
(define-type BinaryTree (U Number BinaryTree))]
@section{Structure Types}