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Document type generalization.

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original commit: eb7c7b0d0be00ba529ee1c7f66a6d5568dd081a8
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Vincent St-Amour 2013-11-18 14:57:51 -05:00
parent d51957fcd3
commit 78f8879e1a
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pkgs/typed-racket-pkgs/typed-racket-doc/typed-racket/scribblings/guide/caveats.scrbl
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@ -93,3 +93,36 @@ of type precision at use sites:
Classes and units are not currently supported in Typed Racket. Support
for classes is under development and will be in a future release.
@section{Type generalization}
Not so much a caveat as a feature that may have unexpected consequences.
To make programming with invariant type constructors (such as @racket[Boxof])
easier, Typed Racket generalizes types that are used as arguments to invariant
type constructors. For example:
@interaction[#:eval the-eval
0
(define b (box 0))
b
]
@racket[0] has type @racket[Zero], which means that @racket[b] ``should'' have
type @racket[(Boxof Zero)]. On the other hand, that type is not especially
useful, as it only allows @racket[0] to be stored in the box. Most likely, the
intent was to have a box of a more general type (such as @racket[Integer]) and
initialize it with @racket[0]. Type generalization does exactly that.
In some cases, however, type generalization can lead to unexpected results:
@interaction[#:eval the-eval
(box (ann 1 Fixnum))
]
The intent of this code may be to create of box of @racket[Fixnum], but Typed
Racket will generalize it anyway. To create a box of @racket[Fixnum], the box
itself should have a type annotation:
@interaction[#:eval the-eval
(ann (box 1) (Boxof Fixnum))
((inst box Fixnum) 1)
]