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clarify docs for unsupplied-arg in ->i

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related to #1539
This commit is contained in:
Robby Findler 2016-12-17 08:42:43 -06:00
parent 21cf407616
commit 23226d4290
1 changed files with 33 additions and 8 deletions
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41
pkgs/racket-doc/scribblings/reference/contracts.scrbl
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@ -1346,16 +1346,41 @@ there is no dependency between two arguments (or the result and an
argument), then the contract that appears earlier in the source text is argument), then the contract that appears earlier in the source text is
evaluated first. evaluated first.
Finally, if all of the identifier positions of the range If all of the identifier positions of the range
contract are @racket[_]s (underscores), then the range contract expressions contract are @racket[_]s (underscores), then the range contract expressions
are evaluated when the function is called and the underscore is not bound are evaluated when the function is called instead of when it returns.
in the range, after the argument contracts are evaluated and Otherwise, dependent range expressions are evaluated when the function
checked. Otherwise, the range expressions are evaluated when the function returns.
returns.
If there are optional arguments that are not supplied, then If there are optional arguments that are not supplied, then
the corresponding variables will be bound to a special value the corresponding variables will be bound to a special value
called @racket[the-unsupplied-arg] value. called @racket[the-unsupplied-arg] value. For example, in
this contract:
@racketblock[(->i ([x (y) (if (unsupplied-arg? y)
real?
(>=/c y))])
([y real?])
any)]
the contract on @racket[x] depends on @racket[_y], but
@racket[_y] might not be supplied at the call site. In that
case, the value of @racket[_y] in the contract on
@racket[_x] is @racket[the-unsupplied-arg]
and the @racket[->i] contract must check for it and tailor
the contract on @racket[_x] to
account for @racket[_y] not being supplied.
When the contract expressions for unsupplied arguments are dependent,
and the argument is not supplied at the call site, the contract
expressions are not evaluated at all. For example, in this contract,
@racket[_y]'s contract expression is evaluated only when @racket[_y]
is supplied:
@racketblock[(->i ()
([x real?]
[y (x) (>=/c x)])
any)]
In contrast, @racket[_x]'s expression is always evaluated (indeed,
it is evaluated when the @racket[->i] expression is evaluated because
it does not have any dependencies).
} }
@defform*/subs[#:literals (any values) @defform*/subs[#:literals (any values)