Re: [racket-users] What is the difference between foo/c and foo? when creating contracts?

2019-06-05 Thread Philip McGrath 
To add a bit to what Alexis said about flat contracts:

On Wed, May 22, 2019 at 1:23 PM Alexis King  wrote:

> Your intuition is right, but let me make it more precise: foo? is used
> when something is a flat contract, and foo/c is used otherwise. Flat
> contracts only check *first order properties* of values, which is a
> technical term that captures what you mean by “general nature.”
>
> The important distinction is that first order properties can be checked
> immediately … Since flat contracts only perform first-order checks, a flat
> contract can be used as a predicate, so you can write (foo? x) to get back
> a boolean. Likewise, all predicates can be used as flat contracts.
>

Some flat contracts use the foo/c naming convention when they include
special error reporting, which can potentially involve a performance cost
beyond a simple predicate. For example, the `xml` module provides both
`xexpr?` and `xexpr/c`:
https://docs.racket-lang.org/xml/index.html#(def._((lib._xml%2Fprivate%2Fxexpr-core..rkt)._xexpr%2Fc))
I write a lot of these kinds of contracts, and I think they're very
effective. In my RacketCon talk last year, I discussed using contracts to
do XML validation and produce error messages good enough for
non-programmers to fix invalid XML files:
https://youtu.be/pv0lLciMI24?t=5479

-Philip

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Re: [racket-users] What is the difference between foo/c and foo? when creating contracts?

2019-05-22 Thread David Storrs 
Great, thanks.  I clearly need to add the section on contracts to my
re-read list.  I thought I understood them, but apparently there are still
bits that I'm missing.

On Wed, May 22, 2019 at 1:23 PM Alexis King  wrote:

> Your intuition is right, but let me make it more precise: foo? is used
> when something is a flat contract, and foo/c is used otherwise. Flat
> contracts only check *first order properties* of values, which is a
> technical term that captures what you mean by “general nature.”
>
> The important distinction is that first order properties can be checked
> immediately, whereas checking of higher-order properties may need to be
> delayed. For example, when you write the contract
>
> (-> integer? string?)
>
>
> then you can check immediately that the value is a procedure (via the
> procedure? predicate), but you have to wait until the function is called to
> check that it is actually passed an integer. Therefore, non-flat contracts
> add proxying wrappers to values that do the deferred checking.
>
> Since flat contracts only perform first-order checks, a flat contract can
> be used as a predicate, so you can write (foo? x) to get back a boolean.
> Likewise, all predicates can be used as flat contracts. This is why the
> foo? naming scheme is used for those contracts, since foo? is the
> historical naming convention in Scheme and its descendants for naming
> predicates. The foo/c naming convention is newer, and is specific to
> contracts, so it is used for contracts and contract combinators that are
> not simple predicates, and therefore cannot use the foo? naming convention.
>
> Alexis
>
> On May 22, 2019, at 12:06, David Storrs  wrote:
>
> Is the idea that foo/c is used only when it will specify structure (e.g.
> (hash/c any/c symbol?)) whereas foo? is used to check that a thing has a
> general nature?
>
>
>

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Re: [racket-users] What is the difference between foo/c and foo? when creating contracts?

2019-05-22 Thread Alexis King 
Your intuition is right, but let me make it more precise: foo? is used when 
something is a flat contract, and foo/c is used otherwise. Flat contracts only 
check first order properties of values, which is a technical term that captures 
what you mean by “general nature.”

The important distinction is that first order properties can be checked 
immediately, whereas checking of higher-order properties may need to be 
delayed. For example, when you write the contract

(-> integer? string?)

then you can check immediately that the value is a procedure (via the 
procedure? predicate), but you have to wait until the function is called to 
check that it is actually passed an integer. Therefore, non-flat contracts add 
proxying wrappers to values that do the deferred checking.

Since flat contracts only perform first-order checks, a flat contract can be 
used as a predicate, so you can write (foo? x) to get back a boolean. Likewise, 
all predicates can be used as flat contracts. This is why the foo? naming 
scheme is used for those contracts, since foo? is the historical naming 
convention in Scheme and its descendants for naming predicates. The foo/c 
naming convention is newer, and is specific to contracts, so it is used for 
contracts and contract combinators that are not simple predicates, and 
therefore cannot use the foo? naming convention.

Alexis

> On May 22, 2019, at 12:06, David Storrs  > wrote:
> 
> Is the idea that foo/c is used only when it will specify structure (e.g. 
> (hash/c any/c symbol?)) whereas foo? is used to check that a thing has a 
> general nature?

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Re: [racket-users] What is the difference between foo/c and foo? when creating contracts?

2019-05-22 Thread Alexis King 
Your intuition is right, but let me make it more precise: foo? is used when 
something is a flat contract, and foo/c is used otherwise. Flat contracts only 
check first order properties of values, which is a technical term that captures 
what you mean by “general nature.”

The important distinction is that first order properties can be checked 
immediately, whereas checking of higher-order properties may need to be 
delayed. For example, when you write the contract

(-> integer? string?)

then you can check immediately that the value is a procedure (via the 
procedure? predicate), but you have to wait until the function is called to 
check that it is actually passed an integer. Therefore, non-flat contracts add 
proxying wrappers to values that do the deferred checking.

Since flat contracts only perform first-order checks, a flat contract can be 
used as a predicate, so you can write (foo? x) to get back a boolean. Likewise, 
all predicates can be used as flat contracts. This is why the foo? naming 
scheme is used for those contracts, since foo? is the historical naming 
convention in Scheme and its descendants for naming predicates. The foo/c 
naming convention is newer, and is specific to contracts, so it is used for 
contracts and contract combinators that are not simple predicates, and 
therefore cannot use the foo? naming convention.

Alexis

> On May 22, 2019, at 12:06, David Storrs  wrote:
> 
> Is the idea that foo/c is used only when it will specify structure (e.g. 
> (hash/c any/c symbol?)) whereas foo? is used to check that a thing has a 
> general nature?

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[racket-users] What is the difference between foo/c and foo? when creating contracts?

2019-05-22 Thread David Storrs 
Is the idea that foo/c is used only when it will specify structure (e.g.
(hash/c any/c symbol?)) whereas foo? is used to check that a thing has a
general nature?

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