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Consider generalizing Bounded
Oct 12, 2010
Tomek Sowiński
Oct 12, 2010
Jason House
Oct 13, 2010
Tomek Sowiński
Oct 13, 2010
Philippe Sigaud
Oct 13, 2010
Tomek Sowiński
```It may sound ridiculous but this is what came to me in a dream last night: why exactly does Bounded have to express an interval? Forget intervals for a minute:

struct Bounded(alias Pred, T);

where Pred is a unary callable returning bool (same as in e.g. filter()).

This opens a host of possibilities:
1. sin(x) - make sure x is in e.g. (-PI/2 + 2*k*PI, +PI/2 + 2*k*PI)
2. constrain a matrix to be non-singular
3. a date variable only assignable to working days
4. don't let a missle fly off the level's bounding box in an FPP shooter
5. work exclusively with non-null references
6. ...

With all the power coming from the abstraction, it may still be worth exposing Bounded(min, max, T), a wrapper over the general form to honor a common usecase. I expect candidates for other wrappers to arise as bounds are laid.

Also, 'Confined' is a name that advertises the more general functionality better and is free of the 2-knots-on-a-ribbon mental payload.

What you think: dream or nightmare?

--
Tomek
```
```Tomek SowiÅ„ski Wrote:

> It may sound ridiculous but this is what came to me in a dream last night: why exactly does Bounded have to express an interval? Forget intervals for a minute:
>
> struct Bounded(alias Pred, T);
>
> where Pred is a unary callable returning bool (same as in e.g. filter()).
...

That reminds me of class invariants.

```
```Jason House napisał:

> Tomek Sowiński Wrote:
>
>> It may sound ridiculous but this is what came to me in a dream last night: why exactly does Bounded have to express an interval? Forget intervals for a minute:
>>
>> struct Bounded(alias Pred, T);
>>
>> where Pred is a unary callable returning bool (same as in e.g. filter()).
> ...
>
> That reminds me of class invariants.

Yeah, only imposed from outside, not inside.

--
Tomek
```
```2010/10/13 Tomek Sowiński <just@ask.me>:
> It may sound ridiculous but this is what came to me in a dream last night: why exactly does Bounded have to express an interval? Forget intervals for a minute:
>
> struct Bounded(alias Pred, T);
>
> where Pred is a unary callable returning bool (same as in e.g. filter()).

We had a discussion 1-2 months ago on wrapping types to add
annotations. This came from Sorted!(predicate, range) which is another
So I think the idea is interesting and could be generalized even
further: it's encoding external information like a contract or an
invariant in a type. Basically, creating a new, richer type from a
base type.

Ideally, we would have a generic wrapper Annotate!(Type, someMoreInfo...), where someMoreInfo can be anything you need to describe the new information: numbers, predicates, etc. But this comes with a bunch of problems.

For Sorted, the (unsolved) problem was to be able to compare two
types, to see if they have 'the same' annotation.
Consider:

bool foo(int i) { return i>0;}

How can the compiler (or library writer) know whether or not
Sorted!(Range, foo) and Sorted!(Range, "a<0") are indeed the same ?

Going back to Annotate now, the difficulty is in standardizing
annotations and in combining them.
Annotate!(Annotate!(T[], hasPositiveElements), isSorted) should be the
same than
Annotate!(Annotate!(T[], isSorted), hasPositiveElements)

I found no elegant way to deal with it. One solution is to wrap each
annotation (say, in a type called Property), append them in a tuple
after the original type and sort them (maybe by name) to have an
annotation list that can be searched and compared at CT.
Like this:

Annotate!(T[], Property!(isSorted, "a<b"), Property!("a<0"))

I also remember having problems with the checking being done at runtime, while the type is a CT entity only. Maybe I couldn't create an enum validating or invalidating the properties, I don't rem

As jason said, this is a bit like contracts, but as you said, they are exposed in the type. Which means they are accessible throughcompile-time introsepction, can be changed also, extracted, reused, etc.

> 5. work exclusively with non-null references

Except, what about a non initialized object? Can you be certain that

NonNull!(someClass) c;

is not null?
```
```Philippe Sigaud napisał:

>> It may sound ridiculous but this is what came to me in a dream last night: why exactly does Bounded have to express an interval? Forget intervals for a minute:
>>
>> struct Bounded(alias Pred, T);
>>
>> where Pred is a unary callable returning bool (same as in e.g. filter()).
>
> We had a discussion 1-2 months ago on wrapping types to add
> annotations. This came from Sorted!(predicate, range) which is another
> So I think the idea is interesting and could be generalized even
> further: it's encoding external information like a contract or an
> invariant in a type. Basically, creating a new, richer type from a
> base type.
>
> Ideally, we would have a generic wrapper Annotate!(Type, someMoreInfo...), where someMoreInfo can be anything you need to describe the new information: numbers, predicates, etc.

Numbers? Mind explaining?

This is interesting, nevertheless.

> But this comes
> with a bunch of problems.
>
> For Sorted, the (unsolved) problem was to be able to compare two
> types, to see if they have 'the same' annotation.
> Consider:
>
> bool foo(int i) { return i>0;}
>
> How can the compiler (or library writer) know whether or not
> Sorted!(Range, foo) and Sorted!(Range, "a<0") are indeed the same ?

It won't, and to me it's acceptable. Although time ago someone proposed foo.body to retrieve ubyte[] with the binary code. So there's hope for slight improvement.

(typo? you meant "a>0"?)

> Going back to Annotate now, the difficulty is in standardizing
> annotations and in combining them.
> Annotate!(Annotate!(T[], hasPositiveElements), isSorted) should be the
> same than
> Annotate!(Annotate!(T[], isSorted), hasPositiveElements)
>
> I found no elegant way to deal with it. One solution is to wrap each
> annotation (say, in a type called Property), append them in a tuple
> after the original type and sort them (maybe by name) to have an
> annotation list that can be searched and compared at CT.
> Like this:
>
> Annotate!(T[], Property!(isSorted, "a<b"), Property!("a<0"))

It's tempting, but we may be trespassing on the language design territory here. Better wait for user-defined annotations in D-to-be?

> I also remember having problems with the checking being done at runtime, while the type is a CT entity only. Maybe I couldn't create an enum validating or invalidating the properties, I don't rem

..ember. ;)

> As jason said, this is a bit like contracts, but as you said, they are exposed in the type.

BTW, his invariant metaphor gave me an idea to replace the bool predicate with a void- returning function to enable different actions on failure:

void check(T t) {
assert(t.foo);
enforce(t.foo);
if (!t.foo) t.invalid = true;
...
}

Confined!(check, T) t;

Even if you stick only to asserts, it's an improvement.

> Which means they are accessible
> throughcompile-time introsepction, can be changed also, extracted,
> reused, etc.

Again, that's what user-defined annotations should be.

If we opt out of the predicate manipulation arms race, 'Confined' seems tractible. Implement it, gain feedback from users, improve. Then, with that hindsight, try to generalize further.

>> 5. work exclusively with non-null references
>
> Except, what about a non initialized object? Can you be certain that
>
> NonNull!(someClass) c;
>
> is not null?

Not sure if I understand... In D refs are initialized to null automatically so the above wrapped class ref is null. I know "= void" leaves a trash pointer which "a !is null" won't trip on. But void init means you want to play dirty so no surprize.

--
Tomek
```