Hey,
> OK. I see the "referent" concept is broader than I had thought. They are
> not just pointers, but (paraphrasing) expressions awaiting evaluation. The
> "referent pattern" is more or less the type of the expression, i.e. the
> type of whatever the expression evaluates to.
Yes. As I see it.
Reference = Referent Pattern + Instruction Patterns + Referents
* Referent Pattern = The most specific description possible of
the structure of the referents of the reference.
* Instruction Patterns = The result of streaming the referents
through the instruction.
* Referents = The objects pointed to be the reference.
Accessing them is a dereference (a manifestation).
Why are references more than just pointers to referents? Kuppitz and I are
realizing that ?all? database optimization can be understood as techniques to
avoid dereferencing. If the reference can tell the VM what to expect from a
dereference, then certain instructions can avoid dereferencing the reference.
Page 17 of the mm-ADT spec has a summary table of the subsequent subsections
that show how foreign keys, schemas, indices, denormalizations, views,
pipelines, and aggregations are all just variations on this theme.
Now, what I realized today what that there are two types of “referent patterns."
* The reference’s referent pattern describes what to expect (locally).
// read-oriented
* The database’s type pattern describes what is legal (globally).
// write-oriented
I didn’t have the second and I need it. Enter a type system. We need a way to
extend the mm-ADT’s type system. Enter you.
> For example, in Haskell
> notation:
>
> sum [1,2,3] :: Int
>
> Here, "sum [1,2,3]" is the reference. The referent is something which has
> yet to be determined (the number 6). We know that the referent's type is
> Int, and we can type-check the expression to be verify that it will produce
> an Int. Another example:
>
> fmap (\n -> "number " ++ show n) $ filter (> 1) [1,2,3] :: [String]
>
> Here, "fmap ... [1,2,3]" is the reference, and the referent is a list of
> strings: ["number 2","number 3”].
Bingo. So there are three types of “right hand sides” to an instruction pattern.
[instruction pattern]->result pattern (a description of what too expect)
[instruction pattern]->result object (the actual result to expect —
memoization-style)
[instruction pattern]->bytecode (a different computation that should be
executed)
…where the first is the second given a specific enough pattern ;). That is, so
the pattern is so specific that its an explicit object. And the last is
something new that I realized this weekend with respects to writing, that might
not be necessary….
> Instruction patterns seem like additional "referents" to me, with the
> difference that they are applied to objects, and that they are composed of
> concrete instructions.
Yes. Though not “concrete” instructions, but instruction patterns. For
instance, what is a database hash-index over the name-attribute?
*person{name:@string, age:@int / // referent pattern
[has,name,eq,$x.@string]->*person{name:x, age:@int} } // instruction pattern
The concrete instruction [has,name,eq,marko] will match the above instruction
pattern and will return a reference to person objects whose name is now known
and whose age is still just some integer.
> Instruction patterns seem like additional "referents" to me
I would argue that referent patterns are actually just instruction patterns as:
[value,name]->@string // instruction pattern
is the same thing as
{name:@string} // referent pattern
…I don’t have an ultra-confident reason why references aren’t just instruction
patterns, but its related to behavioral differences in reading/writing. More
exploration is required.
> If a referent is nullary (has some type "a"), an
> instruction pattern seems unary (has some type "a->b", consuming an "a" and
> producing a "b"). But I need to grok more.
Sorta. The semantics of X->Y are:
“Given a matching X-instruction executed on the reference’s referents,
the result is Y."
If Y is sufficient to solve the computation, the VM avoided dereferencing… and
this is database optimization in a nutshell.
Marko.
http://rredux.com
>
>
> On Sun, May 26, 2019 at 6:51 PM Marko Rodriguez <[email protected]>
> wrote:
>
>> Hello,
>>
>>>> [db][get,’people’] // *{name:@string, age:!gt(20)&!lt(33)}
>>>>
>>>> We have lost information about the “schema.” This is not good as
>>>> compile-time write validation is not possible.
>>>>
>>>
>>> So far, I am thinking: yeah, dealing with schema changes can be tricky.
>>
>> This is not a “schema change” but a greater specification of what the
>> referents are. Again, a reference is defined by its referent pattern (and
>> instruction patterns). The referent pattern is a description of the current
>> instances (referents) while the “schema” is a description of what is legal
>> for all instances (referents). Without “schema,” I lose compile-time
>> validation.
>>
>>> I then create a people-key on the db map that maintains a
>> person-reference.
>>>>
>>>
>>> OK. I think by people-key you mean the primary key for the person type,
>>> i.e. the vertex id. Correct me if I am wrong.
>>
>> No. db.get(‘people’) is the "people table.” RDBMSs are modeled as a map
>> with the keys being the table names and the values being *{:} references to
>> maps (i.e. rows).
>>
>>> I see this as a type plus a constraint. And... you won't be surprised to
>>> hear me say this... you express it with a select statement:
>>>
>>> youngishPeople := σ_{age <= 20 ∧ age >= 33}(people)
>>
>> Well, type definitions like this won’t happen at runtime. The VM will just
>> be able to tell you if the range has been restricted. It won’t create new
>> types. But yea, referent patterns are (now) a type plus a constraint.
>> Before, they were just constraints and that is why I lost schema
>> information at runtime.
>>
>> Take care,
>> Marko.
>>
>> http://rredux.com
>>
>>
