What is a property? • Hacker News Board

Re: What is a property?

#11

Post by gerdesj » Sat, Apr 11, 2026, 11:17 PM UTC

happytoexplain wrote: post #2

Maybe I have a bit of a brain problem, but for me, 90% of the effort that goes into learning anything in tech is spent on identifying which nouns are Nouns, and which are just nouns, and the (often mushy) semantics of how people use those nouns in varying contexts and sub-contexts. I understand the reasons for this complexity - domain-specific terminology is valuable and forms naturally from pre-existing words. It's…

"which nouns are Nouns, and which are just nouns"

English (int al) distinguishes "proper" nouns as a subset of nouns. A proper noun is a name and is capitalized. Hence you might write: King Charles is a king. Now, you also capitalize the first word of a sentence but here King is not the first word of a sentence - King Charles is a name. If you make a small change (indefinite to definite article), you get: King Charles is the King. The second king becomes a moniker and no longer just a description.

English also tends to get capitalization-loopy as soon as religion rocks up (any religion).

You can obviously ignore all that bollocks and write whatever you like, mostly without blushing!

Some other related languages eg German, capitalize all nouns.

Re: What is a property?

#12

Post by blanched » Sat, Apr 11, 2026, 11:23 PM UTC

gerdesj wrote: post #11

happytoexplain wrote: post #2

Maybe I have a bit of a brain problem, but for me, 90% of the effort that goes into learning anything in tech is spent on identifying which nouns are Nouns, and which are just nouns, and the (often mushy) semantics of how people use those nouns in varying contexts and sub-contexts. I understand the reasons for this complexity - domain-specific terminology is valuable and forms naturally from pre-existing words. It's…

"which nouns are Nouns, and which are just nouns" English (int al) distinguishes "proper" nouns as a subset of nouns. A proper noun is a name and is capitalized. Hence you might write: King Charles is a king. Now, you also capitalize the first word of a sentence but here King is not the first word of a sentence - King Charles is a name. If you make a small change (indefinite to definite article), you get: King Charle…

Maybe this is a joke about the dangers of being abstract going over my head, but I don’t think they literally meant they don’t understand capitalization rules :)

Re: What is a property?

#13

Post by DonaldPShimoda » Sat, Apr 11, 2026, 11:23 PM UTC

AlienRobot wrote: post #6

happytoexplain wrote: post #2

Maybe I have a bit of a brain problem, but for me, 90% of the effort that goes into learning anything in tech is spent on identifying which nouns are Nouns, and which are just nouns, and the (often mushy) semantics of how people use those nouns in varying contexts and sub-contexts. I understand the reasons for this complexity - domain-specific terminology is valuable and forms naturally from pre-existing words. It's…

The worst thing is being corrected about minutiae. It's not a "property" it's an attribute/field/member/key/column/variable/getter/function/procedure. Deep down it's all variables. Even the constants are variables from the viewpoint of the CPU that has to load it in its registers. Sometimes I see people saying "in LANG, obj.foo is just 'syntax sugar' for foo(obj)" and I think that technically it has always been "synt…

> It's not a "property" it's an attribute/field/member/key/column/variable/getter/function/procedure.

For what it's worth, to a researcher in the field of programming languages (like the author of the post), these all have distinct unambiguous meanings. At least as far as PL goes, almost every term has a well-defined meaning, but as those terms were adopted into less-than-academic contexts, the meanings have diluted.

"Property" is such a term in the context of programming languages research, and, in particular, it is a very specifically defined term in the realm of property-based testing (no surprise).

> Even the constants are variables from the viewpoint of the CPU that has to load it in its registers.

No; this is not what "variable" means. Registers are properties of the processor, i.e., they are implementation details; variables are an abstract concept from the domain of the formal language specification.

> Sometime along the way we decided that "syntax sugar" means "it means the same thing as" but except for (obj).foo(), which means that the semantics of "syntax sugar" don't mean it's simpler than the phrase it was supposed to replace.

No; this is not what "syntax sugar" means. If a language defines some syntax f and it "expands to" some other syntax g, then f is syntax sugar for g. This is well defined in Felleisen's "On the Expressive Power of Programming Languages" [0]. For example, Python's addition operator `+` is implemented in terms of a method `__add__`; therefore, `a + b` is syntax sugar for `a.__add__(b)`, because the former syntax is built on top of the latter.

Notably, syntax sugar has nothing to do with casts; casts are semantic, not syntactic. There are also no promises about whether syntax sugar makes something "easier"; it's simply the ability to syntactically express something in multiple ways.

[0] direct PDF: https://www2.ccs.neu.edu/racket/pubs/scp91-felleisen.pdf

Re: What is a property?

#14

Post by alemwjsl » Sat, Apr 11, 2026, 11:43 PM UTC

happytoexplain wrote: post #2

Maybe I have a bit of a brain problem, but for me, 90% of the effort that goes into learning anything in tech is spent on identifying which nouns are Nouns, and which are just nouns, and the (often mushy) semantics of how people use those nouns in varying contexts and sub-contexts. I understand the reasons for this complexity - domain-specific terminology is valuable and forms naturally from pre-existing words. It's…

"Property" here comes from mathematics (and I guess similar definition to science). What are the properties of this function? E.g. x * y has the property that x * y = y * x.

So we do import stuff from Mathematics alot.

I think we need better tutorials. It is a hard problem. How to teach something, who is your audience. For an AI tutorial, do I need to explain what inference is? Maybe. If I don't and they don't know it is another weird word.

In terms of brain problem - I have the same thing. It is worse inside the corporate barrier as there is no internet documentation on what the terms mean. I think all we can do is slow down and write cheat sheets until we remember.

Re: What is a property?

#15

Post by happytoexplain » Sat, Apr 11, 2026, 11:57 PM UTC

gerdesj wrote: post #11

happytoexplain wrote: post #2

Maybe I have a bit of a brain problem, but for me, 90% of the effort that goes into learning anything in tech is spent on identifying which nouns are Nouns, and which are just nouns, and the (often mushy) semantics of how people use those nouns in varying contexts and sub-contexts. I understand the reasons for this complexity - domain-specific terminology is valuable and forms naturally from pre-existing words. It's…

"which nouns are Nouns, and which are just nouns" English (int al) distinguishes "proper" nouns as a subset of nouns. A proper noun is a name and is capitalized. Hence you might write: King Charles is a king. Now, you also capitalize the first word of a sentence but here King is not the first word of a sentence - King Charles is a name. If you make a small change (indefinite to definite article), you get: King Charle…

I was being humorous/pithy. I meant e.g. the difference between "property" (the dictionary word) and "property" (the programming word, which is a domain-specific usage of the dictionary word) and "property" (as in Property-Based Testing, an even more domain-specific usage). It's analogous to the concept of regular nouns vs proper nouns, but not the same (which is why I didn't use the term "proper noun").

Re: What is a property?

#16

Post by skybrian » Sat, Apr 11, 2026, 11:58 PM UTC

The property being tested in this example is “after inserting a row into a database table, the same row can be read back again.”

The insert statement isn’t independent of the database because the table needs to exist and its schema has to allow the inserted values. If the database is generated randomly, you need access to it to generate an insert statement that will work.

This is straightforward to do if the library is designed for it. Using my own TypeScript library [1]:

const insertCaseArb: Arbitrary = arb.from((pick) => {

  const db = pick(dbArb);
  const table = pick(arb.of(...db.tables));
  const values = pick(rowArbForTable(table));

  return {
    db,
    tableName: table.name,
    insert: {
      kind: "insert",
      table: table.name,
      values,
    },
  };
 });

Why might that be difficult? Some property testing libraries don’t let you call a pick function directly.

[1] https://jsr.io/@skybrian/repeat-test

Re: What is a property?

#17

Post by kccqzy » Sun, Apr 12, 2026, 1:26 AM UTC

happytoexplain wrote: post #2

Maybe I have a bit of a brain problem, but for me, 90% of the effort that goes into learning anything in tech is spent on identifying which nouns are Nouns, and which are just nouns, and the (often mushy) semantics of how people use those nouns in varying contexts and sub-contexts. I understand the reasons for this complexity - domain-specific terminology is valuable and forms naturally from pre-existing words. It's…

Since we are talking about Haskell, its type system can be explored to answer the question of what is considered a Property by the quickcheck library.

The Property type is a datatype for which the definition is private. So we look at functions that creates Property objects. We find

    class Testable prop where
      property :: prop -> Property

This means anything of the class Testable can be turned into a Property. So let's look at what are the instances of this class. We immediately find

    instance Testable Bool

This means any boolean can be turned into a Property. But more interestingly we find

    instance (Arbitrary a, Show a, Testable prop) => Testable (a -> prop)

This means any function where the argument has a generator and the result can be turned into a Property can itself be turned into a Property. This implies that for example a function of `Int -> Bool` can be turned into a property. And due to currying, `Int -> Int -> Bool` can also be turned into a property.

This basically covers the first half of the article.

We continue to find other functions that returns Property objects. We find

    forAll :: (Show a, Testable prop) => Gen a -> (a -> prop) -> Property

This states that if we have a function where we explicitly provide a generator and it returns anything that can be turned into a Property, it can also be turned into a Property. This is different from the instance above only by providing an explicit generator.

We also find the implication operator:

    (==>) :: Testable prop => Bool -> prop -> Property

This states that if we have a Bool, and anything that can be turned into a property, we also have a property.

You see, the beauty of Haskell and similar languages is that you don't need to first gain an intuitive understanding of those vocabularies of a library. The library designer has built up an abstraction of those vocabularies and Nouns and the compiler is tasked to enforce it. You can get things wrong, and the compiler just yells at you. Or you can just mechanically find things to fill in the blanks ("hole-driven development") without understanding the vocabularies. It's your choice.

Re: What is a property?

#18

Post by happytoexplain » Sun, Apr 12, 2026, 1:33 AM UTC

wizardforhire wrote: post #5

happytoexplain wrote: post #2

Maybe I have a bit of a brain problem, but for me, 90% of the effort that goes into learning anything in tech is spent on identifying which nouns are Nouns, and which are just nouns, and the (often mushy) semantics of how people use those nouns in varying contexts and sub-contexts. I understand the reasons for this complexity - domain-specific terminology is valuable and forms naturally from pre-existing words. It's…

This is literally the difficulties with learning any discipline!

True! I just think it's particularly exaggerated in IT.

Re: What is a property?

#19

Post by opello » Sun, Apr 12, 2026, 2:43 AM UTC

happytoexplain wrote: post #15

gerdesj wrote: post #11

Earlier quoted context omitted.

"which nouns are Nouns, and which are just nouns" English (int al) distinguishes "proper" nouns as a subset of nouns. A proper noun is a name and is capitalized. Hence you might write: King Charles is a king. Now, you also capitalize the first word of a sentence but here King is not the first word of a sentence - King Charles is a name. If you make a small change (indefinite to definite article), you get: King Charle…

I was being humorous/pithy. I meant e.g. the difference between "property" (the dictionary word) and "property" (the programming word, which is a domain-specific usage of the dictionary word) and "property" (as in Property-Based Testing, an even more domain-specific usage). It's analogous to the concept of regular nouns vs proper nouns, but not the same (which is why I didn't use the term "proper noun").

It seems like "jargon" fits the need for a way to label the more specific meaning intended, like "property from objected oriented programming jargon." I think programmers might differ without the more specific description on if OOP, or say, the abstract algebra meaning, of property would be intended, since both seem relevant to different contexts of programming.

Re: What is a property?

#20

Post by BoiledCabbage » Sun, Apr 12, 2026, 4:00 AM UTC

skybrian wrote: post #9

troupo wrote: post #3

Earlier quoted context omitted.

It doesn't help that many texts approach this as a very pseudo-mathematics abstract. It's not a function, it's an implication. It's there are satisfactions of preconditions, there's a thousand different things. Unfortunately, very few texts and tutorials on property-based testing actually tell you how to see what properties are. I have it on paper somewhere in some workshop materials. But online I think this is one o…

Good link. I think that explanation works because it's somewhat closer to providing concrete examples of the kinds of tests you can write.

Buy his book - it's great and exactly like that.

Domain Modeling Made Functional - Scott Wlaschin

In the link above he's described 7 very practical ways to use it. No functional jargon, no mathematical jargon. Just practical useful ideas. And the language choice in the book is irrelevant - the concepts translate well.

There is an alternate universe where he would be well known as the top author on software engineering. His website is great as well.

That said, if you do know a bit of the math his example introduced commutative, invertible, invariance, idempotency, structural recursion & isomorphism - but anyone reading it would never really know and would never need to know. It's just framed as useful applications of tests.