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docs: add explanation to sort primop #14552

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5 changes: 5 additions & 0 deletions src/libexpr/primops.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -4014,6 +4014,8 @@ static RegisterPrimOp primop_sort({

1. Transitivity

If a is less than b; b is less than c. Then it follows that a is less than c
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@Ericson2314 Ericson2314 Nov 13, 2025

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Suggested change
If a is less than b; b is less than c. Then it follows that a is less than c
If a is less than b and b is less than c, then it follows that a is less than c.


```nix
comparator a b && comparator b c -> comparator a c
```
Expand All @@ -4026,6 +4028,9 @@ static RegisterPrimOp primop_sort({

1. Transitivity of equivalence

If a is NOT less than b; b is NOT less then a; a and b are equal according to the comparator.
If b is equal to c then it follows that a is equal to c
Comment on lines +4031 to +4032
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@Ericson2314 Ericson2314 Nov 13, 2025

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Suggested change
If a is NOT less than b; b is NOT less then a; a and b are equal according to the comparator.
If b is equal to c then it follows that a is equal to c
First, let us define *equivalence* with respect to some comparator:
If x is *not* less than y and x is *not* less then y, then x and x are equivalent according to the comparator.
Having done that, now we can define the transitivity of this equivalence:
If a is equivalent to b and b is equivalent to c, then it follows that a is equivalent to c,

I think using different variables here make the prose much less confusing. Your version IMO made the mistake of extending the a: b: ... scope too far: the second a and b are not the same.

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@hsjobeki hsjobeki Nov 13, 2025 •
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Okay, i can make the sentence more correct in that regard. But my other point was to avoid the term transitivity in its own definition. Because at that point i (or the reader) doesn't understand what it means.

I also wonder if we need to define the eq function, when the point is to explain the concept, we shouldn't over formalize, but make sure the required property is understood by keeping the explanation simple.


```nix
let equiv = a: b: (!comparator a b && !comparator b a); in
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@Ericson2314 Ericson2314 Nov 13, 2025

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Suggested change
let equiv = a: b: (!comparator a b && !comparator b a); in
let equiv = x: y: (!comparator x y && !comparator x y); in

equiv a b && equiv b c -> equiv a c
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