Only number an equation if it has a label

[gilded lark]

Is it possible to automatically give an equation a numbering if and only if it has a label?

To be completely clear, I want the following

$ a $
$ b $ <eqa>
$ c $
$ d $ <eqb>

to compile automatically to

a
b (1)
c
d (2)

[rich walrus]

if you want just one label you can make counter(<some-label>)

[gilded lark]

@rich walrus what do you mean?

[grand swift]

?render

#show math.equation:it => {
    if it.fields().keys().contains("label"){
        math.equation(block:true, numbering: "(1)", it)
        // change your numbering style in `numbering`
    } else {
        it        
    }
}

$ sum_x^2 $ 
$ dif/(dif x)(A(t)+B(x))=dif/(dif x)A(t)+dif/(dif x)B(t) $ <ep-2>
$ sum_x^2 $ 
$ dif/(dif x)(A(t)+B(x))=dif/(dif x)A(t)+dif/(dif x)B(t) $ <ep-3>

[grand swift]

I actually got the inspiration from https://discord.com/channels/1054443721975922748/1106309466510590082

[gilded lark]

@grand swift thank you so much!

Oh no! This seems to break references to equations 😦

?render

#show math.equation:it => {
    if it.fields().keys().contains("label"){
        math.equation(block:true, numbering: "(1)", it)
        // change your numbering style in `numbering`
    } else {
        it        
    }
}

$ sum_x^2 $ 
$ dif/(dif x)(A(t)+B(x))=dif/(dif x)A(t)+dif/(dif x)B(t) $ <ep-2>
$ sum_x^2 $ 
$ dif/(dif x)(A(t)+B(x))=dif/(dif x)A(t)+dif/(dif x)B(t) $ <ep-3>
In @ep-2 and @ep-3 we see equations

[small kiln]

An error occurred:

Error: cannot reference equation without numbering
    ╭─[/main.typ:21:4]
    │
 21 │ In @ep-2 and @ep-3 we see equations
    │    ─────  
    │            
    │ 
    │ Help: you can enable equation numbering with `#set math.equation(numbering: "1.")`
────╯

Error: cannot reference equation without numbering
    ╭─[/main.typ:21:14]
    │
 21 │ In @ep-2 and @ep-3 we see equations
    │              ─────  
    │                      
    │ 
    │ Help: you can enable equation numbering with `#set math.equation(numbering: "1.")`
────╯

[gilded lark]

Okay so this works and keeps the clickable links, but the counter has a off-by-one error in the snippet render....

[pale cosmos]

I think there is a PR on this topic by @final pasture

[short shale]

That first if is equivalent to it.has("label") btw

[gilded lark]

?render

#let eqnumbering = "(1)"

#show math.equation:it => {
    if it.has("label"){
        math.equation(block:true, numbering: eqnumbering , it)
    } else {
        it
    }
}

#show ref: it => {
    let el = it.element
    if el != none and el.func() == math.equation {
      link(el.location(),numbering(
        eqnumbering,
        counter(math.equation).at(el.location()).at(0)+1
      ))
    } else {
      it
    }
}

$ sum_x^2 $ 
$ dif/(dif x)(A(t)+B(x))=dif/(dif x)A(t)+dif/(dif x)B(t) $ <ep-2>
$ sum_x^2 $ 
$ dif/(dif x)(A(t)+B(x))=dif/(dif x)A(t)+dif/(dif x)B(t) $ <ep-3>
In @ep-2 and @ep-3 we see equations