Re: [racket-users] "deleting" syntax properties and macro expander cons-merging

[Alex Knauth]

> On Mar 11, 2018, at 2:31 PM, Matthew Flatt  wrote:
> 
> At Sun, 11 Mar 2018 11:27:06 -0400, Alex Knauth wrote:
>> In the expander code I found this [1]:
>> 
>> (define (syntax-property-remove s key)
>>  (if (hash-ref (syntax-props s) key #f)
>>  (struct-copy syntax s
>>   [props (hash-remove (syntax-props s) key)])
>>  s))
>> 
>> Is there a reason why this is not provided as part of the syntax property 
>> operations?
> 
> I see no problem with adding it - done.

Thank you!

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Re: [racket-users] "deleting" syntax properties and macro expander cons-merging

[Matthew Flatt]

At Sun, 11 Mar 2018 11:27:06 -0400, Alex Knauth wrote:
> In the expander code I found this [1]:
> 
> (define (syntax-property-remove s key)
>   (if (hash-ref (syntax-props s) key #f)
>   (struct-copy syntax s
>[props (hash-remove (syntax-props s) key)])
>   s))
> 
> Is there a reason why this is not provided as part of the syntax property 
> operations?

I see no problem with adding it - done.

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Re: [racket-users] "deleting" syntax properties and macro expander cons-merging

[Alex Knauth]

> On Mar 10, 2018, at 6:46 PM, Alex Knauth  wrote:
> 
> The macro expander merges syntax properties between the input of a macro and 
> the output of a macro using `cons`. That means that if there is an 
> identity-macro call like this:
> 
> (identity-macro x)
> 
> where both `x` and `(identity-macro x)` have the property `'prop = 5`, the 
> final result has the property `'prop = (cons 5 5)`.
> 
> To avoid this merging, I would like to "delete" the `'prop` property on the 
> `x`. According to the docs for syntax property, it returns #false if no value 
> is associated with the key. So it seems like the way to "delete" it is to set 
> it to #false.
> 
> However, this method of "deleting" doesn't get rid of the merging. Instead I 
> get `'prop = (cons #false 5)`.

> Is there a better way of deleting properties? 

In the expander code I found this [1]:

(define (syntax-property-remove s key)
  (if (hash-ref (syntax-props s) key #f)
  (struct-copy syntax s
   [props (hash-remove (syntax-props s) key)])
  s))

Is there a reason why this is not provided as part of the syntax property 
operations?

[1]: 
https://github.com/racket/racket/blob/82b5ec8d18a576fd2768ca0983838c5236f43c3d/racket/src/expander/syntax/property.rkt#L53-L57
 


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