[racket-users] datatypes

[Kevin Forchione]

Hi guys,
Is there any function in Racket that will return a symbol representation of a value’s datatype? We can interrogate them with predicates, but it occurs to me that this information must be carried in the object’s syntax somewhere… otherwise the syntax->datum wouldn’t work. Of course I may be overlooking something :)

Kevin

[Alex Knauth]

On Jul 17, 2019, at 12:16 AM, Kevin Forchione <lys...@gmail.com> wrote:

Hi guys,
Is there any function in Racket that will return a symbol representation of a value’s datatype? We can interrogate them with predicates, but it occurs to me that this information must be carried in the object’s syntax somewhere… otherwise the syntax->datum wouldn’t work. Of course I may be overlooking something :)

I'm not sure what you mean by "carried in the object's syntax" or why syntax->datum is relevant. Could you clarify with a concrete example?

Anyway putting aside the syntax part and only looking at values, the `describe` [1] package, in particular the `variant` [2] function might be helpful to you or not depending on what you mean by "datatype".

> (require describe)

> (variant 5)

'fixnum-integer

> (variant (expt 2 65))

'bignum-integer

> (variant 3.0)

'inexact-number

> (variant 'auu)

'symbol

> (variant "oee")

'string

> (variant (list 1 2 3))

'pair

> (variant (list))

'empty-list

> (variant (vector 1 2 3))

'vector

> (variant (λ (x) x))

'procedure

  [1]: https://docs.racket-lang.org/describe/index.html

  [2]: https://docs.racket-lang.org/describe/index.html#%28def._%28%28lib._describe%2Fmain..rkt%29._variant%29%29

Kevin

[Kevin Forchione]

On Jul 16, 2019, at 10:36 PM, Alex Knauth <alex...@knauth.org> wrote:

On Jul 17, 2019, at 12:16 AM, Kevin Forchione <lys...@gmail.com> wrote:

Hi guys,
Is there any function in Racket that will return a symbol representation of a value’s datatype? We can interrogate them with predicates, but it occurs to me that this information must be carried in the object’s syntax somewhere… otherwise the syntax->datum wouldn’t work. Of course I may be overlooking something :)

I'm not sure what you mean by "carried in the object's syntax" or why syntax->datum is relevant. Could you clarify with a concrete example?

Anyway putting aside the syntax part and only looking at values, the `describe` [1] package, in particular the `variant` [2] function might be helpful to you or not depending on what you mean by "datatype”.

Thanks, Alex! That’s exactly what I’m looking for. My remark on syntax->datum. was simply speculation. A better way to put it is: how does variant able to convert a racket value into a symbol representing its datatype? I was thinking that information must be part of the object’s syntax-object, but it doesn’t look like it. Still, variant reports ‘(a . b) as a ‘pair. and that’s amazing. How is it obtaining that information?

Kevin

[Ryan Kramer]

 Still, variant reports ‘(a . b) as a ‘pair. and that’s amazing. How is it obtaining that information?

I just looked it up and yeah, it is pretty cool: (struct->vector '(a . b)) is doing most of the work. See the comments and implementation here: https://github.com/mbutterick/describe/blob/b468661a0e76fbe5ef46fe0e89e030f43f7b3acc/describe/main.rkt#L78