Running a procedure from 'delay' does not capture modified parameters

[LiberalArtist]

@ryanc

One solution would be to replace

(delay ...code goes here...)

with

(let ([saved-db-config-filepath (db-config-filepath)])
  (delay (parameterize ([db-config-filepath saved-db-config-filepath])
    ...code goes here...))

And if your code wants to carry over other parameters, you have to save and restore them too. You can of course create a function or macro that does that for you, but you need to tell it what parameters to capture.

Alternatively, if you want to indiscriminately capture and restore all parameters, you could write:

(let ([saved-paramz (current-parameterization)])
  (delay (call-with-parameterization saved-paramz
           (λ ()
             ...code goes here...))))

@ryanc

(In my RacketCon talk I said that I thought parameters were overused. This is one reason why.)

I've also come to at least a soft version of this view. Two examples that particularly get to me:

1. The way the json library uses (json-null) means that even the result of jsexpr? depends on a relatively obscure piece of mutable state, which makes it difficult/expensive to check as a contract. The typed/json library works around this with wrapper functions that supply #:json-null 'null.
2. I once wrote some #lang web-server code that had a rather bad bug until Jay explained to me on the mailing list that:

   • A continuation captures the values of parameters at the time it is
     captured. So, if your parameterize is outside of the dispatcher that
     calls the continuation, then the code inside the continuation will not
     see the parameter change. That's a pretty important feature of
     continuations.

   • Web parameters would work the same way as normal parameters and not help you.

   • The stateless Web language guarantees that the parameters will have
     values they had when the servlet was initialized.

   • I think that you may want to use a thread-cell as the storage place.