John,

John Soo wrote:
> Others may correct me if I’m wrong here, but during the build 
> phase, network io is off limits. This is since there is not way 
> to reliably guarantee the contents of things gotten over the 
> network remain unchanged between builds, and so would break the 
> immutability guarantees of the package system. 
>
> Again, I could be wrong, I hope others will correct me. 

You are one hundred percent correct.

The exception to this rule are ‘fixed-output derivations’ such as 
url-fetch, which may fetch whatever they want from anywhere as 
long as the end result matches a provided hash.

These same restrictions do not apply to ‘host-side’ Scheme code, 
however.  One can use the phase of the moon to generate a package 
record.

Kind regards,

T G-R