Re: Taming of Pict

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Toby Murray wrote:
> 
> Some quick questions on Pict:
> 
>  - Pict channels are typed. Say one has a channel that is typed to carry
> values of type T. Can one declare this channel so that it is read-only
> (ie. can only be used to read values of type T) or write only (can only
> be used to write values of type T).
>  - If one has a read-write channel (can be read and written to), can one
> get a read-only capability to this channel, and another write-only
> capability to this same channel? Then one could hand the read-only
> capability to one module and the write-only capability to a second
> module, allowing the second to write values to the first, but not vice
> versa.

The Pict tutorial contained some text about subtyping. I have changed it. It now shows a simple
producer-consumer scenario where:
- - The parent process creates a channel
  ( by parenthood rule it thus sees both its ends: its "input end" as well as its "output end")
- - The parent defines two process
  - a consumer process
  - a producer process
  None of them initially sees any of the two ends of the channel
- - The parent process "by introduction" gives the consumer the permission to read from the channel.
- - The parent process "by introduction" gives the producer the permission to write to the channel.

So, hopefully, it aswers the question you asked.

You can read the text here:
http://altair.sk/mediawiki/upload/2/21/Tutorial.pdf
page 29 (Chapter 3)

Subtyping enables us to drop permissions (and authority). It never leads to escalating permissions
(and authority).

Best regards
- --
mk
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