Question about uniqueness typing in Clean

[ptg@owlnet.rice.edu]

> 
> I have a question about uniqueness typing in Clean
> 
> I want to simulate a dual-port RAM, that can read and write at the same clock
> cycle. So I tried to define a function 'ReadWrite', like follows:
> 
> 
> ::RAM:=={:Int:}
> 
> ReadWrite::*RAM Int        Int         Int         -> (*RAM,Int)
> ReadWrite   ram readAdress writeAdress writeValue 
>    =let! readValue=ram.[readAdress]

This let! is the problem.  In the Clean1.0.3 release let! does not handle
observing references properly.  Pattern matching works, though.

>     in   ( {:ram & [writeAdress]=writeValue:},
>            readValue)
> 
> Start=ReadWrite (createArray 1 4711) 0 0 1996
> 
> 
> ReadWrite returns the updated RAM and the read value.
> The Compiler reports 'conflicting uniqueness information due to argument 1 of 
> _update'.
> I'm just beginning using uniqeness typing. Maybe the problem is, that the 
> second argument of the result (the read value) contains a reference into the 
> first argument (the unique array, that represents the RAM). Could I solve the 
> problem by somehow COPYING an element out of the array, so that there is no 
> need for a reference ? Or is there any other solution ?
> 
> 

Try the following:

---
module blah
import StdEnv

::RAM:=={:Int:}

ReadWrite::*RAM Int Int Int -> (*RAM,Int)
ReadWrite ram=:{:[readAddress] = readValue:} readAddress writeAddress writeValue
	= ({: ram & [writeAddress] = writeValue :}, readValue)

Start = ReadWrite (createArray 1 4711) 0 0 1996
---

	Paul