[clean-list] questions about uniqueness

[Peter Achten]

Dear Valery,

At 08:52 PM 10/17/2005, you wrote:
>I'm a bit lost with uniqueness typing. Could anyone help me along ?
>
>/1/
>Start = (a,f 1 a,f 2 a)
>where
>    f :: !Int *{#Int} -> *{#Int}
>    f i t
>    #! t0 = t.[0]
>    = {t & [0] = t0 + i}
>a :: *{#Int}
>a = {i \\ i <- [0..9]}
>The code above produces ({0,1,2},{1,1,2},{2,1,2}), as if 'a' was not 
>unique, but I expected it to be rejected by the compiler, because 'f' is 
>supposed to update its unique argument in place, that is, 'a' should be 
>destroyed by the first application of 'f, and the second one should fail.
>Where am I wrong ? Does 'f' duplicate its array argument despite the 
>uniqueness annotation ?

The reason that you get three arrays is because a is a function that, when 
evaluated, yields a unique array. If you would have used a locally within 
f, then the code would have been rejected (don't forget to remove the type 
of a, otherwise it will be regarded as a local function:

Start = (a,f 1 a,f 2 a)
where
    f :: !Int *{#Int} -> *{#Int}
    f i t
    #! t0 = t.[0]
    = {t & [0] = t0 + i}

// a :: *{#Int}
    a = {i \\ i <- [0..9]}

This correctly produces the error messages:
Uniqueness error [uniqueness_Valery.icl,5,Start]: "a" demanded attribute 
cannot be offered by shared object

>/2/
>What I want actually is a struct that contains a unique array, because the 
>array will be updated in place several time later. The struct itself will 
>be copied, not updated.
>When I try
>:: Pos = {t :: *{Int}}
>I get "inconsistent attribution of type definition", which is fine, since 
>whatever contains something unique has to be unique, but when I try
>:: Pos = *{t :: *{Int}}
>I get "} expected instead of ::".
>How to define this type ?

Your analysis is correct: uniquely attributed objects must reside in unique 
objects. This is indicated by attributing the type constructor as unique:

:: *Pos = { t :: *{Int}}

Regards,
Peter

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