UNITY (programming language)

The programming language UNITY was constructed by K. Mani Chandy and Jayadev Misra for their book Parallel Program Design: A Foundation. It is a rather theoretical language, which tries to focuse on what, and not where, when or how. The peculiar thing about the language is that it has now flow control. The statements in the program run in a random order, until none of the statements any longer may cause change if run. A correct program converges into a fix-point.

All statements are assignments, and are separated by #. A statement can consist of multiple assignments, on the form a,b,c := x,y,z, or a := x || b := y || c := z. You can also have a quantified statement list, for example <# x,y : 0 < x < y < n :: statement>, where x and y are chosen randomly among the values that satisfy the quantification. A quantified assignment is similar. In <|| x,y : 0 < x < y < n :: statement >, the statement is executed simultaneously for all x and y satisfying 0 < x < y < n.

ExamplesBubble sort

Bubble sort the array by comparing adjacent numbers, and swapping them if they are in the wrong order. Using  $\Theta (n)$  time and  $\Theta (n)$  processors.

Program sort2
declare
    n: integer,
    A: array [0..n-1] of integer
initially
    n = 20 #
    <# i : 0 <= i and i < n :: A[i] = rand() % 100 >
assign
    <# k : 0 <= k < 2 ::
        <|| i : i % 2 = k and 0 <= i < n - 1 ::
            A[i], A[i+1] := A[i+1], A[i] if A[i] > A[i+1] > >
end

Merge sort

Merge sort the array using a constant time merge. Using  $\Theta (\log n)$  time and  $\Theta (n^{2})$  processors. In this example, a value may be written many times simultaneously, and values might be duplicated, since we are not carefull with the <= operator. If you expand the a,b quantification, you can get CREW PRAM complexity.

Program sort3
declare 
    n,m: integer,
    A,S,N: array [0..n-1] of integer
initially
    n = 20 #
    m = n ||
    <|| i : 0 <= i and i < n :: A[i], N[i], S[i] = rand()%100, 1, i >
assign
    <|| s : 0 <= s < m - 1 and s % 2 = 0 ::
        <|| a,b : (a = 0 and b = 1) or (a = 1 and b = 0) ::
            <|| sa : 0 <= sa < N[s+a] ::
                A[S[s]+sa] := A[S[s+a]+sa] 
                    if A[S[s+a]+sa] <= A[S[s+b]] ||
                <|| sb : 0 < sb < N[s+b] ::
                    A[S[s]+sa+sb] := A[S[s+a]+sa]
                        if A[S[s+b]+sb-1] <= A[S[s+a]+sa] <= A[S[s+b]+sb] > || 
                A[S[s]+sa+N[s+b]] := A[S[s+a]+sa]
                    if A[S[s+b]+N[s+b]-1] <= A[S[s+a]+sa] > > ||
        S[s/2] := S[s] ||
        N[s/2] := N[s] + N[s+1] > ||
    S[(m-1)/2], N[(m-1)/2] := S[m-1], N[m-1] if m%2 = 1 ||
    m := m/2 + m%2
end

ReferencesK. Mani Chandy and Jayadev Misra (1988) Parallel Program Design: A Foundation.