package Fork(fork, forkL, forkLN) where

import Vector

--@ \subsubsection{Fork}
--@
--@ \index{Enum@\te{Fork} (package)|textbf}
--@ The {\blue} compiler does very aggressive common subexpression
--@ elimination (CSE).  If this has to be circumvented it has to be done
--@ explicitly.  The \te{Fork} package provides various ways to duplicate
--@ values to avoid CSE.
--@

-- XXX m == 2*n, but we can't have contexts on foreign.
foreign vfork :: Bit n -> Bit m = "Fork",("i","o")

--@ Copy a value into two identical values.
--@ \begin{libverbatim}
--@ function Tuple2 #(a, a) fork(a)
--@   provisos (Bits#(a, sa));
--@ \end{libverbatim}
fork :: (Bits a sa) => a -> (a, a)
fork = unpack · vfork · pack

--@ Copy a value into a list with the specified number of elements.
--@ \begin{libverbatim}
--@ function List#(a) forkL(a x)
--@   provisos (Bits#(a, sa));
--@ \end{libverbatim}
forkL :: (Bits a sa) => Integer -> a -> List a
forkL 0 _ = Nil
forkL 1 x = Cons x Nil
forkL n x =
        letseq (x', x'') = fork x
        in  Cons x' (forkL (n-1) x'')

--@ Copy a value into a vector with the specified number of elements.
--@ \begin{libverbatim}
--@ function Vector#(n, a) forkLN(a x)
--@  provisos (Bits#(a, sa));
--@ \end{libverbatim}
forkLN :: (Bits a sa) => a -> Vector n a
forkLN x = toVector (forkL (valueOf n) x)