====== Control flow ======

Complex datapath elements are usually comprised of arrayed versions of
simpler cells. Although arrays can be created directly using arrayed
instantiations, ACT supports looping constructs which can be a
convenient way to create arrayed structures. More complex structures
such as trees can be easily created using recursive instantiations.

===== Loops =====

An example of the loop construct in ACT is shown below:

<code act>
( i : 10 : bool x[i..i]; )
</code>

The variable ''i'' is the loop index and its scope is
delimited by the angle brackets. The colons separate the different parts
of the loop construct. The number ''10'' is an abbreviation for the
range ''0..9''. The body of the loop is the statement ''bool x[i..i];''. The effect of this statement is the same as

<code act>
bool x[0..0];
bool x[1..1];
...
bool x[9..9];
</code>

The body of the loop can contain any ACT body, and therefore can
have multiple statements in it. A more common use of the loop statement
is shown below:

<code act>
register r[1..8];
(i : 1..8 : r[i](in[i],out[i],control); )
</code>

In the example above, registers numbered ''1'' through ''8'' are
created. Their first two parameters are connected to the corresponding
elements of arrays ''in'' and ''out''. However, they have a shared
control that is passed in as the third parameter.

Since loops are part of ACT bodies, they can occur in the body of
another loop. Thus, nested loops are also supported. However, types
cannot be defined in the body of a loop.

A second more general looping construct is borrowed from the guarded
command language.

<code act>
pint i;
i=0;
*[ i < 10 -> bool x[i..i]; i = i + 1; ]
</code>

This builds an array element by element, using the guarded command
syntax for a general while loop. Note that in this case we are
modifying a ''pint'' variable ''i''. This sort of construct can
only be used in the body of a type definition, since ''pint'' types
are immutable in global scopes.

The first form of a loop is a special case of syntactic
replication. The general loop syntax can include a //separator//,
which is used in other contexts.

In production rule bodies, the loop
<code act>
(&i:3: x[i])
</code>
expands to
<code act>
x[0] & x[1] & x[2]
</code>
(notice the absence of a trailing ''&'' is critical for the correct
syntax). The ''&'' symbol separates the body of the loop that is
instantiated for different values of ''i''. 

The syntactic replication construct is written as follows:
<code act>
(sym id : range : body )
</code>
The ''sym'' (symbol) might be empty. ''id'' is a variable that can be used in ''body'', and takes the range specified by ''range''. ''range'' can be either an integer-valued expression or ''start .. end'' to indicate a start and end index. The result of the replication is
<code act>
 body(id set to lo) sym body(id set to lo+1) sym ... sym body(id set to hi)
</code>
where ''lo'' is the starting index of the range, and ''hi'' is the ending index, and ''body(id set to x)'' means ''body'' with the value of ''x'' substituted by the constant ''id''.


===== Selections =====

Conditional execution is supported by the selection statement. The
syntax of a selection statement is:

<code act>
[ boolean_expression -> body 
[] boolean_expression -> body
..
]
</code>
The last Boolean expression in the conditional can be the keyword
''else'', which is short-hand for 'all other guards are false.'

Any one body whose corresponding Boolean expression is true is
executed. For instance, we can create 32 registers with
something special for register 0 as follows:

<code act>
(i : 32 : 
    [ i = 0 -> r0(in[i],out[i],control);
   [] else -> r[i](in[i],out[i],control);
    ]
)
</code>

Boolean expressions can be constructed from Boolean variables, the
constants ''true'' and ''false'', and the Boolean operators
''&'', ''|'', and ''~'' denoting the and, or, and negation
operations respectively.  Numeric expressions can be compared using
''<'', ''< ='', ''>'', ''>='', ''='', and ''!='' for
the operators less than, less than or equal to, greater than, greater
than or equal to, equal to, and not equal to respectively.

===== Recursion =====

Type definitions can be recursive. For instance, the following
definition can be used to create a tree structure.

<code act>
template<pint N>
defproc tree (bool a[N])
{
  [ N = 1 -> leaf l(a[0]);
 [] N > 1 -> tree<N/2> t0(a[0..N/2-1]);
             tree<N-N/2> t1(a[N/2..N-1]);
  ]
}
</code>