While loops


loop_stmt := optlabel "while" sexpr block
loop_stmt := optlabel "repeat" block
loop_stmt := optlabel "until" sexpr block

Here is a while loop:

while x>1 do
  println$ x;

The loop body executes repeatedly until the condition is not satisfied. If the condition is initially unsatisfied the body is not executed.

The repeat loops is an infinte loop equivalent to while true.

The until loop is a while loop with a negated condition.

C style for loop


loop_stmt := optlabel "for" "(" stmt sexpr ";" stmt ")" stmt
loop_stmt := optlabel "for" stmt "while" sexpr ";" "next" stmt block
loop_stmt := optlabel "for" stmt "until" sexpr ";" "next" stmt block

The first two forms execute a statement once which generally assigns a value to a control variable. The expression must be of type bool and is checked to see if the loop should execute. The C form and the while form check the condition whilst the until form checks the negated condition. The next statement is used to increment the control variable.

Integer For Loops


loop_stmt := optlabel "for" sname "in" sexpr "upto" sexpr block
loop_stmt := optlabel "for" "var" sname ":" sexpr "in" sexpr "upto" sexpr block
loop_stmt := optlabel "for" "var" sname "in" sexpr "upto" sexpr block
loop_stmt := optlabel "for" sname "in" sexpr "downto" sexpr block
loop_stmt := optlabel "for" "var" sname ":" sexpr "in" sexpr "downto" sexpr block
loop_stmt := optlabel "for" "var" sname "in" sexpr "downto" sexpr block

These are low level for loops which operate over inclusive ranges. These loops require an integral control variable. The forms with var create the control variable, the other forms require it already exist. The control variable is available after these loops execute.

The block of these forms do not constitue a scope, the loops are implemented with gotos. Therefore you can put labels inside the blocks and goto them, and you can return from the current procedure or function inside the block.

Parallel Loop


loop_stmt := "pfor" sname "in" sexpr "upto" sexpr block

The pfor loop requires the body of the loop to behave independently of other iterations. the range is split into N parts and N pthreads are executed, each one handling a subrange of the loop. The threads run in the system thread pool. N is chosen by the system depending on the thread pool size and/or number of available cores.

pfor loops must not be nested. The reason is that the pfor loop uses the system thread pool. It is safe in general for jobs in the thread pool to enqueue jobs to the thread pool. It is also inefficient because the thread pool already executes N threads concurrently, where N is roughly equal to the number of processor core available.

It is not necessary to initialise the thread pool to use a pfor loops, it will be done automatically. However since the thread pool is used, it must be destroyed to terminate the program.

Generic Loops


loop_stmt := optlabel "for" sname "in" sexpr block
loop_stmt := optlabel "rfor" sname "in" sexpr block
loop_stmt := optlabel "match" spattern "in" sexpr block =>#

The generic for requires an function named iterator. You can provide it directly, or, you can provide any data structure which has an iterator method (that is, a function named iterator which accepts the data structure as an argument). The iterator will usually be a yielding generator and it must return an option type opt[T].

The loops process the Some x values yielded until None is found.

The control variables goes out of scope at the end of the loop.

The for variant uses a goto to loop around.

The rfor variant uses recursion instead. The recursion will be flattened to a goto loop if it is safe, otherwise rfor will create a frame for every iteration.

The match variant sets more than one variable by decoding the argument of the Some constructor.

Labelled loops

Most loops allow an optional label which is written with just a : suffix. You cannot goto such a label. The label is a name for the loop.

Labelled loops support labeled break, continue and redo statements.

 doit: for var i in 1 upto 10 do
   if i == 5 continue doit;
   if i == 7 do
      redo doit;
   if i == 9 break doit;

The continue statement jumps to the start of the selected loop, adjusting the control variable as usual before checking.

The break statement exits the selected loop immediately.

The redo statement restarts the body of the selected loop without adjusting the control variable and without checking it.