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Eve Control Flow

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Conditional

Keyword "if" establish a conditional statement. Also it can be used in expressions. A conditional statement can be a simple statement or a block statement. You can use "if" after a simple statement or before a block statement.

Code pattern

This is syntax for a simple statement with conditional execution. It enable or prevent a statement to be executed. It can not be used for block statements.

# conditioned statement
statement if boolean_variable;
statement if boolean_expression;

Example

Next examples show how we can use conditional to print something when expression is fulfilled.

# conditional print
print "a is even" if (a % 2  = 0);
print "a is odd"  if (a % 2 != 0);

Decision block

In next pattern we use "if/else" keywords to create a decision fork.

decision diagram

# dual path decision
begin [label]:
  ** local scope
  ...
if expression then
  ** true branch
  ...
else
  ** false branch
  ...
done [label];

Notes:

1. The else branch is optional;
2. The label is optional;
3. The local scope is optional;

Ladder

A ladder is a conditional multi-path selector based on exclusive conditions. Only one branch is executed. If no condition is satisfied the alternative branch is executed. Alternative branch is optional.

decision ladder

Example:

#selection ladder
begin [label]:
  ** local variables
  ...
if condition_1 then
  ** first branch
  ...
else if condition_2 then
  ** second branch
  ...
else if condition_3 then
  ** third branch
  ...
[else]
  ** alternative branch
  ...
done [label];

Notes: Previous pattern show a complex situation having several conditions in cascade. Cases should be exclusive and most probable case should be evaluated first. Last block, after else is optional. Also, the local scope is optional.

Match Selector

This is a multi-path selector similar to switch also named: jump table. It two variants: "all" or "one". That is optional keyword. If used, ALL will evaluate all "when" values it may be matching multiple when blocks. If "one" is used (this is the default), only first block that has a matching value will be executed and the rest are ignored even if a second match exist. The second match should not be present though and is probably a mistake!

Match Diagram

Pattern:

# Define a selector variable
class Type = {V1, V2, V3, V4} <: Ordinal;
...
process
  new s := V1; 

  ** create match selector
  match s [all | one] [Type]:
    ** declare locals
    ...
  when V1 then
    ** first path
    ...
    [break];
  when V1, V2 then
    ** second path
    ...
  when [any | other] then
    ** V3 or V4 found
    ...
  done [Type];
return;

Notes:

• options: all or one are exclusive (one) is the default;
• default block (last) is optional: "any" or "other";
• break is necesarry to avoid "fall-trough" with "all" option;
• break is optional with "one" option. First match is executed;
• when you use the Type, the default block becomes mandatory;

The compiler will generate a warning message if not all cases are covered and you do not have use the default block. That is maybe an error but not necesarly. Maybe the type was modified.

Match can be used with any data type not only ordinal. When you use other data type, the Type do not have to be specified. It can be inferred from the variable type.

Unconditional Loop

This is a repetitive block statement. The loop is executed until a "break" statement is encounter. The break can be conditioned by "if". This is called an "interruption". You can use keyword "repeat"with conditional "if" to restart the loop early and skip several statements. This is called a "shortcut".

infinite loop

Syntax:

#infinite loop
cycle [label]:
  ** declare control variable
  ...
loop
  ** loop shortcut
  skip [label] [if condition] ;
  ...
  ** early interruption
  stop [label] [if condition];
  ...
repeat [label] [if condition];

Notes:

• Unconditional loop is executed at least one time;
• The label is optional, that's why is in square brackets;
• The label is used, for nested loops otherwise is not necessary.

While Loop

Execute a block of code as long as one condition is true. When the condition become false the repetitive block stops and block "else" is executed. If the condition is never true only the "else" block is executed.

while loop

Syntax:

#conditional loop
cycle [label]:
  ** declare control variables
  ...
while condition loop
  ** shortcut iteration
  skip if condition;
  ...
  ** early interruption
  stop if condition;
  ...
then
  ** alternative path
  ...
repeat [label] [if condition];

Example:

/** This driver demonstrate
    how to visit members of a List */
driver while_demo:
**  create a shared list
    set this := ("a","b","c","d","e");
process
  ** start unlabeled cycle
  cycle: 
    ** volatile variables
    new i := 0 : Integer; 
    new e : Symbol;       
  while i < this.length() loop
    let e := this[i];
    let i += 1;
    if e  >= "c" then
        write e;
        write ',' if e is not this.tail;
    done;
  then
    write "i = " + i;
  repeat; 
  ** flush the write buffer
  print; -- "c","d","e"
return;

Output:

"c","d","e"
i = 5

For Loop

This statement use one control variables to visit elements in a collection or domain. First use-case of this is to iterate over a range of numbers. In Eve the range is represented as (min..nax).

for ... loop

Pattern:

#for loop syntax
cycle [label]:
    new var := 0;
for var in (min..max) loop
    ** block statements;
    ...
    ** next iteration
    next if condition;
    ...
    ** early interruption
    stop if condition;
    ...
repeat [label];

Notes:

• Control variable is auto-incremented at end of loop;
• Control variable must be declared in local scope;
• Control variable is not available after the loop;

Example of range iteration using step ratio 2:

#example of range iteration
cycle: 
    new i :Integer;
for i in (1..10:2) loop
    ** write only odd numbers
    write i;
    write ',' if (i < 9);
repeat;

print;

Output:

1,3,5,7,9

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