Selection (CASE, IF, MULTI, OVERRIDE, EXCLUSIVE)

The selection operator creates a property that determines for a set of conditions which condition is met, and returns the value of the result corresponding to that condition. If several conditions are met at once, the result corresponds to the first of them in the listed order. If none of the conditions is met, then the value of the created property will be NULL.

All conditions and results are defined as some properties and/or parameters. Accordingly, a condition is met if the value of the property or parameter by which this condition is set is not equal to NULL.

Polymorphic form

The platform also allows you to define a condition and the corresponding result with one property. In this case, the condition may be either matching the property's signature, or the property itself. We will call this the polymorphic form of the operator.

info

Note that the extremum operator and logical operators basically are also varieties of the selection operator (and of its polymorphic form, i.e. the conditions and result are defined by one property)

Mutual exclusion of conditions

The selection operator lets you specify that all its conditions are mutually exclusive — at most one of them is met for any given arguments, so the result no longer depends on their order.

Implicit definition

This operator supports implicit definition, where its conditions and results are added piecemeal across modules. When such an implicitly defined operator is also declared mutually exclusive, its conditions across all modules must not overlap; the platform verifies this at module finalization. For an operator defined inline, the same requirement is taken on trust.

Single form

The single form of the selection operator checks exactly one condition. If this condition is met, the value of the specified result is returned. It is also possible to specify an alternative result whose value is returned if the condition is not met.

The mutual-exclusion option and implicit definition do not apply to the single form.

Result class

The result class is the common ancestor (built-in or user-defined) of its possible results.

Language

To create a property implementing the general form of selection — explicit conditions with results — the CASE operator is used.

In the polymorphic form, MULTI selects the operand whose signature matches the parameter classes; OVERRIDE returns the first operand whose value is not NULL; EXCLUSIVE does the same as OVERRIDE and additionally declares that at most one operand has a non-NULL value.

In the single form, the IF operator returns the result when the condition is met; the IF ... THEN operator additionally accepts an alternative result returned when the condition is not met.

Examples

CLASS Color;
id = DATA STRING[100] (Color);

background 'Color' (Color c) = CASE
    WHEN id(c) == 'Black' THEN RGB(0,0,0)
    WHEN id(c) == 'Red' THEN RGB(255,0,0)
    WHEN id(c) == 'Green' THEN RGB(0,255,0)
;

id (TypeExecEnv type) = CASE EXCLUSIVE
    WHEN type == TypeExecEnv.materialize THEN 3
    WHEN type == TypeExecEnv.disablenestloop THEN 2
    WHEN type == TypeExecEnv.none THEN 1
    ELSE 0
;

nameMulti (Human h) = MULTI 'Male' IF h IS Male, 'Female' IF h IS Female;

CLASS Ledger;
CLASS InLedger : Ledger;
quantity = DATA INTEGER (InLedger);

CLASS OutLedger : Ledger;
quantity = DATA INTEGER (OutLedger);

signedQuantity (Ledger l) = MULTI quantity[InLedger](l), quantity[OutLedger](l);

CLASS Group;
markup = DATA NUMERIC[8,2] (Group);

markup = DATA NUMERIC[8,2] (Book);
group = DATA Group (Book);
overMarkup (Book b) = OVERRIDE markup(b), markup(group(b));

notNullDate (INTEGER i) = OVERRIDE date(i), 2010_01_01;

background 'Color' (INTEGER i) = EXCLUSIVE RGB(255,238,165) IF i <= 5,
                                                   RGB(255,160,160) IF i > 5;

CLASS Human;

CLASS Male : Human;
CLASS Female : Human;

name(Human h) = EXCLUSIVE 'Male' IF h IS Male, 'Female' IF h IS Female;

name = DATA STRING[100] (Book);
hasName (Book b) = TRUE IF name(b);

background (Book b) = RGB(224, 255, 128) IF b IS Book;

countTags (Book b) = GROUP SUM 1 IF in(b, Tag t);

price1 = DATA NUMERIC[10,2] (Book);
price2 = DATA NUMERIC[10,2] (Book);
maxPrice (Book b) = IF price1(b) > price2(b) THEN price1(b) ELSE price2(b);

// if h is of another class, it will be NULL
sex (Human h) = IF h IS Male THEN 'Male' ELSE ('Female' IF h IS Female); 

isDifferent(a, b) = IF a != b THEN TRUE;