decision table and bucketing

Decision tables are a well known technique to specify what output to produce given combination of input values.

Bucketing is a technique where data is allocated into specified number of ‘buckets’ based on values from the inputs. The decision table can be used to express buckets as input expressions.

The decision table belows shows how based on the input ( Temperature, Humidity, WindSpeed), the output ( Sport ) will be determined.

Temperature	Humidity	WindSpeed	SportActivity
[ 20 .. 35 ]	< 85	< 20	Tennis
[ 20 .. 35 ]	< 85	< 10	Cycling
> 20	*	> 20	KiteFlying
> 35	*	*	Chess

Note:

• [ n .. m ] denotes a range of possible values.
• * denotes a wildcard.

ALFA supports decision table expressions to express similar decision making capabilities. The above table can be expressed as below.

library Activities {
    decideSport( temperature : int, humidity : int, wind : int ) : try< string > {
        let sport =
            ( temperature,   humidity,   wind  ) match {
            ( [20 .. 35] ,   < 85    ,   < 20  ) => "Tennis"
            ( [20 .. 35] ,   < 85    ,   < 10  ) => "Cycling"
            ( > 20       ,   *       ,   >= 20 ) => "KiteFlying"
            ( 35         ,   *       ,   *     ) => "Chess"
            }
        return sport
    }
}

ALFA decision table syntax greatly simplifies what would normally would be expressed in several lines of if and else conditions.

Escape analysis will be performed in future to ensure at least a single decision row will be matched.

Syntax:

Note:

• Syntax within < and > is mandatory.
• Syntax within [ and ] is optional.

( < comma seperated n number of comma seperated input expressions >  ) match [ unique | anyof | first | all > ]
{
 (< n comma separated decision-input rules >)  => expression
}

The decision-input rule can be:

• A literal value to match/equal the input
• Not equal to a literal != <value>
• A relative expression - >, >=, < or <= followed by an expression
• A range value ['inclusive value' .. 'inclusive numeric value']
• Excluding a range - not in ['inclusive value' .. 'inclusive numeric value']

The match type of unique | anyof | first | all is optional, and the default is first.

Match type	Result	Comments
unique	Single value	Only a single rule can be satisfied
anyof	Single value	Multiple rules can be satisfied. However, all satisfied rules must generate the same output
first	Single value	Multiple rules can be satisfied
all	List of values	Multiple rules can be satisfied. Outputs returned as a list.

Returns:

The value returned from a decision expression is a try<T> of the output expression.

For match type all, try< list< T > > is returned.

try is returned as the matching may fail due to invalid match. isError can be used to check if the result is valid, and get can be used to read the wrapped value.