[Progress Communities] [Progress OpenEdge ABL] Forum Post: Functional programming in ABL

[Lieven De Foor]

In functional programming languages, functions are first-class citizens. This means you can assign a function to a variable and pass it as parameter to another function or return it from other functions. In ABL there is one construct in which you can specify a method as a parameter, and that is when attaching an event handler to an event: myButton:Click:Subscribe(myButton_Click). We can (ab)use this to have some functional programming capabilities: CLASS FunctionalProgramming.Calculator: DEFINE PRIVATE EVENT Calculator VOID (a AS INTEGER, b AS INTEGER, OUTPUT Result_ AS DECIMAL). METHOD PRIVATE VOID Sum(a AS INTEGER, b AS INTEGER, OUTPUT Result_ AS DECIMAL): Result_ = a + b. END METHOD. METHOD PRIVATE VOID Multiply(a AS INTEGER, b AS INTEGER, OUTPUT Result_ AS DECIMAL): Result_ = a * b. END METHOD. CONSTRUCTOR Calculator(): DEFINE VARIABLE Result_ AS DECIMAL NO-UNDO. Calculator:Subscribe(Sum). Calculatorublish(3, 4, OUTPUT Result_). Calculator:Unsubscribe(Sum). MESSAGE Result_ VIEW-AS ALERT-BOX. Calculator:Subscribe(Multiply). Calculatorublish(3, 4, OUTPUT Result_). Calculator:Unsubscribe(Multiply). MESSAGE Result_ VIEW-AS ALERT-BOX. END CONSTRUCTOR. END CLASS. The boilerplate code (Subscribe/Publish/Unsubscribe) might be able to get isolated in an include file. If anyone has any other creative ideas on how this pattern could be used, please share them here...

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