Maybe Monad AdvPL

Maybe.prg

#include "prelude.ch"
#include "protheus.ch"

/**
 * Monadic implementation for AdvPL
 * @author Marcelo Camargo
 */
Class Maybe From Monad
  Method New( xVal ) Constructor
  Method Bind( bFun )
  Method IsNothing()
  Method Maybe( xDefault, bFun )
EndClass

// Nothing
Class Nothing From Maybe
  Method New() Constructor
EndClass

Method New() Class Nothing
  Return Nil

Method Bind( _ ) Class Nothing
  Return Self

Method IsNothing() Class Nothing
  Return .T.

Method Val() Class Nothing
  Throw "Cannot call Val() of Nothing."

Method Maybe( xDefault, _ ) Class Nothing
  Return xDefault


// Just t
Class Just From Maybe
  Data xValue
EndClass

Method New( xVal ) Class Maybe
  If Type( xVal ) == 'U' .Or. xVal == Nil
    Return Nothing():New()
  EndIf
  Return Just():New( xVal )

Method New( xVal ) Class Just
  ::xValue := xVal
  Return Self

Method Bind( bFun ) Class Just
  Return Maybe():New( Eval( bFun, ::xValue ) )

Method IsNothing() Class Just
  Return .F.

Method Val() Class Just
  Return ::xValue

Method Maybe( _, bFun ) Class Just
  Return Eval( bFun, ::xValue )


nTest := Maybe():New( 10 )
nTest:Bind(Fun ( N ) -> ;
  N * 2 ;
)