TOPIC:

// Early development of FoxPro EVL() Function by Matt Slay.
// Dear community - please help my finish this with any missing data types or
// even determine if there is a better way to implement this Function for X# Runtime.

// Discuss on this X# forum thread: https://www.xsharp.eu/forum/public-vfp/1923-let-s-write-the-evl-function-for-x

Function Evl(uValue, uReturnValue)
	
	Var dataType = ValType(uValue) // Using native X# function call here.
	
	Do Case
	    Case dataType = "C"
		    If Alltrim(uValue) == ""
			    Return uReturnValue
		    Else
			    Return uValue
			Endif
		Case dataType = "N"
			If uValue = 0
				Return uReturnValue
			Else
				Return uValue
			Endif
		Case dataType = "L"
			If uValue
				Return .t.
			Else
				Return uReturnValue
			EndIf
	End Case
	
End Function

*-- This function tests the  EVL() function
Function TestEvl() As Logic
	
	Local varIntZero, varIntNonZero
	Local varDecimalZero, varDecimalNonZero
	Local varEmptyString, VarNotEmptyString
	Local varDate
	Local varDateTime
	Local varObject
	Local varNull
	Local lPassed
	
	lPassed = .t.
	
	varIntZero = 0
	VarIntNonZero = 1

	// Integer 0
	If Evl(varIntZero, 1) = 1
		? "Evl() on integer Zero Passed."
	Else
		? "Evl() on a integer 0."
		lPassed = .f.
	Endif
	
	// Non-zer Integer
	If Evl(varIntNonZero, 0) = 1 
		? "Evl() on a Non-zero Integer Passed."
	Else
		? "Evl() on a Non-zero Integer FAILED!!!"
		lPassed = .f.
	Endif
	
	// Empty String
	If Evl("", 1) = 1
		? "Evl() on empty String Passed."
	Else
		? "Evl() on empty String FAILED !!!"
		lPassed = .f.
	Endif
	
	// Non-Empty String
	If Evl("Not_Empty_String", 1) = "Not_Empty_String"
		? "Evl() on Non-Empty String Passed."
	Else
		? "Evl() on empty String FAILED !!!"
		lPassed = .f.
	Endif

	// Logical False
	If Evl(.f., 1) = 1
		? "Evl() on logical .F. Passed."
	Else
		? "Evl() on logical .F. FAILED !!!"
		lPassed = .f.
	Endif
		
	// Logical True
	If Evl(.t., 1) = .t.
		? "Evl() on logical .T. Passed."
	Else
		? "Evl() on logical .T. FAILED !!!"
		lPassed = .f.
	Endif
	
	Var oObject = Empty{}
	AddProperty(oObject, "TestProperty", 1)
	// Test by passing a dynamically added property on Empty object
	If Evl(oObject.TestProperty, 2) = 1
		? "Evl() on Dynamically added property Passed."
	Else
		? "Evl() on Dynamically added property Failed !!!"
		lPassed = .f.
	Endif
	

	// ToDo: Test Date & Empty Date
	// Todo: Test Null
	// ToDo: Test DateTime & Empty DateTime 
	// ToDo: Figure out all the other data types that need to be tested.
	
	Wait
	
	Return lPassed
	
End Function

FUNCTION EVL(eExpression1 AS USUAL, eExpression2 AS LONG) AS USUAL
FUNCTION EVL(eExpression1 AS USUAL, eExpression2 AS STRING) AS USUAL
FUNCTION EVL(eExpression1 AS USUAL, eExpression2 AS DATE) AS USUAL
FUNCTION EVL(eExpression1 AS USUAL, eExpression2 AS FLOAT) AS USUAL
FUNCTION EVL(eExpression1 AS USUAL, eExpression2 AS CURRENCY) AS USUAL
FUNCTION EVL(eExpression1 AS USUAL, eExpression2 AS OBJECT) AS USUAL

IF IsEmpty(eExpression1 )
   RETURN eExpression2 
ENDIF
RETURN eExpression1 

FUNCTION EVL(eExpression1 AS USUAL, eExpression2 AS USUAL) AS USUAL
IF IsEmpty(eExpression1 )
   RETURN eExpression2 
ENDIF
RETURN eExpression1 

? EVL("abcde", 1234)

? EVL("        ", 1234)

EVL(.NULL., 1234)

? EMPTY(.null.)

MAX(1, .NULL.)
ABS(.NULL.)
LEN(.NULL.)
FV(.NULL., 1, 2)
FV(100, .NULL., 2)
FV(100, 1, .NULL.)

IF ALLTRIM(uValue, 0, " ", CHR(9), CHR(13), CHR(10)) == ""

LOCAL x AS List<String>
x := List<String>{}
x:Add("abc")
x:Add(123) // Error at compile time

FUNCTION EVL(eExpression1 AS USUAL, eExpression2 AS <T>) AS USUAL
If IsEmpty(eExpression1)
  Return eExpression2
Endif
Return eExpression1
....

Case dataType == "C"

  LOCAL emptyChars = " " + CHR(9) + CHR(10) + CHR(13) AS String
  LOCAL testString = uValue.ToString() AS String

  FOR VAR idx = 0 TO testString.Length - 1
    If (emptyChars.IndexOf(testString.Chars[idx]) == -1)
      Return uValue
    Endif
  NEXT

  Return uReturnValue

Function Evl (sValue AS String, uReturnValue)

  LOCAL emptyChars = " " + CHR(9) + CHR(10) + CHR(13) AS String

  FOR VAR idx = 0 TO sValue.Length - 1
    If (emptyChars.IndexOf(sValue.Chars[idx]) == -1)
      Return sValue
    Endif
  NEXT

  Return uReturnValue

End Function

? .NULL.