The fpexprpars unit contains an expression parser. The parser compiles the expression into a node tree, which is type checked after it was compiled. The expression parser handles the following types: String Integer (64-bit) Float (TExprFloat, normally Double) TDateTime Boolean The following operations are allowed: + - / * not and or xor ( ) The binary operations can also be done on integer values, in which case they act on the bits of the integer. In the case of strings addition results in concatenation of the strings. Operator precedence is observed. In case of equal precedence, evaluation order is left-to-right. Normally, both operands of binary operations must have the same type. There are 2 exceptions: The engine will convert integers to float or TDateTime if it detects that one of the nodes is a float or datetime. The engine can be extended with variables and functions. There are over 60 built-in functions, which can be enabled by setting the Builtins property of the expression parser to a set of the following values: bcStrings: Various string routines length copy delete pos lowercase uppercase stringreplace comparetext bcDateTime: Various datetime routines date time now dayofweek extractyear extractmonth extractday extracthour extractmin extractsec extractmsec encodedate encodetime encodedatetime shortdayname shortmonthname longdayname longmonthname formatdatetime bcMath: Various mathematical routines pi cos sin arctan abs sqr sqrt exp ln log frac int round trunc bcBoolean: Various boolean routines shl shr IFS IFF IFD IFI bcConversion : Conversion routines inttostr strtoint strtointdef floattostr strtofloat strtofloatdef booltostr strtobool strtobooldef datetostr timetostr strtodate strtodatedef strtotime strtotimedef strtodatetime strtodatetimedef Additional functions/variables can be added to the Identifiers collection: FP : TFPexpressionParser; The following will define a TODAY variable which has value equal to the date at the moment is is defined: FP.Identifiers.AddDateTimeVariable('TODAY',Date); The following will define a function echodate: Procedure EchoDate(Var Result : TFPExpressionResult; Const Args : TExprParameterArray); begin Result.resDateTime:=Args[0].resDateTime; end; FP.Identifiers.AddFunction('EchoDate','D','D',@EchoDate); The arguments are: Name : Name of the function Result type : Character with result type: I : integer S : String F : FLoat D : TDateTime B : Boolean Argument types : A string with each character the type of argument at that position. Callback : executed when the function is called. This can be a procedural variable or an event (procedure of object). Result and arguments are type-checked. The engine knows 2 built-in functions which are handled specially: IF(Expr,Res1,Res1) Will return Res1 if expr evaluates to True, or Res2 if expr evaluates to False. The types of Res1 and Res2 must be the same, and expr must be a boolean expression. CASE(Tag,Def,Label1,Value1,Label2,Value2,...) Case will examine the value of Tag and compare it with Label1, Label2 etc. till a match is found. It will return Value1, Value2 etc. depending on the match. If no match is found, Def will be returned. From this it follows that 1) The number of arguments is always even and is at least 4. 2) The types of Tag, label1, label2 must be the same; 3) The types of Def, Value1, Value2 must be the same; As soon as the expression is set, it is compiled and checked. Thus FP.Expression:='1*2'; will work. On the other hand FP.Expression:=' 1 * ''a string'''; will result in an exception because 1 and ''a string'' do not have the same type. Getting the result is quite simple FP.Expression:='1*2'; Writeln(FP.AsInteger); This will raise an exception if the type of the result is not integer. In case the expression result type is unknown, it can be examined using the ResultType function, as in : FP.Expression:='Some user-provided expression'; Case FP.ResultType of rtString : Writeln(FP.Evaluate.ResString); rtInteger : Writeln(FP.Evaluate.ResInteger); rtFloat : Writeln(FP.Evaluate.ResFloat); rtBoolean : Writeln(FP.Evaluate.ResBoolean); rtDateTime : Writeln(FormatDateTime('cccc',FP.Evaluate.ResDateTime)); end; Which is equivalent to FP.Expression:='Some user-provided expression'; Case FP.ResultType of rtString : Writeln(FP.AsString); rtInteger : Writeln(FP.AsInteger); rtFloat : Writeln(FP.AsFloat); rtBoolean : Writeln(FP.AsBoolean); rtDateTime : Writeln(FormatDateTime('cccc',FP.AsDateTime)); end;