Interpreter

• Interpreter: A way to include language elements in a program. Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

  

• This pattern doesn’t use very often. It can be thought as a special case of composite pattern.
  1. You need to have a grammar. This is start symbol is Expression. VarExp is leaf. That is a context free grammer
     Expression ::= AddExp | SubExp | VarExp 
     VarExp ::=1|2|3|4|5 
     AddExp ::= Expression + Expression 
     SubExp ::= Expression - Expression
  2. With this grammer, you can design class diagram as below:

     

  3. For this prammer, You need to parse a sentence defined by this grammar. The last purpose of parsing is to calculate the value of expression. So interprete() should be a calculate function. and interprete will be called recursively.
  4. parse function is not key problem to be solved by Interpreter patten. For regular grammar, you can use FST, for simple arithmetic( this example), You can use stack and scan a terminal in an expression from left to right.