Calculator Documentation

Step 4

This is step 4 of the calculator project in Exploration 68 of Exploring C++: The Programmer's Introduction to C++.

This version of the calculator adds functions. In doing so, it changes the var keyword to def to encompass the definition of variables and functions.

This version of the calculator has a more complicated syntax, but still simple enough for a recursive descent parser:

   start ::= statement-list
   statement-list ::= empty | statement-list statement
   statement ::= definition | quit | expression
   definition ::= "def" identifier optional-parameters "=" expression
   optional-parameters ::= empty | "(" optional-identifier-list ")"
   optional-identifier-list ::= empty | identifier-list
   identifier-list ::= identifier | identifier-list "," identifier
   expression ::= multiplicative-expression "+" multiplicative-expression |
                  multiplicative-expression "-" multiplicative-expression
   multiplicative-expression ::= unary-expression "*" unary-expression |
                                 unary-expression "/" unary-expression
   unary-expression ::= "-" primary | "+" primary | primary
   primary ::= number | function-call | "(" expression ")"
   function-call ::= identifier optional_arguments
   optional_arguments ::= empty | "(" optional-expression-list ")"
   optional-expression-list ::= empty | expression-list
   expression-list ::= expression | expression-list "," expression
   

Function arguments are evaluated lazily. There are no variables, only functions, but function definitions can look just like variable assignments:

   def pi = 3.141592653589792
   

so there really isn't any difference between a variable and a function.

The calculator reads expressions from the standard input, evaluates them, and prints the results to the standard output. It's that simple.

The calculator parses expression into a parse tree. The parser uses node objects. The node class uses the pimpl idiom with a tree of node_impl-derived classes.

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